EEXI and EEDI, explained — and how they differ from CII

The International Maritime Organization's energy-efficiency rules come in two flavours that are easy to confuse. There are design indices — which ask how efficient a ship was built to be — and an operational indicator — which asks how efficiently it is actually being run. The EEDI and the EEXI belong to the first group; the CII belongs to the second. All three live inside the energy-efficiency chapter of MARPOL Annex VI, but they answer different questions and they are measured in different ways.

EEDI: the design rule for new ships

The Energy Efficiency Design Index (EEDI) applies to new ships and has been mandatory under MARPOL Annex VI since 2013. It is a pure design measure. For a new ship of a given type and size, the EEDI sets a maximum amount of CO2 the vessel may emit per tonne-mile of transport work, calculated from the design itself — installed engine power, cargo-carrying capacity and a reference speed. A naval architect can therefore know a ship's attained EEDI before the keel is even laid.

Crucially, the required EEDI tightens in phases over time. A ship ordered today must meet a stricter target than one ordered a decade ago, which steadily forces newbuilds to become more efficient — through better hull forms, larger and slower-turning propellers, waste-heat recovery and, increasingly, alternative fuels. The EEDI never changes for a ship once it is set; it is a snapshot of how that hull was designed to perform.

EEXI: the same logic, applied to existing ships

The fleet built before the EEDI bit hard is enormous, so the IMO extended the same design-based logic to ships already in service. The Energy Efficiency Existing Ship Index (EEXI) entered into force on 1 January 2023 and applies to ships of 400 gross tons and above in the categories covered.

The EEXI is a one-off, design-based calculation. Each ship's attained EEXI is worked out from its design and compared against a required value derived from the EEDI framework. A ship that does not meet its required EEXI must take action — most commonly engine power limitation (EPL) or shaft power limitation (ShaPoLi), which caps the maximum power the engine can deliver, or genuine efficiency improvements such as propeller upgrades or energy-saving devices. The key word is once: the EEXI is assessed a single time as a snapshot of the design, not recalculated every year.

CII: the operational rating that changes every year

The Carbon Intensity Indicator (CII) is a different animal entirely. Where EEDI and EEXI judge the blueprint, the CII judges the operation. It measures how efficiently a ship actually transports cargo in service — grams of CO2 per unit of cargo-carrying capacity and nautical mile sailed — and turns that into an annual rating from A (best) to E (worst).

The CII also took effect from 1 January 2023, with ratings first calculated on 2023 operational data. The required CII tightens year on year, so a rating that passes this year may fail next year for the same ship doing the same work. And there are consequences: a ship rated D for three consecutive years, or E for a single year, must submit a corrective action plan. Because it is operational, the CII responds to how a ship is sailed — slowing down, optimising routing, reducing idle time in port and cutting fuel burn all push the rating up the scale.

The distinction that trips everyone up

Here is the heart of it. EEDI and EEXI are design indices — they judge a ship by how it is built, producing a one-off number per hull. CII is an operational index — it judges how the ship is actually run each year, and it changes annually with speed, routing, time in port and everything else a master and operator control.

The practical consequence is that the two can diverge. A ship can hold a perfectly compliant EEXI — its design is efficient enough on paper — and yet earn a poor CII because it is operated inefficiently: sailing fast, taking long ballast legs, or sitting idle with engines running. Design sets the potential; operation delivers the result.

Index	Applies to	Design or operational?	When	One-off or annual
EEDI	New ships	Design	Mandatory since 2013	One-off (set at build)
EEXI	Existing ships, 400 GT+	Design	In force 1 Jan 2023	One-off (snapshot)
CII	Ships in service	Operational	In force 1 Jan 2023	Annual (A–E rating)

The design indices ask what a ship is capable of; the carbon intensity indicator asks what it actually does. A vessel cannot operate its way out of a design number, nor design its way out of how it is run. MARPOL Annex VI, Chapter 4, IMO

How they fit together under MARPOL Annex VI

All three measures live in Chapter 4 of MARPOL Annex VI, the convention's energy-efficiency chapter. Read together they form a single logic: the EEDI raises the floor for new ships, the EEXI drags the existing fleet up to a comparable design standard, and the CII keeps pushing on the part neither design index can reach — the way a ship is sailed day to day. For anyone screening a vessel, the design indices are stable reference points, while the CII is the figure that has to be re-checked each year. They are complementary, not interchangeable.

If you want to place these rules in the wider decarbonisation picture, read our guides to CII ratings, what MARPOL is and the IMO's 2030 and 2050 targets. Then look up any specific ship in the Marifest registry — type a name or IMO number to open the full file, including its tonnage, type and the compliance picture that frames which of these rules apply to it.