CII Attained Calculator: Carbon Intensity Indicator

Formula, assumptions, and limits

Formula

The attained CII for most cargo ships is the Annual Efficiency Ratio (AER): the year’s total CO2 in grams over the year’s transport work, where work is capacity times distance.

The result carries the unit gCO2 / (dwt-nm) for cargo ships, or gCO2 / (gt-nm) for the passenger types that report on gross tonnage. The factor of $10^6$ exists only because fuel mass enters in tonnes while the indicator is quoted in grams; it isn’t a physical correction.

Derivation

The reasoning is short. Burn $F_j$ tonnes of fuel $j$ and you emit $F_j \cdot C_{f,j}$ tonnes of CO2, because $C_{f,j}$ is defined as the CO2 mass per fuel mass for that grade in MEPC.364(79) Appendix 2. Sum across every fuel the ship burned to get total annual CO2. Divide by the transport work the ship performed, which the IMO defines for the AER as capacity times distance, not cargo carried times distance. That last choice is what separates the AER from the EEOI: the AER uses nameplate capacity as a fixed proxy so the indicator does not reward a ship for sailing full and punish it for sailing in ballast. The denominator is therefore a property of the ship and its mileage, never of its cargo intake.

Assumptions

• Capacity is the regulatory capacity for the ship type per MEPC.337(76), not the actual cargo loaded. A part-laden voyage and a full voyage of the same distance contribute identically to the denominator.
• The Cf factors are tank-to-wake. They count the CO2 from burning the fuel on board and ignore the upstream emissions of producing it, which is where the AER and FuelEU Maritime’s well-to-wake intensity diverge.
• Distance is the total distance sailed in the reporting year from the IMO Data Collection System, including ballast and laden legs, port approaches, and any deviation.
• Default Cf values are illustrative. A certified Bunker Delivery Note value governs the regulatory filing where the supplier states a different factor.

Worked example

A 75,000 dwt Panamax bulk carrier sails 90,000 nm in 2025 and burns 9,000 t of heavy fuel oil, Cf 3.114. Work the numbers by hand.

CO2 emitted: $9{,}000 \times 3.114 = 28{,}026$ t, which is $2.8026 \times 10^{10}$ g.

Transport work: $75{,}000 \times 90{,}000 = 6.75 \times 10^{9}$ dwt-nm.

Attained CII: $\frac{2.8026 \times 10^{10}}{6.75 \times 10^{9}} = 4.152$ gCO2/(dwt-nm).

Now the required CII. The bulk-carrier reference line is $a \cdot \text{Capacity}^{-c}$ with $a = 4745$ and $c = 0.622$:

For 2025 the reduction factor is $Z = 0.09$, so required CII $= 4.405 \times (1 - 0.09) = 4.009$. The ratio is $4.152 / 4.009 = 1.036$. Against the bulk-carrier d-vector $(d_1, d_2, d_3, d_4) = (0.86, 0.94, 1.06, 1.18)$ a ratio of 1.036 sits between $d_2$ and $d_3$, so the ship is a C in 2025: compliant, with thin margin.

Hold the ship’s operation steady and watch the line move under it. In 2027 the MEPC.400(83) factor is $Z = 0.13625$, so required CII $= 4.405 \times 0.86375 = 3.805$ and the ratio is $4.152 / 3.805 = 1.091$, above $d_3 = 1.06$: a D. By 2030 the factor is 0.215, required CII falls to 3.458, the ratio reaches 1.201, above $d_4 = 1.18$: an E. Same fuel, same mileage, three different letters, because the required line dropped 13.7 percent across the five years while the attained value did not move. That slide from C to D to E is what the trajectory chart on this page draws.

Edge cases and limits

The AER form is the default, but it is not universal. Cruise ships, ro-pax, ro-ro vehicle carriers, and ro-ro cargo ships report on gross tonnage in the denominator, so their attained value carries the gt-nm unit and is not comparable to a bulker’s dwt-nm figure. LNG carriers above 65,000 dwt use a distinct reference line and the steepest d-vector in the set. The indicator also says nothing about why a value is high: a ship on short coastal hops with many port calls and long maneuvering spells records a worse AER than a deep-sea sister on long ocean passages, even at identical hull efficiency, because port time burns fuel against very little distance. The attained CII is a year’s arithmetic, not a diagnosis.

