Marine Global Fuel Standard (GFS) methodology

Background and history

Mid-term measures pathway

The Global Fuel Standard is the technical pillar of the basket of mid-term measures identified in the 2018 Initial IMO Strategy on Reduction of GHG Emissions from Ships (Resolution MEPC.304(72)) and concretised in the 2023 Revised Strategy (Resolution MEPC.377(80)). The Initial Strategy distinguished short-term measures (entering into force before 2023), mid-term measures (2023-2030) and long-term measures (post-2030). Short-term measures were operationalised in 2021 with the EEXI, the Carbon Intensity Indicator (CII) and the SEEMP Part III regulations adopted by Resolution MEPC.328(76). Mid-term measures were defined as comprising a technical element (a goal-based GHG fuel standard) and an economic element (a market-based measure or pricing mechanism), to be developed in parallel and adopted as a package.

The Revised IMO GHG Strategy of 2023 set the political envelope for the mid-term measures: net-zero GHG emissions from international shipping by or around 2050, indicative checkpoints of at least a 20 percent reduction (striving for 30 percent) by 2030 against a 2008 baseline, and at least 70 percent (striving for 80 percent) by 2040, plus the 2030 target that zero or near-zero GHG emission fuels and energy sources represent at least 5 percent of the international fleet’s energy use (striving for 10 percent). The MEPC instructed the Intersessional Working Group on GHG Emissions from Ships (ISWG-GHG) to deliver draft amendments to MARPOL Annex VI implementing both pillars in time for approval at MEPC 83.

Position of GFS within the Net-Zero Framework

Between MEPC 80 in July 2023 and MEPC 83 in April 2025, the ISWG-GHG and the MEPC plenary debated the relationship between the technical and the economic elements. Three architectural questions dominated the negotiation. First, should the technical element (the Global Fuel Standard) operate in isolation, with the economic instrument layered on top, or should it form an integrated single instrument? Second, should the standard set the WtW intensity bound directly in gCO2eq/MJ, or should it set a percentage reduction from a baseline that is then translated into gCO2eq/MJ? Third, should non-compliance with the standard trigger detention or only economic remediation? The integrated, percentage-from-baseline, economic-remediation-only architecture won out in the final compromise text approved at MEPC 83 by a vote of 63 in favour, 16 against and 24 abstaining.

The Global Fuel Standard sits within the Net-Zero Framework as the engineering instrument that defines what compliance looks like at the level of the individual ship and the individual reporting year. The economic instrument (the Remediation Unit and Surplus Unit mechanism, and the IMO Net-Zero Fund) operates on the outputs of the GFS calculation: a ship that exceeds its Required GFI must surrender RUs in proportion to the excess; a ship that beats the tighter Direct Compliance Threshold receives SUs in proportion to the over-compliance. Without the GFS, the economic instrument has no metric to act on; without the economic instrument, the GFS has no consequence. The two are inseparable in the compliance architecture, but they are formally distinct: the GFS is regulated by Chapter 4 ter, regulations 27ter to 31ter, while the IMO Net-Zero Fund and RU pricing are regulated by separate provisions and a delegated MEPC procedure.

From draft to adoption

The text approved at MEPC 83 will be formally adopted at MEPC 84 in October 2025 under the IMO tacit acceptance procedure of MARPOL Article 16, and enters into force on 1 January 2027 sixteen months after formal adoption (the Article 16 default acceptance interval). First reporting year is 2027, first surrender obligation falls due in the second quarter of 2028 against 2027 emissions, and the first IAPP renewal-survey verification of GFS compliance is required at the next IAPP renewal on or after 1 January 2027. The compliance year for GFS is the calendar year, aligned with the reporting period of the existing IMO Data Collection System (IMO DCS), which is itself the data source for the Attained GFI calculation.

How the GFS computes Attained GFI

The canonical formula

The Attained GFI of a ship in a given calendar year is the energy-weighted average well-to-wake greenhouse-gas intensity of the fuel mix actually consumed during that year, expressed in grams of CO2-equivalent per megajoule of fuel energy (LCV basis):

where the index $i$ runs over the distinct fuel pathways consumed; $M_i$ is the mass of fuel pathway $i$ consumed during the reporting year in tonnes; $\text{LCV}_i$ is the lower calorific value of pathway $i$ in MJ per tonne, taken from MEPC.391(82) defaults or from a certified pathway value supplied by the bunker certificate; and $f_{\text{WtW},i}$ is the well-to-wake emission factor of pathway $i$ in gCO2eq/MJ. The numerator is the total well-to-wake CO2-equivalent emissions in grams; the denominator is the total energy supplied by the fuel mix in megajoules. The ratio is the energy-weighted average intensity, equal numerically to the total emissions divided by the total energy.

Fuel-mix integration

The summation runs over fuel pathways, not fuel types. A pathway is the combination of fuel chemistry (for example, methanol), feedstock (for example, biomethane from anaerobic digestion of agricultural waste), production route (for example, electrolysis from renewable electricity) and end-use cycle (for example, dual-fuel low-pressure two-stroke). The MEPC.391(82) tables enumerate around 60 default pathways covering the principal commercial routes; a ship using a pathway not in the default tables, or wishing to claim a value lower than the default, must obtain a certified pathway value from a Certification Body recognised by its Administration under the Sustainability Certification Scheme established by Chapter 4 ter regulation 28ter.

