The idea in one sentence
Take the ship’s annual CO₂ emissions, divide by its transport-work potential (deadweight or gross tonnage × distance), and compare against a declining yearly target. If you’re below the target, you’re compliant; how far below or above determines whether you score A, B, C, D, or E.
The formula
For most cargo ships CII is the Annual Efficiency Ratio (AER):
Attained CII = Σⱼ (Fⱼ × Cfⱼ × 10⁶) / (DWT × Distance)
[ grams CO₂ per dwt-nautical mile ]
Fⱼ- tonnes of fuel j burned in the reporting year.Cfⱼ- CO₂-per-fuel factor from MEPC.364(79). For VLSFO/HFO it is 3.114, for MGO 3.206, for LNG 2.750, and so on.DWT- deadweight (or gross tonnage for ro-pax, ro-ro vehicle carriers, and cruise ships).Distance- nautical miles travelled in the reporting year per IMO DCS data.
Ro-pax, ro-ro vehicle carriers, and cruise ships use the cgDIST variant (CO₂ / (GT × distance)) instead.
The target: Required CII
The Required CII is a reference line minus an annual reduction factor:
Required CII = a × Capacity^(−c) × (1 − Z)
Where:
aandccome from MEPC.337(76) Table 1, one pair per ship type.Zis the annual reduction per MEPC.338(76): 5 % in 2023, 7 % in 2024, 9 % in 2025, 11 % in 2026. Beyond 2026 the IMO is reviewing further tightening.
The A–E rating
The ratio Attained / Required is compared against four boundaries d1 < d2 < d3 < d4 from MEPC.339(76) - different per ship type:
| Rating | Meaning | Consequence |
|---|---|---|
| A | Major superior performance | Recognition, possible port incentives |
| B | Minor superior performance | - |
| C | Moderate performance | Compliant |
| D | Minor inferior | Three consecutive years → corrective plan |
| E | Inferior | One year → corrective plan |
A D-three-years or any E triggers SEEMP Part III, a mandatory corrective action plan submitted to the flag administration and verified before the next Statement of Compliance is issued.
What you can actually do to improve
CII is an operational metric, so operational levers matter most:
- Slow steaming. Fuel consumption scales roughly with V³; a 10 % speed reduction drops fuel by ~27 % for the main engine (aux consumption is flatter). Huge CII lever.
- Hull cleaning and propeller polishing. Fouling can add 15–30 % to resistance.
- Weather routing. Avoid adverse currents and wind.
- Just-in-time arrival. Eliminate waiting off the port by coordinating with terminals.
- Trim optimisation. A correct draft/trim for the current loading condition shaves ~1–3 %.
- Energy-saving devices. Pre-swirl stators, bulbous-bow retrofit, air lubrication, wind-assist.
- Alternative fuels. LNG (lower Cf), bio-drop-ins, methanol - each has a different CII effect depending on fuel energy density and engine efficiency.
Scope and limitations
- Applies to cargo and passenger ships of ≥ 5,000 GT under MARPOL Annex VI.
- Excludes fishing vessels, offshore rigs, naval ships, and domestic-only ships not subject to MARPOL.
- Ignores methane slip for LNG and upstream fuel emissions (those are picked up by FuelEU Maritime, not CII).
- Trade-lane adjustments are limited - a ship running only ballast legs ends up with a bad CII even if well-operated.
What CII means for charter parties
BIMCO published a 2022 CII Operations Clause that allocates responsibility between owner and charterer for rating-affecting decisions (speed, route, bunkers). A charterer’s instruction to slow-steam may improve CII; an instruction to speed up to catch a laycan may make it worse. Contracts now increasingly reference CII outcomes.
How to compute your ship’s CII
Try our CII Attained calculator - enter ship type, capacity, distance, and each fuel burned in the reporting year, and it returns Attained CII, Required CII, and the A–E rating in one view.