Well-to-Wake Intensity

Background

Why WtW matters

Marine fuels were historically regulated only on the basis of their tank-to-wake (TtW) emissions: the CO2, CH4 and N2O emitted at the funnel from combustion and slip. The TtW basis is the foundation of the IMO Data Collection System (IMO DCS) and was the basis of the original Carbon Intensity Indicator (CII). The TtW basis treats all fuels as equivalent to their direct combustion emissions; for example, LNG as marine fuel has approximately 25% lower TtW CO2 intensity than HFO, and a renewable methanol and a fossil methanol both have the same TtW CO2 intensity (because they are chemically identical at the point of combustion).

The TtW basis fails to capture two important distinctions:

1. Upstream emissions: producing LNG involves significant methane leakage (typically 0.5 to 5% of the gas produced, depending on the production region) and significant CO2 emissions from compression and liquefaction; the WtT contribution to LNG WtW intensity is approximately 15 to 30 g-CO2eq/MJ, comparable in magnitude to the TtW saving over HFO. Producing renewable methanol from green hydrogen and captured CO2 has near-zero upstream emissions; producing fossil methanol from natural gas has significant upstream emissions.
2. Renewability and additionality: a biomass-derived fuel that displaces fossil fuel reduces atmospheric CO2 only to the extent that the biomass would have been planted and grown specifically to produce the fuel (the “additionality” criterion). A fuel produced from renewable electricity is renewable only if the electricity is genuinely incremental to what would otherwise have been produced.

The introduction of the WtW basis in the FuelEU Maritime Regulation (in force January 2025) and in the IMO Net-Zero Framework GFI standard (in force 2027) brings marine fuel regulation into line with land-based transport (where WtW analysis under EU RED has been mandatory since 2009).

Definitions and the system boundary

The total intensity is the sum of the two stages, $GHG_{WtW} = GHG_{WtT} + GHG_{TtW}$, both expressed in g-CO2eq per MJ of fuel energy delivered to and consumed by the ship. The IMO 2024 LCA Guidelines draw the dividing line at the ship’s fuel tank: everything before the tank is well-to-tank (upstream), everything from the tank to the funnel is tank-to-wake (downstream). The guidelines’ generic supply chain runs feedstock extraction or cultivation, early processing at source, transport to the conversion site, conversion to the product fuel, product transport and storage and bunkering, then combustion onboard.

The WtT boundary covers all GHG emissions from feedstock extraction or production, through fuel processing and refining, to delivery to the bunker tanker (or to the vessel directly). MEPC.391(81) decomposes the upstream term as $GHG_{WtT} = e_{fecu} + e_l + e_p + e_{td} - e_{sca} - e_{ccs}$, where $e_{fecu}$ is emissions from feedstock extraction, cultivation and upstream processing, $e_l$ is land-use-change emissions, $e_p$ is fuel-processing emissions, $e_{td}$ is transport, distribution and bunkering emissions, $e_{sca}$ is a credit for soil carbon accumulation, and $e_{ccs}$ is a credit for any carbon capture and storage applied upstream. For a fossil fuel, WtT covers crude extraction, transport to refinery, refining, and transport to bunker. For a biofuel, WtT covers feedstock cultivation (or waste collection), transport to the processing plant, processing, and transport to bunker.

The TtW boundary covers all GHG emissions from combustion of the fuel in the engine, including any unburned fuel released as slip (methane slip for LNG; ammonia slip for ammonia as marine fuel) and the CO2 from any pilot fuel for dual-fuel engines. The IMO formula builds the downstream term from carbon-based emission factors per unit of fuel mass, $C_{fCO_2}$, $C_{fCH_4}$ and $C_{fN_2O}$, each multiplied by its GWP, then divides by the fuel’s lower calorific value (LCV) to land back in g-CO2eq/MJ. The combusted fraction is reduced by the slipped and fugitive fractions: the ship-side methane slip term is $C_{slip\_ship} = C_{slip} \times (1 - C_{fug}/100)$, separating the methane that escapes the engine unburned ($C_{slip}$) from the methane lost as fugitive emissions upstream of combustion ($C_{fug}$). The slipped fuel is then re-counted at its own GWP, so methane that escapes the cylinder is penalised at the full $GWP_{CH_4}$ rather than the lower CO2-from-combustion rate it would have carried had it burned.