Where two or more bunker stems of the same pathway are consumed in the year (for example, three deliveries of MEPC.391(82) default-pathway VLSFO from three suppliers), the masses are summed and a single $M_i$ entered. Where the same fuel chemistry is delivered under two different pathways (for example, default-pathway VLSFO and certified-pathway VLSFO from a different feedstock), each is treated as a separate $i$ index with its own $M_i$, $\text{LCV}_i$ and $f_{\text{WtW},i}$.

Energy basis and emission factor convention

The GFS uses the lower calorific value (LCV, also termed net calorific value) energy basis, not the higher calorific value (HCV) basis that is sometimes preferred in stationary combustion accounting. The choice matches the convention of the IMO LCA Guidelines, the EU FuelEU Maritime Regulation 2023/1805 and the EU MRV Regulation 2015/757. LCV excludes the latent heat of vaporisation of water vapour formed during combustion; for hydrocarbon fuels the LCV is approximately 90 to 93 percent of the HCV, with the gap widest for hydrogen-rich fuels.

The well-to-wake emission factor sums two stages. The well-to-tank (WtT) component covers all upstream emissions from feedstock extraction, processing, transport, distribution and bunkering, in gCO2eq per MJ of fuel delivered to the ship’s tank. The tank-to-wake (TtW) component covers emissions from on-board combustion, including direct CO2 from carbon oxidation, methane slip from internal combustion (especially relevant for LNG dual-fuel engines) and N2O from combustion. The CO2-equivalent uses the AR5 GWP100 metric (CH4 = 28, N2O = 265 against CO2 = 1) consistent with the choice of the IMO LCA Guidelines and the IPCC Fifth Assessment Report. Black carbon and other short-lived climate forcers are not included in the GFI calculation, although the MEPC has indicated that they may be incorporated in a future revision of the LCA Guidelines.

The combined factor is:

with both terms having the same units (gCO2eq/MJ-LCV) and identical GWP100 weighting. The MEPC.391(82) tables present the two terms separately for transparency and for use in regional schemes (FuelEU and EU ETS) that may use a TtW-only convention; the GFS itself uses only the combined WtW value.

Required GFI trajectory 2027-2050

The baseline and the trajectory function

The Required GFI is the year-specific upper bound on the Attained GFI that a ship must not exceed. It is a single number, common to all ships in scope, that decreases each year along a defined trajectory. The 2008 baseline value, set at the technical level by MEPC.377(80) and reaffirmed at MEPC 83, is 93.3 gCO2eq/MJ, derived from the global average WtW intensity of bunker fuels consumed by international shipping in 2008, weighted by the IMO Third GHG Study energy mix. The Required GFI in any given year $y$ is then:

where $r_y$ is the percentage reduction from the 2008 baseline to be achieved in year $y$, set by Chapter 4 ter regulation 29ter and revisable at five-year reviews.

The schedule of reductions

The schedule fixed by MEPC 83 for the period 2027-2035 is set in regulation 29ter and is binding from entry into force. Values for 2036-2050 are indicative, set against the MEPC.377(80) checkpoints of at least 70 percent (striving for 80 percent) reduction by 2040 and net-zero by or around 2050, and will be confirmed at the first five-year review at MEPC 88 in 2030.

The two columns reflect the two-tier model: the Required GFI is the Tier 1 threshold a ship must not exceed without surrendering Remediation Units; the Direct Compliance Threshold is the Tier 2 threshold below which a ship earns Surplus Units. The 13-percentage-point spread between Tier 1 and Tier 2 in 2027 narrows toward 2050 as the framework approaches absolute zero.

Mapping to the MEPC.377(80) checkpoints

The MEPC.377(80) Strategy sets indicative checkpoints in absolute emissions, not in fuel intensity. Translation between the two requires an assumption about the energy demand of the fleet. The 2030 Tier 1 figure of 17 percent reduction in fuel intensity corresponds, on the IMO Secretariat’s central energy-demand projection (3 percent annual fleet-energy growth from 2008 to 2030 followed by a 1 percent annual decline through 2050), to an absolute emissions reduction of about 22 percent against the 2008 baseline, slightly above the at-least-20-percent checkpoint and below the 30-percent striving level. The 2040 Tier 1 figure of 65 percent intensity reduction corresponds to an absolute reduction of about 73 percent, between the at-least-70-percent and striving-80-percent checkpoints. The 2050 Tier 1 figure of 100 percent intensity reduction is consistent with net-zero by or around 2050.

