This article provides general information, not legal, regulatory or financial advice. Requirements depend on the applicable contract, authority and jurisdiction.
How it is calculated
The Carbon Intensity Indicator is an operational efficiency measure applied annually to ships of 5,000 GT and above. It produces a rating from A to E, and the thresholds tighten each year — so a vessel that does nothing differently drifts downward over time by design.
In simplified terms, CII measures grams of CO₂ emitted per unit of transport work:
Attained CII = total CO2 emitted in the year ÷ (deadweight capacity × distance sailed)
The attained value is compared to a required value for the ship type and size, and the ratio determines the band:
| Rating | Meaning |
|---|---|
| A | Major superior performance |
| B | Minor superior |
| C | Moderate — the compliance band |
| D | Minor inferior |
| E | Inferior |
A vessel rated D for three consecutive years, or E for one year, must submit a corrective action plan as part of its Ship Energy Efficiency Management Plan (SEEMP Part III), approved and verified.
Why the metric behaves oddly
The denominator is capacity × distance, not cargo carried. That produces well-known distortions:
| Situation | Effect on CII |
|---|---|
| Long ballast voyage | Distance counts, so ballast legs can help the ratio |
| Waiting at anchor | Fuel burned, no distance — significant harm |
| Short-sea trading with frequent port calls | Manoeuvring and port fuel, little distance — harm |
| Slow steaming | Strong improvement |
| Sailing partially loaded | No penalty; capacity is fixed in the formula |
The single most consequential insight: time spent not moving is the enemy of CII. A ship that steams fast and then waits at anchor for four days does worse than one that arrives just in time at reduced speed — even though the total voyage duration is identical.
The measures that work, in order
1. Speed and arrival management
Power varies roughly with the cube of speed. Reducing speed and eliminating arrive-and-wait is the largest, fastest and cheapest lever available. It requires commercial cooperation — the charterer must accept just-in-time arrival — which makes it a contractual problem as much as a technical one.
2. Hull and propeller condition
Fouling increases required power continuously. Speed loss above 5% is a signal to clean. Coating selection at dry dock, in-water cleaning between dockings, and propeller polishing form the core programme.
3. Reduce time at anchor and in port
Port fuel consumption produces emissions with zero transport work. Shore power where available, careful auxiliary load management, and boiler optimisation all help.
4. Trim and draft optimisation
Small percentages, but continuous and essentially free once the optimum is known for the vessel's loading conditions.
5. Engine and auxiliary efficiency
Turbocharger condition, injector maintenance, auxiliary loading strategy, and avoiding running two generators where one would carry the load.
6. Technical retrofits
Energy saving devices, propeller upgrades, shaft generators, air lubrication. Capital-intensive, and best evaluated at a scheduled docking rather than as a standalone project.
7. Fuel switching
Changes the emission factor directly. Availability, cost and engine compatibility govern.
Managing the rating, not just the emissions
Three practical management points:
Model forward, not backward. The useful number is the projected year-end rating on the current operating profile, updated monthly. A confirmed rating for last year tells you what you can no longer change.
Watch the trajectory across years. Required values tighten annually. A vessel comfortably at C this year may be at D next year on identical operation.
Understand the commercial consequence. CII ratings are increasingly requested by charterers, financiers and cargo owners. Some charterparty forms now include CII cooperation clauses. A D or E rating is a commercial disadvantage before it is a regulatory problem.
How CII, EU ETS and FuelEU interact
They measure different things and reward overlapping but not identical behaviour:
| Regime | Measures | Rewarded by |
|---|---|---|
| CII | CO₂ per capacity-mile | Speed reduction, distance, hull condition |
| EU ETS | Absolute tonnes of CO₂, CH₄, N₂O in scope | Any fuel reduction |
| FuelEU | Well-to-wake GHG intensity of energy used | Lower-carbon fuels, shore power |
Slow steaming improves CII and reduces ETS cost but does nothing for FuelEU intensity. Switching to a low-carbon fuel improves FuelEU and ETS but may not improve CII at all if consumption in energy terms is similar. A coherent plan addresses all three rather than optimising one.
CII is established under MARPOL Annex VI and IMO guidelines; consult the current IMO guidelines and your flag administration for authoritative calculation methods. Impact chart is an indicative model. Reviewed by the Zeaclub Editorial Team, 24 August 2026.
Frequently asked questions
Which ships must calculate CII?
Ships of 5,000 GT and above engaged in international voyages, in line with the IMO DCS data collection framework.
What happens if a ship is rated D or E?
A rating of D for three consecutive years, or E for one year, triggers a requirement for a corrective action plan within SEEMP Part III, subject to verification.
Does slow steaming always improve CII?
Usually, because emissions fall faster than distance. It can be offset if the extra time is spent waiting rather than sailing.
Do charterers care about CII?
Increasingly yes. Ratings feature in vetting, financing and chartering discussions, and cooperation clauses are appearing in charterparties.