CII rating calculator

CII stands for Carbon Intensity Indicator. It is the IMO’s annual measure of how much CO2 a ship emits for the transport work it performs, applied under regulation 28 of MARPOL Annex VI to ships of 5,000 gross tonnage and above. Attained CII is annual CO2 divided by capacity times distance sailed. Compared against the required value for the year, it produces a rating from A to E.

This CII calculator takes deadweight or gross tonnage, distance sailed and the fuel burned over the year. Every ship type covered by regulation 28, with the required value, all four rating boundaries, and what the same vessel rates each year to 2030.

Fuel consumed over the year (tonnes)

Uncorrected CII rating (2026)

C3.80 vs 3.71 required
gCO₂ / dwt·nm

Compliant, and close enough to the boundary that a bad quarter moves it.

On identical operations this vessel drops out of C in 2028, because the required intensity tightens every year while the vessel does not.

This is the uncorrected rating, and it will not match your verifier

A verified CII applies the correction factors and voyage adjustments in MEPC.355(78): ice class and distance sailed in ice, ship-to-ship transfer and shuttle tanker operations, and fuel used for cargo cooling on gas carriers, discharge pumps on tankers, boilers and other auxiliary consumption. Those need voyage-level data this page does not ask for, and they almost always move a vessel in the operator’s favour. Treat the letter above as the floor, not the answer.

See the full breakdown

All four rating boundaries, the reference line and reduction factor behind them, and what this same vessel rates every year to 2030 on unchanged operations.

How a CII rating is calculated

Every constant behind this tool is read from the IMO resolutions themselves rather than from a summary, so the working can be checked line by line.

  1. Fuel becomes CO2. Each fuel’s annual tonnage is multiplied by its carbon factor: 3.114 for HFO, 3.151 for LFO, 3.206 for MDO or MGO, 2.750 for LNG. These are the tank-to-wake DCS factors, which are not the well-to-wake figures FuelEU Maritime uses.
  2. CO2 becomes an intensity. Attained CII is grams of CO2 per capacity-mile. Capacity means deadweight for bulk carriers, tankers, gas and LNG carriers, container, general cargo, refrigerated and combination carriers, and gross tonnage for ro-ro and passenger ships.
  3. The required value comes from 2019. The reference line for each ship type and size is set in IMO resolution MEPC.353(78), adopted 10 June 2022, and reduced by the year’s reduction factor from MEPC.400(83), adopted 11 April 2025: 11% in 2026, then 13.625%, 16.25%, 18.875% and 21.5% through 2030.
  4. Four boundaries decide the letter. The dd vectors in IMO resolution MEPC.354(78) scale the required value into the A/B, B/C, C/D and D/E boundaries for that ship type.

This implementation reproduces the worked example in MEPC.354(78): a bulk carrier with a required CII of 10 gets boundaries of 8.60, 9.40, 10.60 and 11.80, and an attained value of 9.00 rates B.

How to improve a CII rating

CII measures CO2 per unit of transport work, so it improves in only two ways: burn less fuel for the same voyage, or carry the same cargo over fewer idle miles. The levers that move it most in practice are hull and propeller condition, voyage and speed optimisation, time spent at anchor or waiting, and switching to a fuel with a lower carbon factor.

Timing matters more than most operators expect. Because CII is an annual figure, a change made in July has roughly half the effect on the rating that the same change would have had in January. A vessel drifting toward D in the first quarter is a decision; the same vessel in November is an outcome.

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