Tier 1 vs Tier 2 compliance pathways

Tier 1: Required GFI and Remediation Units

A ship whose Attained GFI exceeds the Required GFI for the year must surrender Remediation Units (RUs) in the second quarter of the following year, in a quantity equal to the excess CO2-equivalent emissions. The RU obligation is calculated as:

where $\text{GFI}_{\text{att}}$ and $\text{GFI}_{\text{req}}$ are in gCO2eq/MJ, $E_{\text{total}} = \sum_i M_i \cdot \text{LCV}_i$ is the total annual energy in MJ, and the result $\text{RU}_{\text{owed}}$ is in tonnes of CO2-equivalent. The factor $10^{-6}$ converts grams to tonnes.

One RU represents one tonne of CO2-equivalent and is purchased from the IMO Net-Zero Fund at a price set by MEPC. The price for 2027-2030 is fixed in the Net-Zero Framework Resolution at 100 USD per tonne CO2eq for the Tier 1 portion of the obligation (the portion between Required GFI and Direct Compliance Threshold) and at 380 USD per tonne CO2eq for the Tier 2 portion (the portion above the Direct Compliance Threshold, addressed under Tier 2 below). Prices for 2031 onward are revisable at the five-year reviews.

Tier 2: Direct Compliance Threshold and Surplus Units

The Direct Compliance Threshold (DCT) is the tighter Tier 2 bound. Three regimes coexist in any given year:

1. Over-compliance: $\text{GFI}_{\text{att}} \le \text{GFI}_{\text{DCT}}$. The ship earns Surplus Units (SUs) equal to $(\text{GFI}_{\text{DCT}} - \text{GFI}_{\text{att}}) \cdot E_{\text{total}} \cdot 10^{-6}$ tonnes CO2eq, which it may bank for two years or transfer to another ship in the same fleet under the pooling provision of Chapter 4 ter regulation 30ter.
2. Tier 1 compliance, Tier 2 non-compliance: $\text{GFI}_{\text{DCT}} < \text{GFI}_{\text{att}} \le \text{GFI}_{\text{req}}$. The ship surrenders RUs equal to the gap between Attained GFI and DCT, at the lower 100 USD/tCO2eq Tier 1 price.
3. Tier 1 non-compliance: $\text{GFI}_{\text{att}} > \text{GFI}_{\text{req}}$. The ship surrenders RUs in two parts: the gap between DCT and Required GFI at 100 USD/tCO2eq, plus the gap between Required GFI and Attained GFI at 380 USD/tCO2eq.

The two-tier structure is therefore not a binary pass-or-fail, but a piecewise-linear cost curve. A ship that just exceeds the DCT pays modestly; a ship that exceeds the Required GFI pays a steep marginal price on every gCO2eq/MJ of further excess. The economics push fleets toward the DCT line rather than toward the Required GFI line, accelerating fuel transition relative to a single-threshold design.

Pooling, banking and trading of Surplus Units

Surplus Units may be banked for up to two reporting years from the year of issue. They may be transferred (pooled) between ships under common ownership or operation, subject to declaration in the Statement of Compliance for both donor and recipient. They may also be traded between unrelated parties on the secondary market that the IMO Net-Zero Fund will host from 2028. SUs cannot, however, be used to meet the Tier 2 portion of an RU obligation: a ship whose Attained GFI exceeds the Required GFI must purchase the Tier 2 RUs at the higher 380 USD/tCO2eq price from the Fund directly, regardless of any SUs it holds. This asymmetry, between SU usability for Tier 1 gaps and SU non-usability for Tier 2 gaps, is the principal price-discovery mechanism that anchors the 380 USD ceiling.

Default fuel pathway intensities (MEPC.391(82))

Conventional residual and distillate

The MEPC.391(82) default WtW emission factors for the principal conventional fuel pathways, converted to the GFS units (gCO2eq/MJ-LCV, AR5 GWP100), are summarised below. The values are taken from Table 1 of the LCA Guidelines and rounded to one decimal place; the certified-pathway and bunker-delivery-note values used in actual GFI calculations may differ.

The three LNG entries reflect the materially different methane-slip behaviour of the three principal engine cycles. Otto-cycle medium-speed dual-fuel engines (low-pressure premix combustion) emit 3 to 5 percent of fuel mass as unburned methane in normal operation, equivalent to 8 to 14 gCO2eq/MJ at AR5 GWP100; Otto-cycle slow-speed engines emit 2 to 3 percent; high-pressure Diesel-cycle slow-speed engines (Wartsila X-DF-A, MAN ME-GA) emit below 0.4 percent. The TtW factor in the table reflects the central default of MEPC.391(82); a ship operating a low-slip configuration may apply for a certified pathway value below the default.

Bio and synthetic methane

Negative WtT values reflect the methane-avoidance credit assigned by MEPC.391(82) for biomethane sourced from feedstocks that would otherwise have emitted CH4 to the atmosphere (manure, landfill, anaerobic digestion of organic waste). The credit is capped at the AR5 GWP100 value of the avoided methane and is conditional on certification of the avoidance pathway under the Sustainability Certification Scheme.

Methanol grades

Methanol’s TtW value reflects the carbon content per MJ of LCV; the variation across pathways is concentrated in the WtT term. Fossil methanol exceeds VLSFO on a WtW basis because methanol’s lower LCV per tonne magnifies the impact of upstream natural-gas leakage in the reforming feedstock chain. Only bio-methanol and certain fully-renewable e-methanol pathways achieve WtW intensities materially below distillate diesel.

