CII Rating Explained: The Formula, the 2026 Bands, and How a Voyage Moves It

The Carbon Intensity Indicator is the IMO's report card for ships: every cargo and passenger vessel of 5,000 GT and above gets a letter, A through E, each year. Unlike the EU ETS and FuelEU Maritime, the CII does not send an invoice — but a D or E rating triggers mandatory corrective plans, shows up in vetting and chartering decisions, and increasingly appears in charter-party warranties. Here is the actual math, and how to estimate what a given voyage does to the rating before you commit to it.

#The formula

The attained CII for most ship types is the Annual Efficiency Ratio: total CO₂ emitted in the calendar year divided by deadweight times distance sailed. The required CII starts from a reference line fitted to 2019 fleet data — a function of ship type and size — and is then reduced by the year's reduction factor.

attained AER = annual CO2 (g) / (DWT × distance sailed (nm))
required CII = reference(type, DWT) × (1 − Z)
  where Z = 5% (2023) → 7% (2024) → 9% (2025) → 11% (2026)
rating: attained / required vs boundary factors (d1..d4) → A | B | C | D | E

The boundary factors are published per ship type (MEPC.354). For a bulk carrier, for instance, an attained value under 0.86× required rates A, under 0.94× rates B, up to 1.06× rates C, up to 1.18× rates D, and beyond that E. The bands are tight: a bulker can slide from B to D on a change of trading pattern alone.

#A worked voyage: what one leg contributes

Take a 61,000 DWT supramax on a Rotterdam–Singapore leg: about 8,440 nm, 28 days at 12.5 knots, roughly 2,533 tonnes of CO₂ on VLSFO. That leg alone runs at an AER of about 4.9 gCO₂/dwt·nm. The 2026 required value for that size computes to roughly 4.5 — so this voyage, on its own profile, trades in D territory. Slow the same ship to 11 knots and the fuel drops enough to pull the leg's contribution into the C band. That is the lever: CII rewards exactly the slow steaming that the ETS also rewards, and punishes the fast ballast repositioning that looks free in a spreadsheet.

#Why charterers now care about a ship's letter

• Vetting and cargo access: charterers with ESG commitments screen on ratings; some cargo systems effectively exclude E ships.
• Charter-party clauses: BIMCO's CII clause makes the charterer cooperate in operating the ship compatibly with an agreed rating — speed and routing orders now have a compliance dimension.
• Value and employment: broker reports consistently show C-or-better ships earning stronger employment than D/E sisters; an E rating is a negotiating lever against the owner.
• Corrective plans: three consecutive Ds or one E requires a corrective action plan approved before the ship keeps trading — an administrative burden with commercial shadow.

#The honest criticisms

CII has known distortions — a laden ship waiting at anchorage gets worse, a ship sailing empty but far can look better, and port time counts against the rating even though the ship burns little. The IMO review is expected to refine the metric. None of that changes the practical reality for 2026: the letter exists, counterparties read it, and the inputs — fuel, distance, deadweight — are exactly the ones a voyage estimate already contains. Estimating the CII contribution costs nothing extra once the route and consumption are computed.