Decarbonisation · MARPOL Annex VI
CII calculator guide: attained vs required CII, the A–E rating boundaries and Z factors
Every ship of 5,000 GT and above in the CII ship types gets an A–E Carbon Intensity Indicator rating each year under MARPOL Annex VI regulation 28. The rating compares the ship’s attained CII — grams of CO₂ per tonne-mile of capacity — with a required CII that tightens every year by the reduction factor Z.
This guide gives the formula, the Z factors for 2023–2030, the A–E boundaries, a worked example and what follows a D or E rating.
Updated · Written by SeaReady in our own words. Not affiliated with OCIMF, RightShip, CDI, any MoU or any administration — always check the current official documents.
Required CII reduction factor (Z) by year
| Year of operation | Z (below the 2019 reference) | Required CII | IMO source |
|---|---|---|---|
| 2023 | 5% | 0.95 × reference | MEPC.338(76) |
| 2024 | 7% | 0.93 × reference | MEPC.338(76) |
| 2025 | 9% | 0.91 × reference | MEPC.338(76) |
| 2026 | 11% | 0.89 × reference | MEPC.338(76) |
| 2027 | 13.625% | 0.86375 × reference | MEPC.400(83) |
| 2028 | 16.25% | 0.8375 × reference | MEPC.400(83) |
| 2029 | 18.875% | 0.81125 × reference | MEPC.400(83) |
| 2030 | 21.5% | 0.785 × reference | MEPC.400(83) |
The CII formula, step by step
- Annual CO₂: each fuel burned in the calendar year × its carbon factor CF (MEPC.364(79)). For example heavy fuel oil 3.114, light fuel oil 3.151, diesel or gas oil 3.206, LNG 2.75, methanol 1.375 tonnes of CO₂ per tonne. Most very low sulphur fuel oil is a residual grade and uses the heavy fuel oil factor.
- Capacity × distance: deadweight for bulk carriers, tankers, gas and LNG carriers, container, general cargo, refrigerated cargo and combination carriers (the AER); gross tonnage for ro-ro cargo ship (vehicle carrier), ro-ro cargo ship, ro-ro passenger ship, cruise passenger ship (cgDIST). Multiply by the total distance in nautical miles for the year.
- Attained CII = annual CO₂ in grams ÷ (capacity × distance).
- Required CII = (1 − Z/100) × the reference line for the ship type and size, CIIref = a × capacity−c (MEPC.353(78)).
- Rating: compare the attained CII with the four boundaries d1–d4 × required CII (MEPC.354(78)).
Worked example: a bulk carrier in 2026
| Step | Figure |
|---|---|
| Ship | Bulk carrier, 63,500 DWT, 55,000 nm in 2026 |
| Fuel burned | 5,200 t heavy fuel oil × 3.114 + 400 t gas oil × 3.206 |
| Annual CO₂ | 17,475 t |
| Attained CII | 5.00 g CO₂ per dwt·nm |
| Reference (2019) | 4,745 × 63,500^−0.622 = 4.89 |
| Required CII (2026, Z 11%) | 4.35 |
| Boundaries d1 | d2 | d3 | d4 | 3.74 | 4.09 | 4.61 | 5.13 |
| Rating | D (attained is 15% above required) |
A second D in 2027 and a third in 2028 would trigger a corrective action plan — and Z rises to 13.625% in 2027, so the same voyage pattern rates worse each year. Try your own figures in the CII calculator.
The A–E rating boundaries
| Rating | Attained CII (boundaries for a bulk carrier) | What it means |
|---|---|---|
| A | Below the superior boundary (d1 × required CII; 0.86 for a bulk carrier) | Major superior. No action needed. |
| B | From d1 up to the lower boundary (d2; 0.94) | Minor superior. |
| C | From d2 up to the upper boundary (d3; 1.06). The middle of C is the required CII | Moderate — the required level. |
| D | From d3 up to the inferior boundary (d4; 1.18) | Minor inferior. Three years in a row → corrective action plan. |
| E | At or above d4 (1.18) | Inferior. One year → corrective action plan. |
- Timeline: the ship reports the year’s data by 31 March; the flag State or RO verifies it and issues the Statement of Compliance with the rating by 31 May. See the IMO DCS reporting guide.