Ammonia grades

Ammonia has zero combustion CO2 by chemistry (no carbon in the molecule), but a non-zero N2O slip is permitted in the TtW factor under MEPC.391(82) when N2O exceeds the 0.005 percent of fuel-mass threshold. The default TtW in the table assumes engine designs that meet the threshold; for engines that exceed it, the certified pathway must add the N2O slip at AR5 GWP100. The grey-ammonia WtW value of 130 gCO2eq/MJ exceeds the 2027 Required GFI of 89.57 gCO2eq/MJ and is therefore not a compliance pathway in any GFS year; only blue and green ammonia are compliance-relevant.

See ammonia as marine fuel and methanol as marine fuel for the chemistry, engine technology and bunkering infrastructure that underlie these pathway numbers.

Hydrogen grades

Hydrogen’s high LCV per tonne (3 to 6 times that of conventional fuels) makes it the most energy-dense fuel by mass in the table, but its low volumetric density requires cryogenic or high-pressure storage that constrains commercial deployment. Like ammonia, hydrogen has zero combustion CO2; the WtT term dominates the WtW total.

Formula, assumptions, and limits

Formula

The canonical GFS formula set has three equations.

The Attained GFI for ship $s$ in year $y$:

The Required GFI in year $y$:

The Direct Compliance Threshold in year $y$:

with $r_{1,y} < r_{2,y}$ for all $y < 2050$. The compliance position is then the difference, in tonnes CO2eq, between the ship’s actual emissions and the emissions that would correspond to the Required GFI at the same energy:

Variables and units:

• $M_{i,s,y}$ : mass of fuel pathway $i$ consumed by ship $s$ in year $y$. Tonnes. Range: 0 to 200,000 t/year for the largest container vessels and ULCCs.
• $\text{LCV}_i$ : lower calorific value of pathway $i$. MJ/kg. Range: 18 to 120, with VLSFO at 41.0 and hydrogen at 120.0.
• $f_{\text{WtW},i}$ : well-to-wake emission factor. gCO2eq/MJ-LCV. Range: 0 to 130 across the MEPC.391(82) defaults.
• $E_{\text{total},s,y} = \sum_i M_{i,s,y} \cdot \text{LCV}_i$ : total annual energy consumption. MJ. Range: 10^9 (small coastal ship) to 10^13 (large ULCC).
• $r_{1,y}, r_{2,y}$ : percentage reductions from the 93.3 baseline for Tier 1 and Tier 2. Dimensionless 0 to 1. Sign convention: positive reduction means lower required intensity.
• $\text{Compliance position}$ : tonnes CO2eq. Sign convention: positive = excess emissions = RUs owed; negative = under-baseline = SUs earned (modulo Tier 2 split).

Derivation

Step 1: MARPOL Annex VI Chapter 4 ter regulation 27ter sets the obligation: every ship of 5,000 GT and above on international voyages must compute an Attained GFI in gCO2eq/MJ each calendar year and demonstrate compliance with the Required GFI for that year. The unit is fixed by the Regulation itself.

Step 2: Regulation 28ter directs the calculation to be made on a well-to-wake basis using the IMO LCA Guidelines, currently MEPC.391(82). The LCA Guidelines define the WtW boundary, the emission factor convention and the GWP100 metric.

Step 3: The Attained GFI formula is the consequence of (a) a per-pathway emission factor in gCO2eq/MJ-LCV and (b) an energy-weighted average across pathways. This is mathematically the only consistent way to define a fleet-wide intensity that is also an absolute emission (numerator) divided by an absolute energy (denominator), preserving additivity across pathways and across reporting periods.

Step 4: Regulation 29ter sets the Required GFI in terms of percentage reduction from a 2008 baseline. The 2008 baseline of 93.3 gCO2eq/MJ is the Third IMO GHG Study global average for international shipping bunker fuels, weighted by the 2008 energy mix. Multiplying by $(1 - r_y)$ produces the year-specific Required GFI.

Step 5: The same arithmetic structure applied to a tighter percentage $r_{2,y}$ produces the Direct Compliance Threshold under Tier 2. The piecewise-linear pricing function (100 USD/tCO2eq for the gap between DCT and Required GFI; 380 USD/tCO2eq for the gap above Required GFI) is set by the Net-Zero Framework Resolution.

