CII Rating Calculation: AER Formula and A-E Bands
How the CII rating calculation turns fuel use and distance into an AER, sets A-E bands, and creates port-call work for chandlers, bunker suppliers and repair yards.
The CII rating calculation turns a ship's annual fuel use and distance sailed into one number, its attained annual operational carbon intensity indicator (AER), which is then compared with a reference line and placed on an A-E scale. The carbon intensity indicator (CII) applies to ships above 5,000 gross tonnage, according to The Canadian Journal of Chemical Engineering (2024).
In short: the CII rating calculation converts a ship's annual fuel burn and distance sailed into an AER in grams of CO2 per deadweight tonne-mile. That figure is compared with a required value built from a ship-type reference line, then placed on an A-E scale, with D or E triggering a corrective action plan.
What the CII rating calculation measures
The CII rating calculation measures how much CO2 a ship emits per unit of transport work, expressed as grams of CO2 per deadweight tonne-mile. The result is the attained AER. A lower AER means a more efficient year.
The IMO's target is a 40% cut in greenhouse gas emissions by 2030 against 2008, as set out in the same study on CII data collection for oil tankers.
Two inputs drive the number: fuel consumed and distance travelled. Both come from the ship's noon reports and IMO DCS submissions, which is why data quality sits at the centre of the rating.
The reference line and the A-E scale
The rating compares the attained AER with a required value built from a reference line. The reference line is a formula based on ship type and size, and it tightens each year through a reduction factor.
Bands are set at fixed distances from the required value:
- A sits well below the required value (major outperformance).
- B and C sit around the required value, with C the middle band.
- D and E sit above it, with E the worst band.
A ship rated D for three consecutive years, or E for one year, must submit a corrective action plan in its Ship Energy Efficiency Management Plan (SEEMP).
The bands are not fixed forever. As the reduction factor tightens, a ship that holds the same AER will slide down the scale over time.
Why the calculation is harder than it looks
Fuel consumption and distance are recorded by the crew, and the study on oil tankers describes the data collection process as intricate, involving engineering departments and shipboard management teams. Gaps and inconsistencies in noon reports feed straight into the attained AER.
Verification is mandatory. The same study notes that the achieved annual operational CII must be verified and documented against the prescribed target before a rating is established.
Route and speed changes also move the number. A ship that slows down cuts fuel per mile, while a long ballast leg adds distance without cargo work.
What the rating changes in commercial decisions
CII is now a constraint in fuel purchasing, not just a compliance box. A 2024 study in Multimodal Transportation modelled refuelling for dual-fuel ships under a CII rating limit and carbon tax costs, and found that low sulphur fuel oil (LSFO) refuelling decisions are closely linked to the price difference between LSFO and LNG.
That result matters to bunker suppliers: the stem a buyer chooses can be shaped by the rating the ship needs to protect, not only by the delivered price. Our guide to FuelEU Maritime compliance for bunker suppliers covers the overlapping fuel rules.
Efficiency work feeds the same calculation. A study in IEEE Transactions on Transportation Electrification (2021) developed a data-driven efficiency model for ship powertrains, comparing conventional diesel electric and hybrid systems with batteries, and assessed efficiency from fuel consumption through to the propulsion unit. Less fuel for the same distance lowers the attained AER.
Owners are also buying operational support. According to Hellenic Shipping News (October 2026), WinGD secured three long-term service agreements covering 14 dual-fuel LNG carriers.
Route optimisation is part of the same push. According to Hellenic Shipping News (October 2026), Weathernews and HD Hyundai Marine Solution signed a three-year contract with H-Line Shipping to deploy a route optimisation solution across eight large bulk carriers.
Slow steaming and older tonnage interact. According to Marine Insight (October 2026), ship recycling fell 13% in the first nine months of 2026 as owners kept older vessels trading.
Where service providers fit in the CII cycle
Every input to the CII rating calculation is influenced by work done at a port call. Hull and propeller condition, engine tuning, voyage planning and fuel choice all move the attained AER.
That creates openings before the vessel arrives, not after:
- Hull cleaning and propeller polishing cut drag, which reduces fuel for the same distance.
- Bunker suppliers can position LNG, biofuel and low-sulphur grades against a buyer's rating position.
- Repair yards can quote efficiency-related work alongside scheduled dry-docking.
- Port agents can flag vessels with D or E ratings and connect the technical superintendent early.
The buying roles are specific: technical superintendents, fleet managers and procurement. Our guide to where CII efficiency work lands commercially covers who signs it off.
Timing matters because the calculation runs on a calendar year. Work quoted in Q4 lands in the next reporting period, and the corrective action plan clock starts with the verified rating.
How VesselReach helps
VesselReach turns vessel movements at your target ports into conversations with the people who own efficiency decisions.
- Vessel intelligence: see vessels near your ports and scheduled arrivals, which means you can approach a technical superintendent before the ship berths.
- Decision-maker discovery: find operations, fleet, technical and procurement roles, which means your efficiency pitch reaches the person who buys it.
- Multichannel outreach: staged email, WhatsApp and phone contact, which means follow-ups continue without duplicate approaches.
Book a VesselReach demo to see how it works on your ports.
