CII, EEXI & EEDI: the ship carbon-rating reference

Built for the people who fix, own and operate ships and keep hearing "the vessel is D-rated": charterers, owners, operators and the analysts who price a fixture. Three IMO efficiency rules — EEDI, EEXI and CII — are constantly conflated, and the formulas, boundaries and consequences live scattered across class-society PDFs and MEPC resolutions. We assembled them into one frame, with the reference-line maths and a worked A–E rating.

The rynda voyage-economics calculator computes a voyage's attained CII, the required CII for the year and vessel type, and the resulting A–E rating — so you can reproduce the numbers here.

📖 See also: EU ETS & FuelEU cost reference · BIMCO charter-party clauses · ECA / SECA zones reference

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Contents

• At a glance
• Three measures, not one
• The CII rating — operational A–E grade
• The reference line and the reduction factor
• The A–E boundaries
• A worked CII rating
• What a bad rating actually costs
• EEXI — the one-time design index
• How CII stacks with EU ETS, FuelEU and ECA
• For developers and modellers
• FAQ
• References

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At a glance

The short version, if you read nothing else:

• Three separate IMO efficiency rules, often confused: EEDI (design efficiency of newbuilds), EEXI (a one-time design-efficiency check on existing ships), and CII (an annual operational A–E rating of how efficiently a ship is actually run).
• CII (Carbon Intensity Indicator, MARPOL Annex VI Reg 28) rates cargo, ro-ro and cruise ships ≥ 5 000 GT (gross tonnage) from A (best) to E (worst) each year, based on grams of CO₂ per dwt-mile (deadweight-tonne × nautical mile — the ship's transport work).
• The bar tightens yearly: the required intensity falls −5 % (2023), −7 % (2024), −9 % (2025), −11 % (2026) versus the 2019 reference line.
• A D three years running, or a single E, forces a corrective action plan (SEEMP Part III — an addition to the ship's Ship Energy Efficiency Management Plan). There's no direct fine — the bite is commercial.
• The commercial bite is real: D-rated ships see charter-rate discounts of roughly 5–15 % vs a C, and charterers increasingly write a minimum CII into the charter party or refuse D/E ships outright.
• CII rewards how you sail (speed, routing, idle time); EEXI rewards the ship's design (a one-off). Slow-steaming fixes CII, not EEXI; an engine power limit fixes EEXI, not your annual CII.

This is a reference, not legal or compliance advice. The controlling texts are MARPOL Annex VI Reg 25 (EEXI) and Reg 28 (CII) and the MEPC (IMO Marine Environment Protection Committee) resolutions below; confirm against your class society and verifier.

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Three measures, not one

The single most common mix-up. They answer different questions:

EEDI and EEXI are the same idea — a modelled design efficiency measured once — applied to new and existing ships respectively. CII is the live one that recurs every year and shows up in charter negotiations, so most of this reference is about CII.

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The CII rating — operational A–E grade

CII measures carbon per unit of transport work over the year:

Attained CII (gCO₂ / dwt·nm) = annual CO₂ emitted (g) / (DWT × distance sailed (nm))

Every ship's attained figure is compared to a required figure for its type and size, and the ratio decides the A–E rating. Lower CO₂-per-dwt-mile = better. A ship that slow-steams, avoids idle main-engine hours, keeps a clean hull and sails shorter routes lowers its attained CII.

It applies to cargo, ro-ro and cruise ships ≥ 5 000 GT — the same tonnage threshold as EU ETS/FuelEU, which is why the same ships face all of them at once.

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The reference line and the reduction factor

The required CII is the 2019 fleet reference line for the ship type, tightened by a yearly reduction factor:

Required CII = Reference CII × (1 − reduction_factor(year))
Reference CII = a × Capacity^(−c)      (Capacity = DWT for most types)

The reference-line parameters a and c are IMO-published per ship type (MEPC.353(78)):

(Capacity is DWT for bulk carriers, tankers, container, gas and LNG carriers, but GT for cruise and ro-ro passenger ships. A separate bracket applies below the size thresholds, e.g. gas carrier < 65 000 DWT uses a = 8104.)

The reduction factor steps down each year versus 2019:

So a ship that was exactly "on the line" in 2019 must be 11 % more carbon-efficient in 2026 just to hold the same relative position.