Assumptions

The methodology bakes in the following analytical assumptions, all of which are stated explicitly in MEPC.391(82) or in the MEPC 83 final compromise text:

• Well-to-wake boundary including feedstock extraction, processing, transport, distribution, bunkering and combustion. Lubricants, ship-construction emissions and end-of-life recycling are excluded.
• Lower calorific value (LCV) energy basis, not HCV. Hydrogen-rich fuels are penalised relative to HCV-basis schemes by a few percent.
• AR5 GWP100 weighting for non-CO2 gases (CH4 = 28, N2O = 265). AR6 values (CH4 = 27.9, N2O = 273) are not used; this will be reviewed at MEPC 88 in 2030.
• MEPC.391(82) default values apply unless a certified pathway value is provided by the fuel supplier under the Sustainability Certification Scheme.
• Calendar-year reporting, aligned with the existing IMO DCS reporting period. No partial-year proration except for ships entering or leaving the in-scope fleet mid-year.
• Mass-based fuel accounting, with conversion to energy via LCV. Volumetric measurements at the bunker manifold are converted to mass using the certified density on the bunker delivery note.
• Emission factors are the same for all engine load points within a single fuel pathway, except where the pathway itself distinguishes engine cycle (the three LNG entries) or aftertreatment (selective catalytic reduction reducing N2O slip in ammonia engines).
• Black carbon, organic carbon and other short-lived climate forcers are not counted in the WtW factor. Only CO2, CH4 and N2O contribute.
• Energy from waste-heat recovery, shaft generators or shore power is not credited in the GFS denominator. Only direct fuel energy delivered to the propulsion and auxiliary plant counts.
• Wind-assist propulsion (rotor sails, kites, rigid sails) is credited only via the reduced fuel consumption it produces, not as a direct deduction from the numerator. The IMO is consulting on a separate wind-assist correction factor for the 2030 review.
• EmO (Electricity from On-shore Power Supply) and BioMix fuel substitutes are excluded from the GFS denominator: shore-supplied electricity does not appear in the IMO DCS fuel data and is not counted as energy under the GFS, although it does displace fuel that would otherwise have been burned. This is symmetric with the FuelEU treatment but inconsistent with the EU ETS treatment, which credits zero CO2 to shore power.

Worked example

Consider the bulk carrier MV Cape Pioneer, a Capesize of 180,000 DWT and 25,000 GT, in calendar year 2028.

Annual fuel consumption (from IMO DCS, BDN-reconciled):

• VLSFO, MEPC.391(82) default pathway: 9,000 t/year, $\text{LCV} = 41.0$ MJ/kg, $f_{\text{WtW}} = 91.0$ gCO2eq/MJ.
• Bio-LNG, manure-feedstock, slow-speed Diesel-cycle engine, certified pathway: 1,000 t/year, $\text{LCV} = 50.0$ MJ/kg, $f_{\text{WtW}} = -50.0$ gCO2eq/MJ.

Step 1: Energy contributions (in MJ, after multiplying by 10^3 for kg/t):

• VLSFO: $9{,}000{,}000 \cdot 41.0 = 3.69 \times 10^{8}$ MJ.
• Bio-LNG: $1{,}000{,}000 \cdot 50.0 = 5.0 \times 10^{7}$ MJ.
• Total: $E_{\text{total}} = 4.19 \times 10^{8}$ MJ.

Step 2: Numerator (in gCO2eq):

• VLSFO: $3.69 \times 10^{8} \cdot 91.0 = 3.358 \times 10^{10}$ gCO2eq.
• Bio-LNG: $5.0 \times 10^{7} \cdot (-50.0) = -2.5 \times 10^{9}$ gCO2eq.
• Total: $3.108 \times 10^{10}$ gCO2eq.

Step 3: Attained GFI:

Step 4: Compare against thresholds for 2028:

• Required GFI 2028 = 87.70 gCO2eq/MJ. The Attained GFI of 74.18 is below the Required GFI. No Tier 1 RU obligation.
• DCT 2028 = 75.57 gCO2eq/MJ. The Attained GFI of 74.18 is below the DCT. Surplus Units are earned.

Step 5: Surplus Units earned:

(restated as $1.39 \cdot 419{,}000{,}000 \cdot 10^{-6} = 582$ tCO2eq, with intermediate units consistent.)

Step 6: Economic position. The 582 SUs may be banked, pooled within the same operator’s fleet, or sold on the IMO Net-Zero Fund secondary market. At a hypothetical SU clearing price of 80 USD/tCO2eq (illustrative, not regulated), the SUs are worth approximately $582 \cdot 80 = 46{,}560$ USD.

For comparison, a sister ship MV Cape Reactor burning 100 percent default-pathway VLSFO (10,000 t at 91.0 gCO2eq/MJ) in 2028 would have $\text{GFI}_{\text{att}} = 91.0$ gCO2eq/MJ, exceeding both the 2028 Required GFI (87.70) and the DCT (75.57). The compliance position would split:

• Tier 1 portion (DCT to Required GFI gap, 91.0 to 87.70 only counts above Required GFI; the DCT-to-Required gap is 87.70 to 75.57, so the Tier 1 portion is the full 87.70 to 75.57 = 12.13 g/MJ): $12.13 \cdot 4.10 \times 10^{8} \cdot 10^{-6} = 4{,}973$ tCO2eq at 100 USD = 497,330 USD.
• Tier 2 portion (above Required GFI): $(91.0 - 87.70) \cdot 4.10 \times 10^{8} \cdot 10^{-6} = 3.30 \cdot 410 = 1{,}353$ tCO2eq at 380 USD = 514,140 USD.
• Total RU cost: 1,011,470 USD per year.

The contrast (a 46,560 USD credit for the 10 percent bio-LNG ship versus a 1,011,470 USD obligation for the 100 percent VLSFO ship) is the price signal the GFS is designed to deliver.