Regulatory basis

CII is mandatory under MARPOL Annex VI Reg.28, in force since 1 January 2023 through amendment MEPC.328(76). The Cf carbon factors come from MEPC.364(79) Appendix 2 (the 2022 EEDI calculation guidelines, which carry the agreed fuel-to-CO2 conversion table). The reference-line coefficients $a$ and $c$ are in MEPC.337(76). The reduction factors $Z$ for 2023 to 2026 are MEPC.338(76); the 2027 to 2030 factors, rising 2.625 percentage points a year to 21.5 percent below the reference line by 2030, were added by MEPC.400(83), adopted 11 April 2025 at MEPC 83. The rating-band boundaries d1 to d4 are the current G4 values in MEPC.354(78), the 2022 amendment that revised the original MEPC.339(76) bands for the ro-ro types.

Common errors

1. Wrong capacity basis. Entering gross tonnage for a bulk carrier, or deadweight for a cruise ship, throws the attained value off by an order of magnitude. Match the basis to the ship type per MEPC.337(76).
2. Cargo carried instead of nameplate capacity. The AER uses the ship’s regulatory capacity, not the tonnes loaded. Using actual cargo gives the EEOI, a different indicator with a different denominator.
3. Counting only laden distance. The AER denominator is total distance sailed in the year, ballast legs included; dropping the ballast miles understates the work and inflates the attained value.
4. Stale or default Cf factors in a filing. The pre-filled MEPC.364(79) values are defaults; a certified BDN figure governs the regulatory submission where the supplier states a different factor.
5. Reading a single year as durable. A C with thin margin in 2025 is a D in 2027 with no change in operation. The attained value is fixed by how the ship ran; the rating is not, because the required line moves.

How to use this calculator

1. Select the ship type. It sets the reference-line coefficients, the capacity basis shown on the capacity field (DWT or GT), and the rating bands d1 to d4.
2. Enter the ship capacity in the unit the field displays: deadweight in tonnes for cargo ships, gross tonnage for passenger and ro-ro vehicle carriers.
3. Enter the distance travelled in the reporting year in nautical miles, from the IMO DCS data.
4. Select the reporting year to apply that year’s reduction factor Z.
5. Add a fuel row for each fuel burned, entering the mass in tonnes; the Cf factor pre-fills from MEPC.364(79). Use + Add fuel for multi-fuel ships.
6. Read the attained CII, the required CII for the year, and the A to E rating. The trajectory chart holds the attained value constant and shows how the required line tightens through 2030.

Capacity basis by ship type

The denominator basis is fixed per ship type and is the single input most often entered wrong. The list below is the MEPC.337(76) mapping the calculator applies.

A bulker’s attained CII in gCO2/(dwt-nm) and a cruise ship’s in gCO2/(gt-nm) are not comparable figures; each is graded only against its own ship-type reference line and d-vector.

Cf carbon factors

The carbon factor converts fuel mass to CO2 mass. These are the tank-to-wake MEPC.364(79) Appendix 2 values the calculator pre-fills; a certified BDN value overrides the default in a regulatory filing.

The distillates carry a higher Cf than the residual fuels because they hold more carbon per tonne, so a ship that switches from HFO to MGO for emission control area compliance raises its attained CII slightly even at identical energy delivered. LNG’s lower Cf is the headline number behind dual-fuel newbuilds, though the tank-to-wake factor does not see methane slip, which a well-to-wake regime such as FuelEU Maritime does.

When to use this calculator and when to use a sibling

This page builds the attained CII from raw fuel and distance, which is the job when the year’s DCS data is in hand and the operational number does not yet exist. Three sibling tools take it from there. The CII required calculator returns only the target line for a ship type, capacity, and year, useful for setting a fixture target before any fuel is burned. The CII rating calculator starts from an attained CII you already hold and returns the A to E letter and the distance to the adjacent bands. The attained-versus-required calculator holds the attained value steady and tightens the line across the reporting years, the projection view of the same arithmetic. All four share one CII module, so they cannot disagree on a boundary.

CII against the EU’s two carbon regimes

A deep-sea ship trading to Europe answers to three carbon rules at once, and they measure different things. The IMO CII this page computes grades operational carbon intensity against a tightening line, with an administrative corrective plan as the consequence and no direct price. The EU ETS prices the CO2 emitted on EEA-scope voyages in allowances, a euro bill that scales with tonnes, handled by the EU ETS calculator. FuelEU Maritime grades the well-to-wake intensity of the energy per megajoule with a euro penalty for missing the target, handled by the FuelEU intensity calculator. A ship can post a clean CII C and still carry a heavy ETS bill simply by trading a lot, or hold a strong CII while failing FuelEU on a fuel chain the CII’s tank-to-wake factor never sees. Optimizing one regime in isolation usually worsens another.