For biofuels, the TtW CO2 emission is conventionally counted as zero (the carbon released by combustion was originally absorbed from the atmosphere by photosynthesis), so the WtW intensity of a biofuel equals its WtT intensity. This convention is challenged by the Indirect Land-Use Change (ILUC) literature, which argues that diversion of land or biomass to fuel production indirectly causes additional CO2 emissions from forest clearance or grassland conversion elsewhere; the EU RED applies an ILUC additivity factor for some feedstock categories.

Global warming potentials and the AR5 100-year basis

GHG emissions of different gases are normalised to a CO2-equivalent basis using global warming potentials (GWPs). The IPCC publishes successive revisions of GWP values, and the two regulatory regimes that drive WtW accounting in 2025 do not use the same set. This is the single fact that most often trips up a first compliance calculation: a methanol or LNG bunker can carry two slightly different WtW numbers depending on which rulebook you open.

The IMO 2024 LCA Guidelines (Resolution MEPC.391(81), adopted 22 March 2024) define the CO2-equivalent of a fuel’s combustion gases at the GWP-100 horizon as $g_{CO2eq(100y)} = 1 \times gCO_2 + 28 \times gCH_4 + 265 \times gN_2O$. The guidelines also tabulate a GWP-20 set, $g_{CO2eq(20y)} = 1 \times gCO_2 + 84 \times gCH_4 + 264 \times gN_2O$, for sensitivity work, but the GFI compliance metric runs on the 100-year values. These are the IPCC AR5 fossil-origin GWP-100 figures.

The FuelEU Maritime Regulation (Regulation (EU) 2023/1805) uses a different set in its Annex II tank-to-wake weighting: $GWP_{CO_2} = 1$, $GWP_{CH_4} = 25$, and $GWP_{N_2O} = 298$. These are the IPCC AR4 (Fourth Assessment Report) 100-year values, carried over to keep FuelEU consistent with the EU Emissions Trading System and the EU Monitoring, Reporting and Verification (MRV) regime that feeds it. The 3-point gap on methane (25 versus 28) and the 33-point gap on nitrous oxide (298 versus 265) mean that a high-methane-slip LNG vessel scores marginally worse under FuelEU’s CH4 weighting is lower but its N2O weighting is higher, so the two effects partly offset and the net divergence on a typical marine fuel is under 1 g-CO2eq/MJ. For ammonia, where N2O dominates the TtW term, the FuelEU N2O = 298 weighting produces a materially higher WtW figure than the IMO N2O = 265 weighting at the same measured slip rate.

The choice of GWP-100 rather than GWP-20 is a policy choice, not a measurement choice. Because methane has an atmospheric lifetime of roughly 12 years, the integration period changes the ranking of fuels: under GWP-20, LNG carries a higher WtW intensity than HFO once methane slip and upstream methane leakage are counted, because the CH4 weight jumps from 28 to 84; under GWP-100, LNG stays below HFO under typical leakage assumptions. Both the IMO GFI metric and the FuelEU limit run on the 100-year horizon, so a fuel that looks clean on the regulatory scorecard can still be a near-term warming liability under a 20-year lens.

The GWP* metric, a modified GWP that better reflects the temperature impact of short-lived gases relative to long-lived ones, appears in academic literature but is not used in either the FuelEU or the IMO regulatory framework.

The fossil baseline and reduction trajectories

FuelEU Maritime fossil baseline

The FuelEU Maritime Regulation specifies a fossil baseline of 91.16 g-CO2eq/MJ, derived from a weighted average of the WtW intensity of the dominant marine fuels in 2020 (HFO, VLSFO, LSMGO and MGO). The reduction trajectory (relative to this baseline) is:

Compliance is on a per-vessel-per-calendar-year basis. Non-compliance triggers a pooling, multiplier and penalty regime in which the company can either pay the penalty (currently EUR 2,400 per t VLSFO-equivalent of compliance deficit) or pool the deficit with other compliant vessels in the same company or via a multi-company pooling agreement.

