The IMO Energy Efficiency Framework (EEF) is the foundation of shipping’s emissions reductions today and is the first and most immediate lever for reducing emissions from international shipping.

Long before fuel standards and carbon pricing entered the regulatory landscape, the International Maritime Organization (IMO) focused on a simpler question: how efficiently ships use energy. The EEF is the result of that work: a set of technical and operational regulations that reduce the amount of energy required to transport cargo by sea.

Together with the IMO Net-Zero Framework (NZF) , the IMO EEF forms one of the two regulatory pillars delivering the IMO’s 2023 GHG Strategy.

Find more information from the IMO about the EEF here.

The IMO’s formal work on reducing GHG emissions from ships began in the early 2000s with a focus on technical and operational efficiency.

This work led to amendments to MARPOL Annex VI and, in 2013, the entry into force of the world’s first global energy efficiency regulations covering an entire industry sector. These regulations introduced:

• the Energy Efficiency Design Index (EEDI) for new ships, and

• the Ship Energy Efficiency Management Plan (SEEMP) for all ships.

Since then, the framework has been expanded and strengthened, extending efficiency requirements to existing ships and introducing operational performance ratings.

The IMO EEF combines design standards with operational performance measures. Together, these instruments ensure that ships are both built and operated efficiently.

The EEDI sets an index translating to a minimum energy efficiency standard for new ships. It expresses efficiency as grams of CO2 per tonne‑mile by ship size and reference speed.

Since coming into force in 2013, the EEDI has been tightened through phased reductions:

• Phase 0: Baseline

• Phase 1: 10% improvement

• Phase 2: 20% improvement

• Phase 3 (from 2025): 30–50% improvement, depending on ship type

As a technology‑neutral measure, the EEDI allows designers to choose the most suitable options for how the ship will meet the required target.

Introduced in 2023, the EEXI applies the same design‑based logic as EEDI to existing ships, extending equivalent requirements across the global fleet.

EEDI and EEXI thus cover the technical characteristics of the ship at the design stage, not taking into consideration actual operational performance.

Good design does not guarantee efficient operation. The CII measures the annual operational energy efficiency of ships. A ship’s attained CII for a given year is calculated using the ship’s reported fuel consumption data, which is converted into an estimate of CO2 emissions using IMO-defined conversion factors, then divided by the vessel’s cargo capacity and distance traveled. In this way, the CII formula links energy consumption and emissions to the transport work the ship has delivered.

Once the attained CII has been calculated, it is compared against the required CII for that year, and each ship is assigned an annual rating from A (best performance) to E (worst performance). The rating associated with a given attained CII value decreases over time – so, for example, the same CII value could result in a ‘C’ rating in 2024 but a ‘D’ rating in 2026. This setup incentivizes continuous improvements in efficiency as time goes on.

Ships with low ratings  – D for three consecutive years, or E for one year – must develop corrective action plans under SEEMP Part III, linking operational performance directly to regulatory follow‑up.

The SEEMP is the operational backbone of the framework and serves as the enforcement mechanism for the CII. It requires ships to plan, monitor, and document how energy efficiency and carbon intensity are managed when ships are in operation.

Under IMO guidelines, the SEEMP consists of three parts:

• Part I: Improving operational energy efficiency

• Part II: Fuel consumption data collection and reporting

• Part III: Managing and improving carbon intensity in line with the CII

Energy efficiency in shipping is not a single measure: it covers actions across three interconnected dimensions, each targeting a different way of reducing the energy a ship needs to do its work:

• Improved energy conversion: Making better use of the energy already on board. Rather than consuming more fuel, ships can extract more useful work from each unit of energy by minimizing what is lost between the engine and the propeller.

• Reduced energy demand: Lowering the energy a ship needs in the first place. Examples include reducing hydrodynamic resistance, lowering speed, or retrofitting the ship with energy-saving devices. In this way, less energy is required to move the same amount of cargo over the same distance.

• Economy of scale: Transporting more cargo with less energy use. Larger vessels can reduce the amount of energy needed to transport one unit of cargo.

The EEF and the NZF have the potential to address different sides of the same challenge: reducing greenhouse gas emissions from shipping.

• The EEF regulates the amount of energy that ships require for their operations.

• The NZF promotes switching to more sustainable and low–emissions energy sources.

Read our newsletter on energy efficiency and the IMO NZF  for further discussion of the intersections between the two frameworks.

Together, these regulations can ensure that shipping’s transition to net zero can be both energy‑efficient and fuel‑agnostic, reducing emissions faster and at a lower cost.