CII: The Grade That Follows the Ship

9 min read

What you'll take away Explain how the attained CII is calculated, how the A–E rating works, how the requirement tightens to 2030, and what a poor rating triggers.

Measurement, the first two lessons showed, was scaffolding. In 2023 the IMO built on it: the Carbon Intensity Indicator takes the fuel data every large ship already reports under DCS and turns it into a grade — A to E, recalculated every year, recorded on the ship's Statement of Compliance. The IMO publishes no ranking of ships — its database stays anonymised — but the grade travels with the vessel, and charterers, financiers and vetting platforms routinely ask to see it. It converted carbon efficiency from an internal metric into a commercial reputation.

The arithmetic

CII applies to ships of 5,000 GT and above of the major commercial types. The attained CII is the year's CO₂ emissions divided by transport work — deadweight × distance sailed for bulk carriers, tankers, container ships and most cargo types; gross tonnage instead of deadweight for ro-ro and cruise ships. The result is grams of CO₂ per capacity-mile.

Note carefully what the denominator is not: cargo actually carried. A vessel sailing in ballast still accrues distance at full deadweight in the formula — an intentional simplification with real operational consequences, since anything that burns fuel without covering miles (waiting, adverse weather, a fouled hull) worsens the attained value.

The required CII is where the regulatory pressure lives. Each ship type has a reference line fitted to the 2019 fleet; the requirement is that line reduced by a factor Z that grows every year: 5% in 2023, 7% in 2024, 9% in 2025, 11% in 2026 — and, as adopted at MEPC 83 in 2025, 13.625% in 2027 stepping annually to 21.5% by 2030. A ship whose operations stand still is, by construction, a ship whose rating deteriorates.

The bands, and the teeth

Attained over required places the vessel in a band: C straddles the requirement, A and B sit better, D and E worse, with the boundaries set per ship type. The rating is stamped onto the DCS Statement of Compliance each year.

The formal teeth: a ship rated D for three consecutive years, or E even once, must develop a corrective action plan — folded into SEEMP Part III and approved — before its Statement of Compliance is issued. The commercial teeth arrived faster than the formal ones: charterers screen ratings in vetting, cargo owners carry their own supply-chain carbon targets, and a poor grade quietly narrows a ship's market long before any regulator acts.

Where the rating is really made

Because CII is computed from actual fuel burned, it is decided by the themes of this Academy's other courses: the resistance the hull presents, and the data discipline that measures it. A fouled hull burning 15–35% more fuel for the same distance is a rating problem before it is a maintenance one. Trim, speed profiles and routing all print directly onto the attained value. And the rating rests on reported data — so the fleet that measures precisely knows its trajectory months in advance, while the fleet that doesn't finds out its grade when the letter arrives.

The grade made carbon performance a fact the market can demand to see. The next lesson is about the regime that made it expensive.

Primary references: MARPOL Annex VI and IMO guidelines G1–G5 (MEPC resolutions); MEPC 83 (2025) for the 2027–2030 reduction factors; ClassNK, SEEMP / DCS / CII outline.

Check yourself

1. For a bulk carrier, the attained CII is calculated as:
2. The required CII tightens each year through a reduction factor Z against the 2019 reference. The 2026 value of Z is:
3. Which rating outcomes trigger a mandatory corrective action plan?
4. Why can hull fouling drag down a CII rating?
5. How is the required CII for a given year derived?
6. What value does the reduction factor Z reach in 2030?
%

Type a number in %.

7. Which ship types use gross tonnage rather than deadweight in the CII denominator?

Select all that apply.

8. A ship sailing in ballast drops out of the CII calculation until it loads cargo.