IMO Net-Zero Framework GFI baseline

The IMO Net-Zero Framework, adopted at MEPC 83 in April 2025, specifies a fossil baseline of approximately 93.3 g-CO2eq/MJ (slightly different from FuelEU due to a different fossil-fuel weighting). The reduction trajectory (relative to this baseline) is:

A vessel meeting the direct compliance threshold has zero GFI deficit; a vessel between the direct and indirect thresholds has a Tier 1 remedial unit liability; a vessel above the indirect threshold has both Tier 1 and Tier 2 remedial unit liability. The remedial-unit pricing is set by an IMO-administered fund, with prices in the order of USD 100 to USD 380 per t CO2eq of deficit by 2030 (subject to MEPC review).

EU RED III shipping target

The EU Renewable Energy Directive Recast (RED III) Article 25 specifies that a minimum of 1.0% of marine fuel energy sold to ships in EU ports must be from Renewable Fuels of Non-Biological Origin (RFNBO) by 2030, rising to higher percentages thereafter. RFNBO is a defined category that requires:

• The fuel must be produced from renewable electricity, with strict additionality and temporal correlation criteria (the renewable electricity must be matched on an hourly basis from new renewable installations).
• The fuel must achieve at least 70% GHG savings compared to the fossil comparator (95.6 g-CO2eq/MJ for marine fuel), corresponding to a maximum WtW intensity of approximately 28.7 g-CO2eq/MJ.

Eligible RFNBO marine fuels include: green hydrogen, green ammonia (from green hydrogen), e-methanol (from green hydrogen + captured CO2), e-LNG (from green hydrogen + captured CO2). See RFNBO under EU rules for the full eligibility framework.

WtW intensity by fuel type

Conventional fossil fuels

The values above are representative central estimates from the FuelEU Maritime Annex II default values (Annex II provides a tabulated set of default WtW intensities for use in compliance calculations where no certified bunker delivery note is available). Annex II is updated periodically by the European Commission as the Joint Research Centre (JRC) refines the default values.

LNG WtW intensity depends critically on the engine type and methane slip rate:

• Spark-ignition (SI) engines (Wartsila DF, Caterpillar 3500 series): no methane slip in normal operation; lowest WtW intensity.
• High-pressure dual-fuel (HPDF) engines (MAN ME-GI, Win GD X-DF): low slip (typically 0.2 to 0.5% of fuel); low WtW intensity.
• Low-pressure dual-fuel slow-speed (LBSI) engines (Win GD RT-flex DF, MAN ME-LGI): medium slip (typically 1.0 to 2.5% of fuel).
• Low-pressure spark-ignition (LPSI) engines (Wartsila 50DF generation 1, Caterpillar 3600 series): higher slip (typically 2.5 to 5.0% of fuel); the slip can be sufficient to make LNG WtW intensity comparable to or higher than HFO. This was the principal “methane slip” controversy of the early 2020s.

See methane slip from LNG dual-fuel for a full treatment.

Methanol

Methanol can therefore range from higher WtW intensity than HFO (fossil methanol from natural gas) to near-zero WtW intensity (e-methanol with strict RFNBO accounting). The economic and regulatory value of a methanol bunker is highly dependent on the certified production pathway. See methanol as marine fuel for the full pathway analysis.

Ammonia

Ammonia TtW intensity is dominated by N2O slip from the engine, which has a GWP of 265. A 1% N2O slip rate (calculated as N2O emitted per N2O equivalent burned) translates to approximately 6.6 g-CO2eq/MJ of TtW intensity, comparable in magnitude to the WtT intensity of green ammonia. Engine manufacturers (MAN Energy Solutions, Wartsila, Win GD) are actively developing N2O catalysts and combustion strategies to minimise N2O slip; current commercial ammonia engines (2024) achieve approximately 0.05 to 0.5% N2O slip. See ammonia as marine fuel and N2O emissions from marine engines for full analyses.