- Corrective action plan: after a D for three consecutive years or one E, the SEEMP gets a plan of corrective actions, verified before the Statement of Compliance is issued.
- SEEMP Part III (regulation 26.3) holds the calculation method, the required CII for the next three years and how the ship will meet it — follow the 2024 SEEMP guidelines, MEPC.395(82) as amended.
EU and UK schemes alongside CII
- EU MRV, EU ETS and FuelEU Maritime run alongside the IMO rules for voyages to and from EU ports, with their own reports and dates — compared side by side in the IMO DCS guide. FuelEU applies to ships above 5,000 GT.
- UK: UK MRV has been revoked (SI 2026/245); check the current UK ETS maritime guidance for voyages to and from UK ports.
Questions seafarers ask
How do you calculate a ship’s CII?
Attained CII = annual CO₂ ÷ (capacity × distance). Annual CO₂ is each fuel burned in the calendar year multiplied by its carbon factor (for example 3.114 t CO₂ per tonne for heavy fuel oil, 3.206 for diesel or gas oil). Capacity is deadweight for most cargo ships (the AER) and gross tonnage for cruise, ro-ro and ro-ro passenger ships (cgDIST). Distance is the total nautical miles in the year. Compare the result with the required CII for that year and ship type.
What is the CII reduction factor for 2026?
Z = 11% for 2026 (MEPC.338(76)), so the required CII is 89% of the 2019 reference line. MEPC.400(83), adopted in April 2025, set 13.625% for 2027, 16.25% for 2028, 18.875% for 2029 and 21.5% for 2030. The rating for 2026 is reported by 31 March 2027 and shown on the Statement of Compliance due by the end of May 2027.
What are the CII rating boundaries A to E?
MEPC.354(78) sets four boundaries per ship type as multiples of the required CII: superior (d1), lower (d2), upper (d3) and inferior (d4). For a bulk carrier they are 0.86, 0.94, 1.06 and 1.18. Below d1 is A, d1–d2 is B, d2–d3 is C, d3–d4 is D and d4 or more is E.
What happens if a ship is rated D or E?
A ship rated D for three years in a row, or E once, must add a plan of corrective actions to its SEEMP (MARPOL Annex VI regulation 28). The revised SEEMP goes for verification no later than one month after the data is reported, and the Statement of Compliance is not issued until the plan has been verified.
Which ships need a CII rating?
Ships of 5,000 GT and above of the types in regulation 28: bulk carriers, gas carriers, tankers, container ships, general cargo ships, refrigerated cargo carriers, combination carriers, LNG carriers, ro-ro cargo ships (vehicle carriers), ro-ro cargo ships, ro-ro passenger ships and cruise passenger ships.
What did MEPC.412(84) change?
MEPC.412(84) amends the CII calculation guidelines (G1, MEPC.352(78)): the distance used is now the total distance both under way and not under way, while the supply-based capacity × distance is still used for the rating. MEPC.413(84) amends the 2024 SEEMP guidelines (MEPC.395(82)). Neither replaces the earlier guidelines outright.
Calculate your ship’s CII in SeaReady
Open the offline CII calculator: enter the ship type, DWT or GT, distance and the fuel burned, and see the attained and required CII, the four boundaries and the A–E rating for the year.
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Related guides
- IMO DCS reporting guideReg. 27 deadlines, MEPC.385(81) data fields, BDN reconciliation and IMO DCS vs EU MRV, ETS and FuelEU.
- Ship bunkering & ECA fuel changeoverPre/during/post bunkering checks, MEPC.182(59) samples, BDNs and 0.10% ECA changeover.
- MARPOL ECA & NOx Tier III guideMARPOL Annex VI 7 IMO ECAs, NOx Tier III changeover logging, SCR/EGR compliance, MEPC.340(77) EGCS and BDN verification.
- PSC inspection checklistThe inspection start to finish, every deficiency area, risk windows and the CIC.