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The A–E boundaries

The rating is the ratio attained ÷ required, bucketed by IMO "d-value" boundaries — and the boundaries differ by ship type (MEPC.354(78)), so you must apply the right column:

C is the pass mark — being at or a little under the required line. Gas carriers, ro-ro and cruise ships have their own d-vectors again — always apply the set for the ship's exact type, because using the bulker column on a tanker mis-rates it.

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A worked CII rating

Panamax bulk carrier, 75 000 DWT, calendar year 2026. Over the year it emits 19 000 t CO₂ and sails 63 000 nm (laden — loaded — plus ballast — sailing empty to reposition). This is a bulker, so read the bulk-carrier boundary column above.

Reference CII  = 4745 × 75 000^(−0.622)        ≈ 4.41 gCO₂/dwt·nm
Required 2026  = 4.41 × (1 − 0.11)             ≈ 3.92 gCO₂/dwt·nm
Attained       = 19 000 × 10⁶ g / (75 000 × 63 000)
               = 1.9×10¹⁰ / 4.725×10⁹          ≈ 4.02 gCO₂/dwt·nm
Ratio          = 4.02 / 3.92                    ≈ 1.03  → within C (≤ 1.06)

A C, but only just — a slightly slower average speed or fewer idle days would move it toward B; a dirty hull or more ballast steaming would tip it into D. That knife-edge is why operators model CII per voyage before fixing, not once a year in hindsight.

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What a bad rating actually costs

There is no direct fine for a poor CII. The consequences are procedural and, above all, commercial:

• Corrective action plan (SEEMP Part III). A ship rated D for three consecutive years, or E for a single year, must add a corrective action plan to its Ship Energy Efficiency Management Plan, stating the measures, timeline and expected effect.
• Statement of Compliance / PSC. Without a valid SEEMP Part III and Statement of Compliance, a ship theoretically risks Port State Control attention.
• Charter-rate discount. Industry estimates put D-rated ships at roughly 5–15 % below a comparable C on hire, and E-rated ships can become effectively uncharterable in some trades.
• Charter-party terms. Major charterers increasingly specify a minimum CII (e.g. "C or better") in the fixture, and the BIMCO CII Operations Clause allocates the operational control and data-sharing needed to hold a rating — because on a time charter the charterer controls speed and routing, but the owner carries the rating.

The takeaway for a fixture: a ship's CII trajectory is now a commercial term, not just a compliance box.

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EEXI — the one-time design index

EEXI (Energy Efficiency Existing Ship Index, Reg 25) applied a design-efficiency requirement to the existing fleet (ships ≥ 400 GT) from 2023. Unlike CII it is assessed once — at the first annual/renewal survey on or after 1 January 2023 — and certified, not re-rated each year.

• The ship's attained EEXI (a modelled gCO₂/capacity-mile from installed power, reference speed and fuel) must be ≤ the required EEXI = the EEDI reference line reduced by a type/size-specific factor.
• The commonest way to comply on an existing ship is an Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi) — capping the engine's maximum power so the modelled efficiency passes. The limit can often be overridden in an emergency.
• EEXI is a design figure — it says nothing about how the ship is actually run. A ship can pass EEXI and still earn a poor CII by steaming fast, and vice-versa.

EEDI is the same measure for newbuilds (contracts from 2013), tightened in phases. Together EEDI/EEXI police the ship's design; CII polices its operation.

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How CII stacks with EU ETS, FuelEU and ECA

CII is one of four independent regimes that can hit the same voyage — don't confuse them (full comparison in the EU ETS & FuelEU reference):

They can pull against each other, but watch the units on the classic example: burning cleaner-sulphur MGO in an ECA raises CO₂ per tonne (3.206 vs 3.114) yet MGO is more energy-dense, so a voyage burns fewer tonnes and its CO₂ per mile is roughly unchanged — MGO does not meaningfully worsen CII. The genuine conflict is elsewhere: e.g. slow-steaming to improve CII lengthens the voyage. Optimising one layer in isolation can quietly worsen another — which is why a combined per-voyage model matters.