Biofuels

Biofuel WtW intensity depends critically on the feedstock and the ILUC accounting:

• Annex IX Part A feedstocks (waste cooking oil, animal fats Cat 1/2, lignocellulosic biomass, palm oil mill effluent): low WtW intensity (typically 5 to 20 g-CO2eq/MJ); double-counted under FuelEU Maritime (each MJ counts as 2 MJ for compliance).
• Annex IX Part B feedstocks (used cooking oil, animal fats Cat 3, molasses): moderate WtW intensity; capped at a fleet-wide share of bio-feedstock.
• Food and feed crops (rapeseed, soya, palm, maize, sugar beet, sugar cane): high WtW intensity due to ILUC; maximum 7% share of EU transport energy under RED III; further restricted for marine.

See biofuels in shipping and RFNBO under EU rules for full treatments.

Hydrogen

Hydrogen WtW analysis is similar in structure to ammonia (which is itself produced from hydrogen); the principal differences are the absence of the ammonia synthesis step and the lower theoretical N2O slip rate (because hydrogen contains no nitrogen).

Captured CO2 and OCC

Onboard carbon capture (OCC) interacts with WtW analysis as follows: the captured CO2 is excluded from the TtW intensity provided it is permanently stored or utilised in a way that does not subsequently re-release the CO2. A 70% capture rate on HFO combustion reduces the effective TtW CO2 from approximately 79.5 g-CO2eq/MJ to approximately 23.9 g-CO2eq/MJ, with the WtW intensity falling from approximately 93 to approximately 37 g-CO2eq/MJ (after accounting for the energy penalty of the capture system).

Calculation methodology

FuelEU Maritime calculation

Under FuelEU Maritime, the annual GHG intensity of energy used onboard is calculated as:

where $E_i$ is the energy of fuel category $i$ used by the vessel (in MJ) and $WtW_i$ is the WtW intensity of fuel category $i$ (in g-CO2eq/MJ). The sum is over all fuel categories used by the vessel during the calendar year.

The WtW intensity values are taken from:

1. Bunker delivery note (BDN) certified values if available and traceable.
2. FuelEU Annex II default values if certified BDN values are not available.

For RFNBO fuels, a multiplier of 2 is applied to the energy (each MJ of RFNBO counts as 2 MJ for compliance), with the multiplier applying through 2034. For onshore power supply at berth, the energy is counted at zero g-CO2eq/MJ (where the shore power is from the EU electricity grid, with appropriate residual mix accounting).

The compliance check is: $GHG_{intensity}$ is compared to the maximum permitted intensity for the calendar year (e.g. 89.34 g-CO2eq/MJ in 2025); if the vessel exceeds the maximum, a compliance deficit is calculated as:

The deficit can be:

• Pooled with surplus from other vessels in the same company or via a third-party pooling agreement.
• Banked for use against a future-year deficit.
• Penalised at EUR 2,400 per t VLSFO-equivalent (the default penalty rate, increasing by 10% per consecutive year of non-compliance).

See FuelEU penalties, pooling and multipliers for the full mechanic.

IMO Net-Zero Framework GFI calculation

The IMO Net-Zero Framework GFI calculation is structurally similar to the FuelEU Maritime calculation but with different default values, different multipliers and a different penalty / remedial-unit mechanic.

The vessel’s annual GFI is:

Compliance is tested against two thresholds: a direct compliance threshold and an indirect compliance threshold. A vessel meeting the direct compliance threshold has no liability. A vessel between the direct and indirect thresholds has a Tier 1 remedial unit liability, calculated as the deficit at a Tier 1 price (set by the IMO Fund). A vessel above the indirect threshold has additional Tier 2 remedial unit liability at a higher Tier 2 price.

The remedial units can be:

• Surrendered as actual emission reductions from a low-carbon fuel use elsewhere, evidenced by certified credits.
• Purchased from the IMO Fund at the Tier 1 / Tier 2 prices.
• Banked for future use within prescribed limits.

The IMO Net-Zero Framework GFI compliance interacts with FuelEU Maritime compliance for vessels operating in EU ports: the EU has indicated it will treat IMO Tier 1 / Tier 2 remedial unit surrender as evidence of FuelEU compliance, but the detail is yet to be confirmed.

JRC and IPCC default values

The principal source of WtW default values for FuelEU is the JRC (Joint Research Centre) of the European Commission, which maintains a detailed WtW analysis for all common transport fuel pathways. The JRC values feed into FuelEU Annex II.

