CII Rating Explained: A–E Scale, How to Calculate It & How to Improve

What does CII measure?

CII measures the carbon intensity of a ship's operations — how much CO₂ it emits relative to the transport work it performs. A smaller number (less CO₂ per unit of transport work) is better. CII is calculated annually using actual operational data, not design specifications.

The IMO introduced CII under MARPOL Annex VI (Regulation 28) as part of the Short-Term GHG Reduction Measures, effective 1 January 2023. It applies to ships of 5,000 GT and above engaged in international voyages.

How is CII calculated?

The attained CII is calculated using the Annual Efficiency Ratio (AER) for most ship types:

For passenger ships, cruise ships, and Ro-Ro vessels, CGT (compensated gross tonnage) or gross tonnage × distance is used instead of DWT. The MEPC.338(76) resolution sets the exact capacity and transport work definitions for each ship type.

How is the A–E rating determined?

Each ship type (bulk carrier, tanker, container, general cargo, etc.) has a required CII value for a given year, calculated from a reference line that is tightened annually:

• 2023: baseline reference (reduction factor 5%)
• 2024: 7% reduction factor
• 2025: 9% reduction factor
• 2026: 11% reduction factor

Your attained CII is then compared to the required CII. Rating boundaries (the exact d₁–d₄ values) vary by ship type and size. The IMO publishes the correction factors and reference lines in MEPC.338(76).

Our free CII calculator performs this calculation for all major ship types using the current IMO reference lines.

What happens if your vessel gets a D or E rating?

Under MARPOL Annex VI, the consequences of a D or E rating escalate over time:

• Rating D for three consecutive years: the shipping company must submit a corrective action plan to the flag state, with specific measures to achieve at least a C rating. The plan is reviewed by the Administration.
• Rating E in any year: same immediate obligation — a corrective action plan is required the following year.

While the IMO has not (yet) imposed automatic port state control detention for poor CII ratings, many port states and charterers now request CII certificates as part of due diligence. A D or E rating can impact charter negotiations and insurance premiums.

How to improve your CII rating

CII improvement strategies fall into three categories:

Operational measures (immediate impact)

• Slow steaming — reducing speed by 10% can cut fuel consumption by up to 27% (cubic relationship). The most effective single measure.
• Weather routing optimisation — choosing routes that minimise head seas, reduce resistance, and shorten total distance.
• Trim and draft optimisation — ensuring the vessel operates at optimal trim for each loading condition.
• Reducing idle time and waiting at anchor — fuel burned at anchor counts against CII but produces zero transport work.
• Just-in-time arrival — coordinating with ports to reduce unnecessary high-speed transits followed by waiting.

Technical measures (medium-term)

• Propeller polishing and hull cleaning to reduce resistance
• Engine tuning and waste heat recovery
• Air lubrication systems
• Wind-assisted propulsion (rotorsails, kite sails)

Alternative fuels (long-term)

• Switching from HFO to LNG, methanol, or biofuels reduces the CO₂ factor (CF) applied to fuel consumption
• Biofuels (UCOME, FAME) with verified GHG reduction credits can significantly lower CII

CII and EU MRV: how they relate

CII and EU MRV use similar underlying data (fuel consumption, distance travelled) but serve different purposes and have different scopes:

• EU MRV: EU/EEA regulation, covers all voyages to/from EU ports, submits to THETIS-MRV, used for EU ETS allowance calculation
• IMO CII: global MARPOL regulation, covers all international voyages, rating reported to flag state, published in ship's SEEMP

VesselComply collects fuel and voyage data once and uses it to generate both EU MRV reports and CII ratings, eliminating duplicate data entry.