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For developers and modellers

• Per-voyage CII: the voyage-economics calculator computes the voyage's attained CII, the required CII for the year and vessel type, the A–E rating and the delta to target — alongside EU ETS, FuelEU and the ECA premium in one TCE (Time Charter Equivalent — the voyage's net daily earnings).
• Compute it yourself: attained = annual_CO₂_g / (DWT × distance_nm); required = a × DWT^(−c) × (1 − reduction_year); rating = bucket attained/required by the d-values above. The CO₂ per tonne of fuel comes from the EU-MRV/IMO factors (3.114 HSFO/VLSFO, 3.206 MGO, 2.75 LNG).
• Reference line by type: use the (a, c) table above (MEPC.353(78)); apply the ship's own type d-values for the A–E cut.

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FAQ and common confusions

Q: What is a CII rating?
A: The Carbon Intensity Indicator is an annual A-to-E grade (A best) of how much CO₂ a cargo, ro-ro or cruise ship ≥ 5 000 GT emits per dwt-mile of transport work, versus an IMO required line that tightens each year. C is the pass mark; D or E signals an inefficient ship.

Q: How do I improve a CII rating?
A: Lower CO₂ per dwt-mile — mostly by operating the ship better: slow-steam, cut idle main-engine hours in port, keep the hull and propeller clean, optimise routing and trim, and avoid unnecessary ballast steaming. Speed is the single biggest lever. (Design fixes like an engine power limit change EEXI, not your annual CII.)

Q: What's the difference between EEXI and CII?
A: EEXI is a one-time design check (installed power, reference speed) certified in 2023. CII is an annual operational rating (actual CO₂ per dwt-mile) that recurs every year and produces an A–E grade. A ship can pass EEXI and still rate D or E on CII.

Q: Is there a fine for a D or E rating?
A: No direct fine. A D three years running or a single E forces a corrective action plan (SEEMP Part III), and the ship risks PSC scrutiny without a valid Statement of Compliance. The real cost is commercial — lower hire and charterers avoiding the ship.

Q: What is the pass mark?
A: C. A/B are better than required; C is broadly on the line; D/E are below it. Most charter clauses ask for "C or better".

Q: Does slow-steaming improve CII?
A: Yes — less fuel burned per mile lowers CO₂ per dwt-mile, so speed is the biggest operational CII lever. It does not change EEXI (a design figure).

Q: On a time charter, who is responsible for the CII rating?
A: The owner holds the rating, but the charterer controls speed and routing — the tension the BIMCO CII Operations Clause exists to manage (control, data-sharing, and sometimes rating warranties).

Q: Does switching to low-sulphur MGO in an ECA worsen my CII?
A: Not really — it's a common myth. MGO's CO₂ factor is higher per tonne (3.206 vs 3.114), but MGO also carries more energy per tonne, so a voyage burns fewer tonnes of it. CII counts CO₂ per mile, which tracks energy, not fuel mass — so the switch is roughly CII-neutral (marginally better, if anything). The "MGO worsens my carbon" belief comes from reading the per-tonne factor without adjusting for its higher energy content.

Q: Which ships does CII apply to?
A: Cargo, ro-ro and cruise ships of 5 000 GT and above, worldwide — the same threshold as EU ETS and FuelEU.

Q: Does the required CII keep tightening?
A: Yes — −5/−7/−9/−11 % for 2023–2026 versus the 2019 line, with steeper reductions expected for 2027+ once MEPC sets them. A ship standing still gets a worse rating each year.

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References

• MARPOL Annex VI — Reg 23/25 (EEXI attained/required), Reg 28 (CII), Reg 20–21 (EEDI).
• IMO MEPC.337(76) — the CII framework; MEPC.352–355(78) — CII reference lines, reduction factors, rating boundaries (d-values) and the operational guidelines.
• IMO MEPC.334(76) / EEXI guidelines — attained/required EEXI.
• BIMCO — CII Operations Clause for Time Charter Parties.
• Class-society CII/EEXI/SEEMP Part III guidance (DNV, Lloyd's Register, ClassNK, Bureau Veritas).
• rynda voyage-economics calculator.

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Document version

• 2026-07 — first public version. EEDI/EEXI/CII in one frame, with the 2026 reference lines, reduction factors and A–E boundaries. Reduction factors for 2027+ are set by MEPC and will be added when published.