Why EEDI exists
Shipping consumes around 300 million tonnes of fuel a year and emits nearly 3 % of global anthropogenic CO₂. Before EEDI, there was no mandatory design-phase carbon metric for new ships. Operators could order any hull, any engine, any powertrain - efficiency was left to the market.
EEDI, adopted into MARPOL Annex VI in 2011 and in force since 1 January 2013, changes that. Every new ship above certain size thresholds must compute an Attained EEDI at the design stage and prove it is no worse than the Required EEDI for its type, size, and phase.
What EEDI measures
EEDI is a ratio:
EEDI = CO₂ emissions per transport work
= (grams of CO₂) / (tonne × nautical mile)
More precisely, for a ship at 75 % MCR (maximum continuous rating) on its design draft, EEDI sums the CO₂ from the main engine(s), auxiliary engines, and shaft generators, subtracts credits for innovative energy-efficient technologies, and divides by the product of the ship’s capacity and its reference speed.
A bulk carrier moving 80,000 tonnes at 14 knots with a good EEDI emits fewer grams of CO₂ per tonne-mile than one with a bad EEDI - regardless of how the efficiency was achieved (better hull, better propeller, LNG, wind assist, air lubrication, or any combination).
Attained vs Required
- Attained EEDI: the ship’s actual design value. Calculated from the engine data, the fuel’s Cf conversion factor (from MEPC.364(79)), the capacity, and the reference speed.
- Required EEDI: a moving target. For each ship type, IMO publishes a reference line of the form
a × DWT^(−c)(coefficients in MEPC.231(65), updated by MEPC.328(76)). The required value isreference × (1 − phase_factor), and the phase factor has tightened in steps - Phase 0 (0 %), Phase 1 (10 %), Phase 2 (20 %), Phase 3 (30 % or more depending on ship type and size).
Phase 3 applies to most ships contracted from 1 April 2022; some categories already moved earlier. In effect, a 2025 newbuild container ship must be ~30 % more carbon-efficient per TEU-mile than a similar 2013 design.
Which ships are in scope
EEDI applies to new-build ships of 400 GT and above in the following categories: bulk carriers, gas carriers, tankers, container ships, general cargo, refrigerated cargo, combination carriers, ro-ro, ro-pax, LNG carriers, and cruise ships with non-conventional propulsion. Ships with diesel-electric, turbine, or hybrid propulsion are covered too. Small domestic craft and military vessels are excluded.
EEXI - the existing-ship sibling
EEDI only binds new ships. In 2023 the IMO introduced EEXI (Energy Efficiency Existing Ship Index) to extend the same logic to the existing fleet. EEXI uses the same formula but the reference speed is derived at 75 % MCR with the engine either physically limited (EPL) or shaft-limited (ShaPoLi). An existing ship that cannot meet EEXI without modification must install one of those limitations before its next survey.
Computing EEDI in practice
- Identify the capacity metric - DWT for most types, GT for passenger/cruise/ro-pax.
- Look up the reference line coefficients - from MEPC.328(76) Table 1.
- Apply the phase reduction factor - which depends on ship type, size band, and contract date.
- Calculate attained EEDI - sum main/auxiliary CO₂ terms, subtract innovative-tech credits, divide by capacity × reference speed.
- Check the margin - attained must be ≤ required.
Our EEDI Attained calculator (shipping in Phase 1) walks through each step with regulatory citations inline.
The bigger picture
EEDI targets design-phase CO₂ per tonne-mile. It does not address methane slip, short-lived climate pollutants, lifecycle emissions, or how the ship is actually operated. For those, IMO has layered on CII (operational carbon intensity), SEEMP (Ship Energy Efficiency Management Plan), and the pending IMO GHG strategy net-zero-around-2050 target. EU adds EU ETS and FuelEU Maritime on top. EEDI is the foundation the rest is built on.