Frequently asked questions
How is the CII rating calculated?
The CII rating calculation divides a ship's annual CO2 emissions by its transport work (deadweight times distance) to give the attained AER. That figure is compared with a required value from a ship-type reference line, then placed on an A-E scale.
What triggers a corrective action plan?
A ship rated D for three consecutive years, or E for one year, must submit a corrective action plan in its SEEMP. The plan sets out how the ship will improve its rating.
Which ships does CII apply to?
CII applies to ships above 5,000 gross tonnage, according to The Canadian Journal of Chemical Engineering (2024).
How can a port agent use CII ratings to win work?
A port agent can flag vessels with D or E ratings and connect the technical superintendent early. VesselReach helps by showing scheduled arrivals and the decision-makers behind each vessel.
Do bunker suppliers need to track CII ratings?
Yes. A 2024 study in Multimodal Transportation found that LSFO refuelling decisions are closely linked to the LSFO-LNG price difference under a CII rating limit, so fuel choice can be shaped by the rating a ship needs to protect.
The cii rating calculation method turns a ship's annual fuel use and distance into an AER, then compares it with a required value built from a reference line. A lower AER means a more efficient year. Ships above 5,000 gross tonnage must comply. A D rating for three years or an E for one year requires a corrective action plan in the SEEMP. Service providers can use the rating to time hull cleaning, fuel offers and repair quotes before the vessel arrives.
Frequently asked questions
How is the CII rating calculated?
The CII rating calculation divides a ship's annual CO2 emissions by its transport work (deadweight times distance) to give the attained AER in grams of CO2 per deadweight tonne-mile. That figure is compared with a required value from a ship-type reference line, then placed on an A-E scale. Source: The Canadian Journal of Chemical Engineering (Wiley)
What must a SEEMP corrective action plan contain?
A SEEMP corrective action plan must set out how the ship will improve its CII rating, and it is required after a D rating for three consecutive years or an E rating for one year. The plan sits inside the Ship Energy Efficiency Management Plan and is triggered by the verified rating.
When does the D or E rating clock start?
The D or E rating clock starts with the verified rating, not with the calendar year. The attained annual operational CII must be verified and documented against the prescribed target before a rating is established, so the corrective action plan obligation follows verification. Source: The Canadian Journal of Chemical Engineering (Wiley)
Which ships does CII apply to?
CII applies to ships above 5,000 gross tonnage, according to The Canadian Journal of Chemical Engineering (2024). Ships below that threshold are outside the rating. Source: The Canadian Journal of Chemical Engineering (Wiley)
Do bunker suppliers need to track CII ratings?
Yes. A 2024 study in Multimodal Transportation found that LSFO refuelling decisions are closely linked to the LSFO-LNG price difference under a CII rating limit, so the stem a buyer chooses can be shaped by the rating a ship needs to protect, not only by delivered price. Source: Multimodal Transportation (Elsevier BV)
How can a port agent use CII ratings to win work?
A port agent can flag vessels with D or E ratings and connect the technical superintendent early, before the vessel arrives. VesselReach shows scheduled arrivals and the decision-makers behind each vessel, so the efficiency conversation starts ahead of the port call.
Sources
- Data-Driven Efficiency Modeling and Analysis of All-Electric Ship Powertrain: A Comparison of Power System Architectures, IEEE Transactions on Transportation Electrification, 27 October 2021
- Ship refueling optimization for dual-fuel ships considering carbon intensity indicator rating limit and uncertain fuel prices, Multimodal Transportation (Elsevier BV), 19 June 2024
- Data collection framework for enhanced carbon intensity indicator ( CII ) in the oil tankers, The Canadian Journal of Chemical Engineering (Wiley), 10 July 2024
- Weathernews and HD Hyundai Marine Solution Sign First Commercial Contract for OSR-OW, with H-Line Shipping, Hellenic Shipping News, 16 October 2026
- Only 273 Ships Sold For Scrap In 9 Months As Record Freight Rates Keep Old Vessels Sailing, Marine Insight, 13 October 2026
- WinGD secures long-term service contracts covering 14 dual-fuel LNG carriers across three major shipowners, Hellenic Shipping News, 26 October 2026
The cii rating calculation is simpler than it looks once you separate the two steps. First, the ship's annual CO2 emissions are divided by its transport work to give the attained AER. Second, that number is compared with a required value built from a reference line and a yearly reduction factor, and the gap places the ship in a band from A to E. What makes it hard is the data behind it: fuel and distance come from crew records that must be verified. For a chandler, bunker supplier or repair yard, the practical point is that hull condition, fuel choice and voyage planning all move the number, so efficiency work is a sales opening before the vessel arrives.
The most useful point for a supplier is that the CII rating calculation rewards work done before the vessel arrives. Hull and propeller condition, engine tuning, voyage planning and fuel choice all feed the attained AER, so efficiency work quoted at a port call lands in the same calendar-year reporting period. Bunker suppliers can position LNG, biofuel and low-sulphur grades against a buyer's rating position, and port agents can flag D or E rated vessels early to the technical superintendent.
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This article was written with AI assistance from the sources listed above. Facts are linked to their original publishers; please consult the sources before acting on them.