CII ratings (A–E), explained

For decades, a ship's environmental credentials were judged mostly on how it was built. The Carbon Intensity Indicator (CII) changed that. Since 1 January 2023 it measures how a ship is actually operated — and then turns that performance into a single letter grade that owners, charterers and ports can read at a glance.

The CII sits inside MARPOL Annex VI, the IMO convention chapter that governs air pollution and energy efficiency from ships. It applies to most cargo ships, roll-on/roll-off passenger ships (RoPax) and cruise ships of 5,000 gross tons and above — the same tonnage threshold that already triggers mandatory fuel-consumption reporting under the IMO Data Collection System (DCS).

What the indicator actually measures

At its core the CII is a ratio. It divides a ship's total annual carbon dioxide emissions by the transport work it performed — its cargo-carrying capacity (expressed in deadweight or gross tonnage, depending on ship type) multiplied by the distance it sailed. The result is the grammes of CO2 emitted to carry one tonne of capacity one nautical mile.

This figure — the ship's attained CII — is not a forecast or a design assumption. It is calculated from the real fuel burned and the real distance covered over a calendar year, drawn from the data each ship already reports under the IMO DCS. A ship that idles at anchor with engines running, takes inefficient routes, or sails part-laden will see its attained CII rise.

From a number to a letter

Each ship is also given a required CII for the year — the level it is expected to meet. The required value is derived from a 2019 reference line and then tightened annually by a reduction factor, so the bar moves down every year. Comparing the attained value against the required value, and against fixed boundaries around it, produces the rating band:

Rating	Band	What it means
A	Major superior	Well above the required efficiency — a leading performer.
B	Minor superior	Better than required; comfortably compliant.
C	Moderate	Around the required level — the target most ships aim for.
D	Minor inferior	Below required; tolerated short-term but flagged.
E	Inferior	Well below required — immediate corrective action triggered.

Because the reduction factor steepens over time, a ship that earns a C today will need to keep improving simply to hold that same C in later years. Standing still means sliding down the scale.

The consequences of a poor grade

The CII has teeth, though gentler ones than a hard cap. A ship rated D for three consecutive years, or E for a single year, is required to develop a corrective action plan as part of its Ship Energy Efficiency Management Plan (SEEMP) — the document, mandatory under MARPOL, in which a ship sets out how it will improve. That plan must be verified, and the ship is expected to work its way back to a C rating or better.

The CII does not ban a ship from sailing because it scored an E — but it puts that E on the record, and a charter market that is watching its own emissions reads that record carefully. IMO MARPOL Annex VI, Carbon Intensity Indicator framework

That commercial pressure is now the sharpest edge of the rule. Charterers under emissions scrutiny of their own increasingly want B or C tonnage, and CII clauses — pioneered by BIMCO in standard charterparty wording — now allocate responsibility for a vessel's rating between owner and charterer. A poor grade can quietly shrink the pool of cargo a ship is offered.

CII is not EEXI or EEDI

It is easy to confuse the CII with the IMO's design-efficiency measures, but they answer different questions:

• CII is operational — how the ship is run, measured year after year from real fuel and distance data.
• EEDI (Energy Efficiency Design Index) and EEXI (its in-service equivalent for existing ships) are technical — how efficiently the hull and machinery were designed and built, calculated once.

A modern, well-designed ship with a strong EEXI can still earn a weak CII if it is operated badly — and a basic ship run carefully can score better than its design suggests. The CII has been criticised for exactly this complexity, particularly how it treats slow-steaming and idle time, and the IMO has it under review. Owners watching the debate should still treat the rating as a live commercial factor.

Marifest tracks the ship-identity and ownership backbone that every CII record sits on. You can open any vessel's full file in the registry and cross-reference it against the wider regulatory picture on the compliance screen, alongside how the CII fits with the broader IMO 2030 and 2050 GHG targets and the regional FuelEU Maritime regime.