The GFI compliance calculator reproduces this calculation in an interactive form, taking as input the fuel mix, year and RU price, and returning the Attained GFI, the year-specific thresholds, and the compliance position in both physical (tCO2eq) and monetary (USD) units. The GFI attained calculator computes only the intensity, useful for tendering and bunker-procurement modelling.

Edge cases and limits

• Ships under 5,000 GT are excluded from the GFS entirely, by Chapter 4 ter regulation 27ter paragraph 1. They remain subject to the energy-efficiency provisions of Chapter 4 (EEDI for new ships, EEXI for existing ships, SEEMP), but not to the GFI metric or the RU/SU mechanism. The 5,000 GT threshold matches the existing IMO DCS scope and the EU MRV scope.
• Domestic voyages are excluded. Only fuel consumed on international voyages, defined by reference to the existing MARPOL Article 2 and IMO DCS scoping, contributes to $M_i$. A ship that performs both domestic and international voyages must apportion fuel between the two using the same method as for IMO DCS, typically a port-call-based allocation.
• Ice-class ships receive a correction factor on the Required GFI, equal to the EEDI ice-class correction $f_j$ from MARPOL Annex VI Regulation 21 paragraph 4.1. The correction inflates the Required GFI by 1 to 7 percent depending on ice class, recognising the additional propulsion power required to operate in ice.
• Passenger ships are not subject to a different threshold but are subject to a different baseline-derivation process at the next five-year review, in recognition of the materially different energy profile of passenger operations. MEPC 86 in 2028 will deliberate the proposal.
• Voyage-mix rounding can shift a ship between tiers in marginal years. The Statement of Compliance specifies that the Attained GFI is to be calculated to two decimal places (gCO2eq/MJ) and rounded toward the higher integer; the Required GFI and DCT are likewise to two decimals. A ship reporting an Attained GFI of 87.71 in 2028 (above 87.70 by 0.01 and within rounding) is treated as non-compliant and owes a small RU obligation.
• Null-energy denominators arise for ships that record zero international fuel consumption in a year (laid up, in repair, on domestic-only voyages). The GFI formula is undefined in this case. The Statement of Compliance treats null-energy years as non-applicable and carries forward any banked SUs by their normal two-year limit; no RU obligation arises.
• Certified-pathway override of defaults is a one-way ratchet: a ship may always claim a default pathway value, but to claim a value lower than the default it must hold a Certified Pathway Statement issued by a recognised Certification Body. Loss of certification (for example, withdrawal of certification mid-year) reverts the pathway to the default.
• Fuel-mix changes mid-year are accommodated by the summation: each pathway is summed across all bunker stems in the year, and the formula is applied once to the aggregate. There is no requirement to compute separate Attained GFIs for different periods within a year.
• Wind-assist energy is not credited directly. Rotor sails, kite sails and rigid wing sails contribute to compliance only through the reduced fuel consumption they produce. The MEPC has indicated that a wind-assist correction factor in the GFS denominator may be considered at the 2030 review.
• Onshore power supply during port calls displaces fuel that would otherwise have been burned by the auxiliary engines or boilers. Under the GFS, this fuel simply does not appear in the IMO DCS data and so does not enter the GFI calculation. The displaced fuel is therefore implicitly credited at the Attained GFI of the rest of the year’s mix, not at zero. Ships seeking to maximise the OPS benefit should bunker low-carbon fuel for the remainder of the year.

Regulatory basis

• MARPOL Annex VI Chapter 4 ter (regulations 27ter to 31ter), as approved by MEPC 83 on 11 April 2025 and to be formally adopted at MEPC 84 in October 2025. Effective date 1 January 2027 under the tacit acceptance procedure.
• Resolution MEPC.391(82): 2024 Guidelines on lifecycle GHG intensity of marine fuels (the IMO LCA Guidelines), adopted at MEPC 82 in October 2024 and incorporated by reference in Chapter 4 ter regulation 28ter.
• Resolution MEPC.377(80): 2023 IMO Strategy on Reduction of GHG Emissions from Ships (the Revised IMO GHG Strategy), adopted at MEPC 80 in July 2023.
• Resolution MEPC.328(76): 2021 amendments to MARPOL Annex VI introducing EEXI, CII and SEEMP Part III, the antecedent short-term measures with which GFS coexists.
• MEPC 83 final report, MEPC 83/15, including the Net-Zero Framework Resolution and the agreed RU price schedule.
• Sustainability Certification Scheme: Chapter 4 ter regulation 28ter paragraph 4 and MEPC.391(82) Annex 3, defining the Certification Body recognition process.