The principal source of WtW default values for the IMO regime is the IMO Guidelines on Life Cycle GHG Intensity of Marine Fuels. The first version was adopted as Resolution MEPC.376(80) on 7 July 2023 at MEPC 80, which fixed the well-to-wake structure (WtT plus TtW), the gas boundary (CO2, CH4, N2O), and the GWP-100 weighting set. It was revoked and replaced less than a year later by Resolution MEPC.391(81), the 2024 LCA Guidelines, adopted 22 March 2024 at MEPC 81. The 2024 version added the detailed term-by-term formulas for both the WtT and TtW stages, the GWP-20 sensitivity set, the Fuel Lifecycle Label (which records fuel type, feedstock type and feedstock carbon nature), and the default and certified-value framework. The IMO LCA Guidelines draw on JRC, IPCC, the GREET model (Argonne National Laboratory), Ecoinvent and other recognised LCA data sources.

For owners and charterers seeking to establish a non-default WtW value for a specific fuel batch, the principal certifiers are:

• ISCC EU (International Sustainability and Carbon Certification): the dominant certifier for EU RED-compliant biofuels.
• REDcert: an alternative EU RED certifier.
• Bonsucro: a sugar-cane specific certifier.
• Roundtable on Sustainable Biomaterials (RSB): a multi-feedstock certifier.
• Argus, Platts, S&P Global: index providers for certified fuel WtW intensity.

IMO default pathways versus EU default emission factors

The two regimes both fall back on tabulated defaults when a vessel cannot prove a fuel-specific value, but the tables are not interchangeable. FuelEU Maritime carries its defaults in Annex II of Regulation (EU) 2023/1805: a list of WtT factors, TtW carbon factors ($C_f$), and methane-slip coefficients ($C_{slip}$) by fuel and engine class, weighted with the AR4 GWP set (CH4 = 25, N2O = 298). The European Commission can amend Annex II by delegated act to add new energy sources or to adjust existing factors as the JRC refines them. The IMO regime instead resolves a fuel to a default well-to-wake pathway defined in the 2024 LCA Guidelines, weighted with the AR5 GWP set (CH4 = 28, N2O = 265), and labelled with the Fuel Lifecycle Label so that feedstock origin travels with the number.

The practical consequence: the same physical bunker can produce two different WtW intensities, one for the FuelEU balance and one for the IMO GFI, and a company trading into EU ports must track both. FuelEU’s actual-value route is also narrower than the IMO’s. Under FuelEU, a ship may only depart from the Annex II TtW default for sustainable biofuels and synthetic fuels, not for fossil fuels, and only with a certified value from a recognised voluntary scheme. Fossil bunkers are pinned to the default factors. The IMO framework is more permissive on certified pathway values but requires the Fuel Lifecycle Label and supporting documentation.

A second divergence is the treatment of biofuel combustion CO2. Both regimes count the TtW CO2 of a sustainable biofuel as zero on the photosynthetic-recapture argument, so a biofuel’s WtW intensity collapses to its WtT term. The EU then applies the Annex IX A and B feedstock classification from the Renewable Energy Directive, with a fleet-wide cap on Annex IX B feedstocks and exclusion of high-ILUC food and feed crops; the IMO leans on its sustainability themes and the feedstock-carbon field of the Fuel Lifecycle Label. A used-cooking-oil FAME that qualifies as Annex IX A under the EU rules and counts double under FuelEU may carry a different effective weighting under the IMO GFI, so a charterer cannot assume one certificate satisfies both.

How the WtW figure drives the FuelEU compliance balance

The single WtW number per bunker is the input that decides whether a vessel runs a surplus or a deficit. FuelEU compares the energy-weighted annual WtW intensity against the year’s limit, which steps down from 89.34 g-CO2eq/MJ in 2025 (a 2% cut on the 91.16 g-CO2eq/MJ reference) toward 18.23 g-CO2eq/MJ in 2050 (an 80% cut). The compliance balance is the limit minus the attained intensity, multiplied by total energy used: positive means surplus, negative means deficit.

