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TL;DR: CII is calculated by dividing a ship's annual CO₂ emissions by its transport work capacity multiplied by distance sailed. The result, in grams of CO₂ per capacity-mile, is compared against a required value for the ship's type and size, and that comparison sets the A–E rating.
That is the whole idea. The rest of this guide unpacks the CII calculation for a ship step by step the formula (sometimes called the CII index), the data behind it, and how the rating bands work ending with a worked example.
The CII formula, broken down
The CII calculation is one fraction:
Attained CII = annual CO₂ emissions (grams) ÷ (capacity × distance sailed)
The unit is grams of CO₂ per capacity-mile for most cargo ships, gCO₂ per deadweight-tonne-nautical-mile. It answers a simple question: how much carbon does this ship emit to move one tonne of its capacity one mile?
Two details matter. First, CO₂ is not measured directly it is derived from fuel consumption, with each fuel type multiplied by a fixed IMO emission factor (heavy fuel oil ≈ 3.114 tonnes of CO₂ per tonne burned, marine gas oil ≈ 3.206, LNG ≈ 2.750).
Second, capacity means the ship's design capacity, not the cargo actually carried. A bulk carrier uses deadweight tonnage whether it sails full or in ballast which is why empty legs hurt the rating.
The inputs you need
Only three ingredients go into the CII calculation, all from a calendar year of operation:
- Fuel consumption by fuel type — from noon reports, flow meters and bunker delivery notes; converted to CO₂ via the emission factors.
- Distance sailed — total nautical miles over the year, from the ship's logs.
- Capacity — deadweight tonnage for bulkers, tankers and container ships; gross tonnage for cruise and RoPax vessels.
Simple list, high stakes: because the same data feeds EU MRV, IMO DCS and the EU ETS bill, an error in the fuel figure moves every number downstream of it.
Attained vs required: how the A–E bands are set
The formula above gives only half the picture. The grade comes from comparing two numbers:
- Attained CII — what the ship actually did over the year, straight from the formula above.
- Required CII — the IMO's target for a ship of that type and size in that year, set from a 2019 baseline and tightened annually (11% below baseline by 2026, then 2.625% more each year from 2027).
- The ratio decides the grade — attained ÷ required. Comfortably below 1.0 earns an A or B, around 1.0 is a C, and progressively above it falls to D and E, with the exact boundaries set per ship type.
Because the required value tightens every year, the same attained CII earns a worse grade over time. The full rating context what the grades mean and what happens at D or E — lives in our hub on CII compliance in shipping.
A worked CII calculation example
Take a 60,000 DWT bulk carrier over one year. It burns 5,000 tonnes of heavy fuel oil and sails 50,000 nautical miles.
- Step 1 — CO₂: 5,000 t fuel × 3.114 = 15,570 tonnes of CO₂, or 1.557 × 10¹⁰ grams.
- Step 2 — transport work: 60,000 DWT × 50,000 nm = 3 × 10⁹ dwt·nm.
- Step 3 — attained CII: 1.557 × 10¹⁰ ÷ 3 × 10⁹ = 5.19 gCO₂/dwt·nm.
If the required CII for that ship and year is 5.0, the ratio is 1.04 a C rating, but with little room to spare. On identical performance next year, with the required value tightened again, the same ship slides towards a D.
The formula is simple knowing it early is the hard part
The CII calculation itself takes minutes. The difficulty is that the official answer arrives once a year, after the year it describes is already over and a standalone CII calculator only tells you where the ship was, not where the year is heading.
That is why operators increasingly run the calculation continuously. Platforms like ZeroNorth's Emission Analytics compute attained CII per vessel as voyages complete and forecast the year-end rating, so a ship trending from C to D is a mid-year alert rather than a year-end surprise.
Because in CII, the formula was never the hard part. Knowing your answer at the right time is and that is the part that pays.
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