Common errors

• Mixing tank-to-wake (TtW) and well-to-wake (WtW) in the emission factor. The GFS uses WtW only. A common error is to apply the EU MRV TtW factor (which excludes upstream emissions) to the GFS calculation, understating the intensity of LNG and methanol pathways and overstating the relative advantage of green hydrogen.
• AR4 instead of AR5 GWP for methane and nitrous oxide. AR4 GWP100 is CH4 = 25, N2O = 298; AR5 is CH4 = 28, N2O = 265. The IMO LCA Guidelines specify AR5; using AR4 understates LNG slip emissions by about 11 percent and overstates ammonia N2O slip by about 12 percent.
• Double-counting EU ETS surrender as if the EUA surrender for a voyage offsets the GFS RU obligation for the same voyage. The two are independent: EUAs go to the EU ETS auction; RUs go to the IMO Net-Zero Fund. A ship operating between Rotterdam and Singapore in 2027 owes both, and the IMO and EU Commission are consulting on a partial offset mechanism for 2028 onward, but no offset exists in 2027.
• Misclassifying voyage segments between domestic and international. The GFS scope follows IMO DCS, which uses the port-of-call definitions of the IMO Ship Identification Number. Mis-classification of a domestic leg as international (or vice versa) shifts both the energy denominator and the emission numerator and can flip the compliance position.
• SOx-content drift confounding GFI. A ship that switches from VLSFO (0.5 percent S) to HFO with a scrubber retains a similar GFI on a WtW basis (since SOx does not enter GWP100), but the bunker delivery note may show a different LCV, slightly shifting the energy denominator. The GFI is sensitive to LCV at the second decimal place; bunker certificate accuracy matters.
• Forgetting the bio-pathway certification expiry. Bio-LNG, bio-methanol and biodiesel pathways with negative WtT factors require a current Certified Pathway Statement. If certification lapses mid-year, the Attained GFI must be recomputed using the default pathway for the uncertified portion.
• Overlooking the methane-slip distinction across LNG engine types. The three MEPC.391(82) LNG entries (Otto medium-speed, Otto slow-speed, Diesel slow-speed) differ by up to 16 gCO2eq/MJ in WtW intensity. Applying the wrong row to a ship’s actual engine cycle can shift the Attained GFI by 5 to 10 gCO2eq/MJ.
• Treating Surplus Units as fungible against Tier 2 obligations. SUs may offset only Tier 1 (DCT-to-Required) gaps, not Tier 2 (above Required) obligations. The asymmetry is the principal price-discovery mechanism, and overlooking it leads to systematic under-provisioning of Tier 2 RU purchases.

Interaction with regional regimes

FuelEU Maritime overlap

FuelEU Maritime (Regulation (EU) 2023/1805) imposes a parallel WtW intensity reduction on energy used at, or to or from, EU ports, with reductions of 2 percent in 2025, 6 percent in 2030, 14.5 percent in 2035, 31 percent in 2040, 62 percent in 2045 and 80 percent in 2050 against the same 2008 baseline. The GFS Required GFI is more demanding than FuelEU in 2027 (4 percent reduction versus 2 percent in 2025) and remains broadly aligned through 2050. The EU has stated that, once the GFS is in force, it will consider FuelEU as fulfilled by GFS compliance for the WtW portion, retaining FuelEU only for the additional pooling, multiplier and shore-power provisions. See FuelEU penalties, pooling and multipliers for the structural detail.

The technical convergence (both schemes use WtW gCO2eq/MJ, both use AR5 GWP100, both use LCV energy basis, both use a 93.3 baseline) means the GFI calculation is shared, but the compliance positions differ because the trajectories differ. A ship trading exclusively to and from EU ports in 2027 would compute one Attained GFI and compare it against two Required GFIs, owing FuelEU penalty if it failed the 2 percent FuelEU reduction and an RU obligation if it failed the 4 percent GFS reduction; in 2030, when GFS sits at 17 percent versus FuelEU at 6 percent, the FuelEU obligation becomes irrelevant for any ship that meets the GFS Tier 1.

EU ETS double-counting concerns

EU ETS for shipping, under Directive (EU) 2023/959, requires a ship to surrender EUAs covering 100 percent of CO2 emissions from intra-EU voyages and 50 percent of emissions from voyages to or from a non-EU port, plus 100 percent of emissions during port calls. The EU ETS uses a TtW CO2-only metric, in contrast to the GFS WtW CO2eq metric. A ship operating between Hamburg and New York in 2027 is therefore exposed simultaneously to:

• IMO GFS RU obligation on the WtW CO2eq emissions of all international fuel use, computed against the 2027 Required GFI of 89.57 gCO2eq/MJ.
• EU ETS EUA obligation on 50 percent of the TtW CO2 emissions of the Hamburg-to-New-York voyage and 100 percent of the port-call emissions, at the prevailing EUA price.
• FuelEU penalty exposure on the WtW CO2eq emissions of the same voyage if it fails the 2 percent reduction.

The triple exposure is not a double or triple count of the same emissions, because each scheme defines a different metric and a different scope. A single tonne of CO2 emitted on a Hamburg-to-New-York voyage produces an obligation under all three. The IMO MEPC and the European Commission have committed at MEPC 83 to begin consultations on partial offsetting from 2028 onward, with the likely outcome that EUA surrender will offset 50 percent of the GFS Tier 2 RU price for in-scope voyages, but no formal mechanism exists in the 2027 reporting year.