Small movements in the WtW figure move real money. A 4,000-vessel-year fleet burning a typical 200,000 GJ each carries roughly 800 PJ of energy; at the 2025 limit, a 1 g-CO2eq/MJ improvement in average attained intensity is worth about 800 tonnes of CO2eq of compliance balance across that fleet. The FuelEU penalty for an unresolved deficit is set in Annex IV at EUR 2,400 per tonne of VLSFO energy equivalent of the deficit, computed by dividing the deficit (in g-CO2eq) by the product of the attained intensity and the VLSFO energy content of 41,000 MJ per tonne, then multiplying by 2,400. The rate rises 10% for each consecutive year a ship stays in deficit. Because the penalty scales with the size of the gap, the marginal value of shaving a few g-CO2eq/MJ off a fuel’s WtW figure (by sourcing a lower-slip LNG cargo, blending in an Annex IX A biofuel, or switching to a certified RFNBO) is the avoided penalty plus the value of any surplus that can be pooled or banked. The FuelEU GHG intensity calculator and the FuelEU penalties, pooling and multipliers article work through the balance mechanics in detail.

The RFNBO incentive sharpens the picture. Through 2034 each MJ of RFNBO counts twice toward the FuelEU energy total, so a near-zero-intensity e-fuel both lowers the attained intensity and inflates the denominator, producing surplus faster than its physical energy share. From 2034 a separate 2% RFNBO subtarget of yearly energy used onboard applies unless the Commission finds production capacity insufficient. A fuel that fails the 70% GHG-saving test never qualifies as RFNBO regardless of its absolute WtW number, so the WtW calculation is both the compliance metric and the eligibility gate.

Key uncertainties and policy debates

Methane slip measurement

The actual in-service methane slip rate of LNG dual-fuel engines is a major source of uncertainty in WtW intensity estimates. Engine bench tests typically give lower slip rates than in-service measurements (because bench tests use clean fuel, optimised injection timing and steady-state load); in-service measurements are confounded by load variation, fuel composition variation and engine wear. The FuelEU Annex II default values use representative averages from a large in-service measurement campaign coordinated by SEA-LNG and the JRC; individual vessels can establish a vessel-specific value through certified continuous emission monitoring.

N2O slip from ammonia engines

The N2O slip rate from ammonia engines is currently poorly characterised because so few commercial ammonia engines are in operation. Initial estimates from engine manufacturer bench tests give 0.05 to 0.5% N2O slip; in-service validation data are expected from approximately 2026 to 2028 as the first commercial ammonia engines accumulate operating hours.

ILUC and biofuel sustainability

The ILUC accounting for biofuels is contested. The EU RED III applies an ILUC factor for some feedstock categories (e.g. palm oil) that effectively renders them ineligible for compliance counting; for other categories, the ILUC value is zero or low. Recent academic literature challenges several of the EU ILUC values and argues for a more granular spatial accounting; the EU has indicated it will review the ILUC framework periodically.

Hourly matching of renewable electricity

The RFNBO definition requires hourly matching of renewable electricity to hydrogen production, with the renewable electricity coming from new (additional) renewable installations within geographical proximity to the hydrogen plant. The hourly matching requirement is significantly stricter than monthly or annual matching and substantially raises the cost of compliant RFNBO production. The framework has been challenged by hydrogen industry groups arguing for monthly matching; the EU has so far retained the hourly requirement.

Black carbon and Arctic considerations

The WtW framework focuses on long-lived greenhouse gases (CO2, CH4, N2O) but excludes short-lived climate forcers (SLCFs) such as black carbon, which has a particularly high regional warming impact in the Arctic. The IMO is developing separate measures for black carbon control on Arctic routes. See black carbon and Arctic shipping for a full treatment.

Implications for owners, charterers and insurers

Owners

Vessel owners must understand WtW intensity to make informed bunkering, engine specification and retrofit decisions. The optimum strategy depends on the vessel’s trading pattern (FuelEU and EU ETS exposure) and on the residual life of the vessel (for IMO Net-Zero Framework compliance from 2027).

Charterers

Charterers in long-term time charters increasingly require BIMCO CII clauses and equivalent FuelEU clauses that allocate the regulatory cost of WtW non-compliance between owner and charterer. Spot voyage charterers face a more diffuse regulatory cost passthrough.

Insurers

Marine insurers (P&I clubs and hull underwriters) have begun incorporating WtW intensity into risk assessment for Poseidon Principles and Sea Cargo Charter signatories, and into climate-transition disclosures.