What the rating triggers

A D rating for three consecutive years, or a single E in any year, obliges the operator to develop a SEEMP Part III corrective action plan, filed with and verified by the flag administration, setting out how the ship returns to at least C. The plan is an administrative obligation, not a euro penalty in the EU ETS sense, but it is enforceable and the Statement of Compliance the DCS regime issues depends on it. The asymmetry at the bottom of the scale is deliberate: a single E pulls the trigger at once, while D is tolerated for two years before the third consecutive D acts. Charterers increasingly write a minimum-C warranty into period fixtures, which is why the attained value this page produces is now a commercial number and not only a compliance one. See the corrective action plan and the SEEMP Parts I, II and III for the procedural detail.

About this CII attained calculator

This tool serves ship operators, technical superintendents, charter and operations desks, and classification surveyors who report a vessel under the IMO Data Collection System. It computes the Annual Efficiency Ratio form of the attained CII set by MARPOL Annex VI Reg.28: it sums each fuel’s mass burned times its MEPC.364(79) carbon factor, scales to grams, and divides by capacity times distance sailed during the reporting year. The required CII is the MEPC.337(76) reference line, capacity raised to the power $-c$, scaled by $a$ and reduced by the annual factor Z, 9 percent in 2025 rising to 21.5 percent by 2030. The ratio of attained to required is then read against the MEPC.354(78) d1 to d4 boundaries to assign the grade.

The rating carries a regulatory consequence: a D for three consecutive years, or an E in any single year, forces a SEEMP Part III corrective action plan filed with the flag administration. The result cards report the attained value, the required value, and the grade. The rating trajectory chart plots the attained CII as a constant line against the required CII, which tightens each year, with the A to E bands shaded behind it, so a reader can see the year a currently compliant ship slips a grade. Use it to test the effect of a fuel switch, a speed reduction, or a change in reported capacity before the number reaches a regulatory filing.

The arithmetic is the MEPC.337/354/400 chain in the shared CII module, the same code behind the CII required, CII rating, and attained-versus-required calculators, so the four agree on every boundary by construction rather than by a house interpretation of the resolutions.

Further reading

• What is CII? Carbon Intensity Indicator explained
• CII corrective action plan and SEEMP Part III
• SEEMP Parts I, II and III
• CII required calculator
• CII rating calculator
• CII attained vs required calculator

Frequently asked questions

What is the attained CII?

The attained CII is a ship's actual operational carbon intensity for a reporting year, measured as grams of CO2 per unit of transport work. For most cargo ships it is the Annual Efficiency Ratio: the total CO2 from the year's fuel burn, divided by deadweight times distance sailed. It is the number that is compared against the required CII to set the A to E rating.

What capacity do I use, deadweight or gross tonnage?

Deadweight (DWT) for most cargo ships: bulk carriers, tankers, container ships, gas and general cargo. Gross tonnage (GT) for passenger types: ro-pax, ro-ro vehicle carriers, and cruise ships. The capacity basis is fixed per ship type by MEPC.337(76), and mixing the two is the most common reason an attained CII comes out wrong by an order of magnitude.

Which Cf carbon factors does the attained CII use?

The Cf conversion factors from MEPC.364(79) Appendix 2, the tank-to-wake mass of CO2 per mass of fuel. Heavy fuel oil and VLSFO are 3.114, marine gas oil and MDO are 3.206. The calculator pre-fills these, but a certified Bunker Delivery Note value should replace the default in a regulatory filing where the supplier states a different factor.

Why does my attained CII not change but my rating gets worse?

The attained CII reflects how the ship was run, so a steady operating profile gives a steady attained value. The required CII tightens every year through the reduction factor Z, from 9 percent below the reference line in 2025 to 21.5 percent by 2030. A ship holding its attained value steady slides toward worse bands as the required line drops beneath it.

Is the attained CII the same as EEXI or EEDI?

No. EEXI and EEDI are design-stage technical indices, calculated once from installed power and a reference speed. The attained CII is an operational measure recomputed every year from the fuel actually burned and the distance actually sailed, so a technically efficient ship can still post a poor attained CII by trading inefficiently.