How to use the CII calculator
- Pick the ship type and enter its capacity. Cargo ships use deadweight (DWT); ro-ro, ro-pax and cruise ships use gross tonnage (GT).
- Enter the distance travelled in the calendar year in nautical miles, as reported under the IMO Data Collection System (DCS).
- Add each fuel burned in the year in tonnes, from bunker delivery notes or the DCS report. Include fuel used in port and at anchor: it counts towards CO₂ even though no distance is sailed.
- Choose the rating year. The result shows the attained and required CII, the A–E rating and how the rating changes up to 2030 if the ship keeps operating the same way.
How the CII is calculated
The Carbon Intensity Indicator measures grams of CO₂ emitted per unit of transport capacity per nautical mile. IMO calls this the annual efficiency ratio (AER) for most cargo ships. It is calculated with the IMO CII guidelines G1 to G4:
The attained CII uses the ship's actual capacity. For three size ranges (bulk carriers of 279,000 DWT and above, LNG carriers below 65,000 DWT and vehicle carriers of 57,700 GT and above) the reference line uses a capped capacity instead, as shown in the table below.
Reduction factors (Z) by year
| Year | 2023 | 2024 | 2025 | 2026 | 2027 | 2028 | 2029 | 2030 |
|---|---|---|---|---|---|---|---|---|
| Z (below 2019 line) | 5% | 7% | 9% | 11% | 13.625% | 16.25% | 18.875% | 21.5% |
2023–2026 from MEPC.338(76); 2027–2030 adopted by resolution MEPC.400(83) on 11 April 2025, rising 2.625 points a year to 21.5% in 2030.
Reference line parameters (MEPC.353(78))
| Ship type | Size | Capacity | a | c |
|---|---|---|---|---|
| Bulk carrier | 279,000 DWT and above | 279,000 | 4,745 | 0.622 |
| below 279,000 DWT | DWT | 4,745 | 0.622 | |
| Gas carrier | 65,000 DWT and above | DWT | 14405E7 | 2.071 |
| below 65,000 DWT | DWT | 8,104 | 0.639 | |
| Tanker | — | DWT | 5,247 | 0.610 |
| Container ship | — | DWT | 1,984 | 0.489 |
| General cargo ship | 20,000 DWT and above | DWT | 31,948 | 0.792 |
| below 20,000 DWT | DWT | 588 | 0.3885 | |
| Refrigerated cargo carrier | — | DWT | 4,600 | 0.557 |
| Combination carrier | — | DWT | 5,119 | 0.622 |
| LNG carrier | 100,000 DWT and above | DWT | 9.827 | 0.000 |
| 65,000 to 100,000 DWT | DWT | 14479E10 | 2.673 | |
| below 65,000 DWT | 65,000 | 14779E10 | 2.673 | |
| Ro-ro cargo ship (vehicle carrier) | 57,700 GT and above | 57,700 | 3,627 | 0.590 |
| 30,000 to 57,700 GT | GT | 3,627 | 0.590 | |
| below 30,000 GT | GT | 330 | 0.329 | |
| Ro-ro cargo ship | — | GT | 1,967 | 0.485 |
| Ro-ro passenger ship | — | GT | 2,023 | 0.460 |
| Ro-ro passenger ship — high-speed craft | — | GT | 4,196 | 0.460 |
| Cruise passenger ship | — | GT | 930 | 0.383 |
Rating boundaries (MEPC.354(78))
The four boundaries are the required CII multiplied by exp(d1) to exp(d4). Below the first boundary the ship is rated A; at or above the last it is rated E.
| Ship type | A | B | B | C | C | D | D | E |
|---|---|---|---|---|
| Bulk carrier | 0.86 | 0.94 | 1.06 | 1.18 |
| Gas carrier, 65,000 DWT and above | 0.81 | 0.91 | 1.12 | 1.44 |
| Gas carrier, below 65,000 DWT | 0.85 | 0.95 | 1.06 | 1.25 |
| Tanker | 0.82 | 0.93 | 1.08 | 1.28 |
| Container ship | 0.83 | 0.94 | 1.07 | 1.19 |
| General cargo ship | 0.83 | 0.94 | 1.06 | 1.19 |
| Refrigerated cargo carrier | 0.78 | 0.91 | 1.07 | 1.20 |
| Combination carrier | 0.87 | 0.96 | 1.06 | 1.14 |
| LNG carrier, 100,000 DWT and above | 0.89 | 0.98 | 1.06 | 1.13 |
| LNG carrier, below 100,000 DWT | 0.78 | 0.92 | 1.10 | 1.37 |
| Ro-ro cargo ship (vehicle carrier) | 0.86 | 0.94 | 1.06 | 1.16 |
| Ro-ro cargo ship | 0.76 | 0.89 | 1.08 | 1.27 |
| Ro-ro passenger ship | 0.76 | 0.92 | 1.14 | 1.30 |
| Cruise passenger ship | 0.87 | 0.95 | 1.06 | 1.16 |
CO₂ conversion factors (Cf)
| Fuel | t CO₂ per t fuel |
|---|---|
| HFO — ISO 8217 RME to RMK | 3.114 |
| LFO — ISO 8217 RMA to RMD | 3.151 |
| Diesel / gas oil (MGO, MDO) — DMX to DMB | 3.206 |
| LNG | 2.750 |
| LPG — propane | 3.000 |
| LPG — butane | 3.030 |
| Methanol | 1.375 |
| Ethanol | 1.913 |
Worked example: Ultramax bulk carrier
A 63,500 DWT bulk carrier sails 75,000 nm in 2026 and burns 6,500 t of HFO and 500 t of marine gas oil:
- CO₂ = 6,500 × 3.114 + 500 × 3.206 = 21,844 t
- Attained CII = 21,844 × 10⁶ ÷ (63,500 × 75,000) = 4.587 gCO₂/dwt·nm
- Reference CII = 4,745 × 63,500^−0.622 = 4.886
- Required CII 2026 = 4.886 × (1 − 0.11) = 4.348
- Attained ÷ required = 1.055, inside the C band (0.94–1.06), so the ship is rated C.
With the same operation the ship drops to D in 2027 (required CII 4.220, ratio 1.087) and to E in 2030. To stay at C in 2027 it would need to cut CO₂ by about 2.5%, for example through speed optimisation, hull cleaning or less time idling at anchor.
What happens with a D or E rating?
Under MARPOL Annex VI regulation 28, a ship rated D for three consecutive years, or E in any single year, must develop a corrective action plan as part of its Ship Energy Efficiency Management Plan (SEEMP Part III) and have it approved. Some ports, charterers and financiers also look at CII ratings, so a falling rating can cost business before it triggers a corrective plan.
SENA Ship Management helps owners monitor CII through the year and plan improvements. Talk to our team about your fleet, or estimate the effect of a speed change with the fuel consumption calculator.