Banks and financiers

Ship-finance banks signed up to the Poseidon Principles report annual portfolio WtW intensity against a science-based reference trajectory; vessels with high WtW intensity contribute negatively to the bank’s portfolio score.

Limitations

A WtW figure is only as good as the data behind it, and several limitations recur in practice. First, the two regimes do not produce one number. The IMO GFI uses AR5 GWP-100 (CH4 = 28, N2O = 265); FuelEU uses AR4 GWP-100 (CH4 = 25, N2O = 298). Quoting “the WtW intensity” of a fuel without naming the rulebook is incomplete, and reconciling a charterer’s FuelEU figure with an owner’s IMO figure is a recurring source of dispute.

Second, the default tables are conservative central estimates, not the truth for a specific batch. FuelEU Annex II and the IMO default pathways are designed to be used where no certified value exists, and they sit deliberately on the high side so that a vessel claiming a lower value must prove it. The actual-value route is narrow: FuelEU permits departures from the TtW default only for sustainable biofuels and synthetic fuels, never for fossil bunkers, and only with certification from a recognised scheme. An owner who assumes a real bunker beats the default without certification will be assessed at the default.

Third, methane slip and N2O slip are the largest in-service uncertainties. Engine bench tests run on clean fuel at steady-state load and report lower slip than real voyages with load swings, fuel-composition variation and engine wear. The slip term enters TtW at the full GWP of the slipped gas, so a 1-point error in the slip percentage can shift an LNG or ammonia WtW figure by several g-CO2eq/MJ, enough to flip a compliance balance. Vessel-specific values from certified continuous monitoring are the only way to close this gap, and they are not yet standard.

Fourth, the boundary excludes things that matter to climate. The WtW metric covers only CO2, CH4 and N2O at the GWP-100 horizon. It excludes black carbon and other short-lived climate forcers, excludes the GWP-20 near-term warming view, and treats biofuel combustion CO2 as zero on a recapture assumption that the ILUC literature contests. A fuel that scores well on the regulatory WtW scorecard can still carry near-term or land-use climate costs the metric does not see.

Finally, the figures and trajectories here reflect the rules as adopted through early 2024 (MEPC.391(81)) and 2023 (Regulation (EU) 2023/1805). Annex II is amendable by delegated act, the IMO LCA Guidelines have already been revised once (MEPC.376(80) to MEPC.391(81)), and default factors will move as the JRC and IMO refine them. Always work from the current version of the regulation and the latest default tables, not from a cached number. The values in this article are for orientation; a compliance submission must use the certified or default factor in force for the reporting year. None of this is legal or commercial advice.

See also

Additional calculators:

• GFI Attained - WtW Intensity from Fuel Mix
• HFO Well-to-Wake Calculator
• LNG Well-to-Wake Calculator - Otto MS / Otto SS / Diesel
• FuelEU - RFNBO Double Count

Additional formula references:

• Fueleu Ghg Intensity
• Emis Methane Slip Co2Eq
• Fueleu Rfnbo Double Count

Regulatory and reporting frameworks

• MARPOL Annex VI
• IMO Net-Zero Framework
• IMO GHG Strategy
• EEXI, EPL and ShaPoLi
• SEEMP I, II, III
• CII corrective action plan
• EU MRV Regulation
• EU ETS for shipping
• FuelEU Maritime
• FuelEU penalties, pooling and multipliers
• UK ETS for shipping
• China DCS
• IMO DCS vs EU MRV
• CARB at-berth rule
• Emission control areas
• NOx Tier I, II, III
• IMO 2020 sulphur cap
• RFNBO under EU rules

Voluntary frameworks

• Poseidon Principles
• Sea Cargo Charter
• RightShip GHG Rating
• Green Shipping Corridors
• BIMCO CII clauses
• EUA market mechanics for shipping
• Voluntary carbon credits in shipping

Marine fuels

• LNG as marine fuel
• LNG fuel system
• Methanol as marine fuel
• Ammonia as marine fuel
• Biofuels in shipping
• Heavy fuel oil
• Marine gas oil
• Methane slip from LNG dual-fuel
• N2O emissions from marine engines
• Black carbon and Arctic shipping