Port-state vs flag-state enforcement

The GFS is enforced by the flag State at the IAPP renewal survey, with port-State control supplementing flag-State authority where reasonable grounds exist. The Statement of Compliance and the Confirmation of Continuous Compliance issued under regulation 31ter are the documentary basis for enforcement. A ship without a valid Statement of Compliance is detainable under MARPOL Article 5 paragraph 2 in any port-State control inspection in any Party State.

The contrast with EU ETS and FuelEU, which are enforced by the Member State of administration (allocated by the EU Commission), produces three potentially distinct enforcement authorities for the same ship in the same year: the flag State for GFS, the EU Member State of administration for EU ETS and FuelEU, and the port State for any of the three. The Net-Zero Framework Resolution explicitly notes that the IMO and the EU Commission will jointly develop guidance on coordinated enforcement to avoid duplicative administrative burden.

Implementation timeline and verification

Entry into force

Formal adoption is scheduled for MEPC 84 in October 2025. Under MARPOL Article 16 the tacit acceptance interval runs sixteen months, which sets entry into force at 1 January 2027. The first reporting year is therefore 2027, with first surrender obligation on 30 June 2028 against 2027 calendar-year emissions. The IAPP renewal-survey verification cycle catches each ship at its next IAPP renewal on or after 1 January 2027; for ships whose IAPP renews in late 2026, the first GFS verification falls on the renewal in 2031 (five years later), but this does not exempt the ship from the annual reporting and surrender obligations through the Statement of Compliance.

Reporting flow

Fuel consumption data flows through the existing IMO Data Collection System (IMO DCS), augmented by pathway and certification data from the bunker delivery note. The flow is:

1. Bunker delivery (BDN) records the fuel pathway, mass, LCV, density and (where applicable) certified pathway value at the bunker manifold.
2. The ship’s engineer logs daily and voyage fuel consumption in the IMO DCS bunker logbook, allocating consumption to the relevant pathway.
3. At year-end, the data is aggregated into the IMO DCS annual report, transmitted to the flag Administration by 31 March of the following year.
4. The flag Administration verifies the data and issues a Statement of Compliance setting out the Attained GFI, the Required GFI, the DCT, the compliance position and the RU obligation or SU credit.
5. RU surrender (or SU issuance) takes place by 30 June of the following year via the IMO Net-Zero Fund clearing system.
6. The Confirmation of Continuous Compliance is issued and accompanies the IAPP certificate.

Verification at IAPP renewal

The first GFS verification occurs at the first IAPP renewal survey on or after 1 January 2027. The Recognised Organisation (typically the classification society holding the IAPP delegation) reviews the Statement of Compliance for the previous calendar year, the bunker delivery notes, the IMO DCS records and any certified pathway statements, and confirms or queries the Attained GFI calculation. Discrepancies trigger a re-calculation and, where necessary, a re-issue of the Statement of Compliance and an adjustment to the RU surrender. Repeated material discrepancies trigger a flag-State investigation under MARPOL Article 6 and may result in withdrawal of the IAPP certificate.

The cycle then repeats annually, with each year’s Statement of Compliance reviewed at the next IAPP intermediate or annual survey, until the next IAPP renewal at five-year intervals. The intermediate-survey check is a desktop review; only the renewal survey involves an on-board audit of the bunker logbook and the IMO DCS records.

See also

• IMO Net-Zero Framework
• IMO GHG Strategy
• MARPOL Annex VI
• MARPOL Convention
• FuelEU Maritime explained
• FuelEU penalties, pooling and multipliers
• EU ETS for shipping
• IMO DCS vs EU MRV
• What is CII
• CII corrective action plan
• Slow steaming and CII
• Ammonia as marine fuel
• Methanol as marine fuel
• IMO 2020 sulphur cap

References

• IMO press briefing: Net-Zero Framework approved at MEPC 83 (April 2025).
• Resolution MEPC.391(82): 2024 Guidelines on lifecycle GHG intensity of marine fuels (LCA Guidelines).
• MEPC 83 final report, MEPC 83/15, including the Net-Zero Framework Resolution.
• Resolution MEPC.377(80): 2023 IMO Strategy on Reduction of GHG Emissions from Ships.
• MARPOL Annex VI: Prevention of Air Pollution from Ships, consolidated text (2024 edition).
• Regulation (EU) 2023/1805: FuelEU Maritime.
• IPCC Fifth Assessment Report (AR5), Working Group I: Climate Change 2013, GWP100 metrics.
• IMO short-term measures: EEDI, EEXI, SEEMP and CII regulations under MARPOL Annex VI Chapter 4.
• Directive (EU) 2023/959: extension of the EU ETS to maritime transport.
• IMO Data Collection System (DCS): Regulations 27 and 28 of MARPOL Annex VI Chapter 4.
• Resolution MEPC.328(76): 2021 amendments introducing EEXI, CII and SEEMP Part III.

Related calculators

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• Slow Steaming - Fuel Savings per Voyage
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• UN1202 - Diesel fuel (UN2760) (Class 3)
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Related formulas

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• Fuel Cf conversion factors (MEPC.364(79))
• ECA Fuel-Cost Premium
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