Carbon Intensity Indicator: How CII Ratings Work in Practice

What the Carbon Intensity Indicator Means for Your Ship

The Carbon Intensity Indicator (CII) is the International Maritime Organization’s (IMO) way of measuring how much carbon dioxide a vessel emits per unit of cargo moved over a calendar year. From 1 January 2023, all ships over 5 000 gross tonnage must calculate an annual CII, compare it to a tightening target, and receive a rating from A to E.

A rating of A shows superior performance, C is the minimum acceptable level, and D or E signals underperformance that triggers a corrective action plan.

How CII Is Calculated – The Numbers Behind the Rating

Most cargo ships use the Annual Efficiency Ratio (AER), expressed in grams of CO₂ per deadweight tonne‑nautical mile. Fuel consumption for the year is converted to CO₂ using fuel‑specific factors; the result is divided by deadweight multiplied by distance sailed. For vessels where deadweight is a poor proxy—cruise ships, ro‑ro carriers, and similar types—a gross‑tonnage‑based version of the same ratio is used.

The required CII comes from a 2019 reference line for the ship type and size band, reduced by an annual factor that increases each year. A ship that does nothing will drift down through the bands over time. The denominator problem means the metric rewards distance, not work done. Time at anchor, drifting, slow steaming while waiting for a berth, and long ballast legs all burn fuel while adding little or no distance, pushing the ratio higher.

Reporting, Ratings, and What a Bad Band Triggers

Attained CII is calculated for the calendar year (1 January–31 December). Within three months of year‑end, the ship reports the attained annual operational CII and resulting rating to its flag administration electronically, using the existing IMO Data Collection System fuel reporting. The consequence appears in the ship management system rather than as a fine.

A ship rated D for three consecutive years, or E in a single year, must submit a corrective action plan (CAP) showing how it will reach at least a C rating. The CAP is incorporated into the Ship Energy Efficiency Management Plan (SEEMP) and verified before the vessel can continue trading with a compliant document.

Why the Market Feels the Pressure

Regulatory penalties are modest, but the commercial penalty is significant. Charterers, cargo owners, and financiers increasingly screen on CII band. A D or E rated ship faces a narrower charterer pool and weaker freight rates. Lenders under the Poseidon Principles and cargo owners under their own reporting frameworks apply the same filter. Because CII depends on how the ship is employed and the charterer controls employment on a time charter, responsibility had to be allocated contractually.

BIMCO has published a CII operations clause for time charter parties that sets out how owner and charterer share the obligation, cooperate on data, and handle a rating shortfall. Fixing a modern time charter without addressing CII leaves an argument waiting to happen.

Levers That Move the Rating – What You Can Do

• Speed – Fuel consumption rises steeply with speed. A modest reduction produces a disproportionate improvement. It is the strongest single lever and the one most constrained by the charter party.
• Hull and propeller condition – Fouling can add double‑digit percentages to power demand. Planned in‑water cleaning and propeller polishing are among the cheapest gains available.
• Trim and ballast optimisation – Small changes in trim alter resistance measurably, and modern trim guidance is cheap to apply.
• Voyage planning and weather routing – Avoiding adverse current and heavy seas cuts fuel for the same distance.
• Just‑in‑time arrival – Slowing to arrive when a berth is free is better than steaming fast and then burning fuel at anchor, though it depends on port and charterer cooperation.
• Auxiliary and boiler load – Cargo heating, inert gas, and hotel load count in the same fuel total and are often overlooked.

Data Quality – The Foundation of a Reliable Rating

The rating is only as good as the bunker and distance figures behind it. Common problems include inconsistent bunker survey practice, distance reported through the water instead of over the ground, unrecorded fuel transfers between tanks, and gaps around dry dock and off‑hire periods. Verification bodies see the same errors repeatedly, and a correction found late in the year cannot be undone by any operational effort.

What to Check Before the Next Reporting Year

1. Confirm the reduction factor and guidelines applicable to the coming year rather than assuming the values used at entry into force still stand, because the regulation itself provides for review of the framework.

2. Check three things on board: that noon report distance and consumption definitions match the verifier expectations, that hull cleaning is scheduled against measured performance rather than a fixed interval, and that the SEEMP Part III targets reflect the trade the ship is actually in.

3. Review the SEEMP to ensure the CAP is integrated and verified.

Next Steps for Shipowners and Operators

To stay ahead, integrate CII monitoring into your regular performance reviews, use BIMCO’s CII operations clause in new time charter agreements, and schedule regular hull cleaning and trim optimisation. For detailed guidance on hull performance monitoring and fuel measurement, see Marine Engineering on Marine Insight 360.

Sources and Further Reading

• EEXI and CII, ship carbon intensity and rating system
• IMO Rules on ship carbon intensity and rating system enter into force
• IMO Carbon Intensity Indicator
• Lloyd’s Register Carbon Intensity Indicator (CII) insights
• DNV

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