Operational and technical efficiency

• Wind-assisted propulsion
• Air lubrication systems
• Just-in-time arrival
• Weather routing
• Trim optimisation
• Slow steaming
• Bulbous bow retrofits
• Energy-saving devices
• Battery-hybrid propulsion
• Onboard carbon capture
• Cold ironing / shore power

Engines and machinery

• Marine diesel engine
• Marine gas turbine
• Marine propeller
• Exhaust gas cleaning system

Hydrostatics and stability

• Hull form design
• Block coefficient
• Hydrostatics and Bonjean curves
• Trim and list
• Metacentric height
• Free surface effect
• Intact stability
• Damage stability
• Ship resistance and powering

Conventions, codes and class

• SOLAS Convention
• MARPOL Convention
• Ballast Water Management Convention
• Hong Kong Convention
• COLREGs Convention
• ISM Code
• ISPS Code
• Classification society

Calculators

• WtW intensity calculator
• FuelEU GHG intensity calculator
• GFI compliance calculator
• Methane slip GWP calculator
• Biofuel ILUC calculator
• SEEMP Measures Combined calculator
• EEXI Required calculator
• CII Attained calculator
• Calculator catalogue

References

• IMO Resolution MEPC.391 (in adoption 2025): Life Cycle Assessment Guidelines for Marine Fuels. International Maritime Organization, 2025.
• IMO Resolution MEPC.328(76): 2021 Revised MARPOL Annex VI. International Maritime Organization, 2021.
• IMO Resolution adopted MEPC 83 (April 2025): IMO Net-Zero Framework. International Maritime Organization, 2025.
• Regulation (EU) 2023/1805 of the European Parliament and of the Council of 13 September 2023 on the use of renewable and low-carbon fuels in maritime transport (FuelEU Maritime). Official Journal of the EU, 2023.
• Regulation (EU) 2023/959 of the European Parliament and of the Council of 10 May 2023 amending Directive 2003/87/EC (EU ETS Maritime). Official Journal of the EU, 2023.
• Directive (EU) 2023/2413 of the European Parliament and of the Council of 18 October 2023 amending Directive (EU) 2018/2001 (RED III). Official Journal of the EU, 2023.
• Joint Research Centre (JRC). Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context (JEC WTW Report v5). European Commission JRC, 2020.
• Argonne National Laboratory. GREET Model: Marine Module. Argonne National Laboratory, 2023.
• IPCC. Fifth Assessment Report (AR5) Working Group I: Climate Change 2013 - The Physical Science Basis. Intergovernmental Panel on Climate Change, 2013.
• DNV. Maritime Forecast to 2050. DNV Energy Transition Outlook, 2023.
• ICCT. Greenhouse gas emissions from global shipping, 2013 to 2015. International Council on Clean Transportation, 2017.

Further reading

• IRENA. A pathway to decarbonise the shipping sector by 2050. International Renewable Energy Agency, 2021.
• ICCT. The cost of zero-emission ships and shipping. International Council on Clean Transportation, 2022.
• ICS. Catalysing the Fourth Propulsion Revolution. International Chamber of Shipping, 2022.
• SEA-LNG. LNG as a Marine Fuel - The Investment Opportunity. SEA-LNG, 2023.
• Methanol Institute. Methanol as a Marine Fuel. Methanol Institute, 2023.
• World Bank. The Role of LNG in the Transition Toward Low and Zero Carbon Shipping. World Bank Group, 2021.

Related calculators

• Wake Fraction & Thrust Deduction
• System - Main Cargo Pump: Centrifugal deep-well
• Wake Fraction (Harvald 1983)
• Wake Fraction - Taylor Approximation
• Pilot Boat Wake - Height & Angle
• Pilot Boarding - Wake Risk
• Offshore - Well logging - wireline
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Related formulas

• Wake Fraction & Thrust Deduction
• System - Main Cargo Pump: Centrifugal deep-well
• Wake Fraction (Harvald 1983)
• Wake Fraction - Taylor Approximation
• Pilot Boat Wake - Height & Angle
• Pilot Boarding - Wake Risk
• Offshore - Well logging - wireline
• Offshore - Well logging - LWD