How to Calculate a Ship’s CII Rating in 2026: Formula, Rating Bands and Improvement Strategies

A ship’s Carbon Intensity Indicator (CII) rating measures its operational carbon intensity over a reporting year. Under the International Maritime Organization (IMO) framework, applicable ships receive a rating from A to E, based on their attained annual operational CII compared with the required CII for their ship type, size and reporting year.

For shipowners and fleet managers, calculating CII accurately is essential for understanding vessel performance, preparing verified compliance reports and identifying opportunities to reduce fuel consumption and emissions.

In 2026, CII calculations continue to rely on verified operational data, including fuel consumption and distance travelled. The required performance level also reflects the reduction factor applicable to the reporting year.

What Is a Ship’s CII Rating?

The Carbon Intensity Indicator measures the relationship between a ship’s carbon dioxide emissions and its transport work. It helps assess how efficiently a vessel operates in relation to the capacity measure and distance travelled.

The IMO’s operational CII framework applies to ships of 5,000 gross tonnage and above within its defined scope under MARPOL Annex VI.

The rating has five categories:

• A: Major superior performance
• B: Minor superior performance
• C: Moderate performance
• D: Minor inferior performance
• E: Inferior performance

A rating of C or better is the performance level to aim for. A ship rated D for three consecutive years, or E in a single year, must develop a corrective action plan under the applicable IMO requirements and its Ship Energy Efficiency Management Plan (SEEMP).

Official reference: IMO — EEXI and CII Frequently Asked Questions.

How to Calculate a Ship’s CII Rating

CII calculation involves determining the vessel’s attained CII, calculating its required CII for the reporting year, and comparing the two using the applicable ship-specific rating boundaries.

Step 1: Collect the required operational data

Start with the vessel’s verified annual records, including:

• Fuel consumed by fuel type
• Applicable fuel-specific CO₂ conversion factors
• Distance travelled during the reporting period
• The ship’s relevant capacity measure
• Any applicable regulatory corrections or voyage adjustments

The data should be consistent across noon reports, bunker delivery notes, fuel records, engine or flowmeter data and voyage documentation.

Step 2: Calculate the attained annual operational CII

For many cargo ships, the calculation is based on annual CO₂ emissions divided by capacity multiplied by distance travelled.

A simplified expression is:

Attained CII = Annual CO₂ emissions ÷ (Capacity × Distance travelled)

For most relevant ship types, capacity is expressed in deadweight tonnes (DWT). Certain ship types use gross tonnage or another prescribed capacity measure under the applicable IMO methodology.

The detailed calculation must follow the relevant IMO guidelines, including the correct fuel conversion factors and any permitted adjustments. The simplified expression above is intended to explain the principle, not replace the formal calculation.

Step 3: Calculate the required CII for 2026

The required CII is derived from the applicable reference value and the reduction factor for the reporting year.

Required CII = Reference CII × (1 − Annual reduction factor)

The reference value depends on the vessel’s type and capacity. For 2026, the applicable reduction factor is 11% relative to the relevant reference CII value under the current reduction-factor schedule.

This means a vessel’s required CII is stricter than its unreduced reference value. Use the applicable IMO guidelines and ship-specific coefficients when preparing an actual compliance calculation.

Step 4: Compare attained CII with required CII

Once both values have been calculated, compare the attained CII against the required CII and apply the correct rating boundaries for the vessel’s category.

A lower attained CII generally indicates lower CO₂ emissions per unit of transport work. However, the A–E rating cannot be assigned accurately from the attained figure alone; the ship type, required CII and applicable rating boundaries all matter.

For this reason, shipowners should avoid applying a single universal set of rating thresholds to every vessel.

What Do the A–E CII Rating Bands Mean?

Rating

Meaning

Operational response

A

Major superior

Maintain performance and document effective practices

B

Minor superior

Sustain efficiency and monitor performance

C

Moderate

Meet the required performance level and track trends

D

Minor inferior

Investigate causes and monitor consecutive-year results

E

Inferior

Prepare for the required corrective-action process

The rating indicates operational carbon-intensity performance, not simply the total amount of fuel consumed. Vessel size, ship type, capacity, distance and the applicable reference methodology all influence the result.

A vessel may consume more fuel in absolute terms than a smaller ship while still achieving a better CII rating relative to its transport work.

What Can Affect a Ship’s CII Rating?

Several operational and data-quality factors influence the final result.

Fuel consumption accuracy: Missing bunker records, inconsistent fuel quantities or incorrect fuel classifications can affect calculated CO₂ emissions.

Voyage planning: Route selection, avoidable detours and operational delays can influence fuel use and distance travelled.

Speed and operating practices: Appropriate speed optimisation and coordination between commercial and technical teams may help reduce fuel consumption.

Hull and propeller condition: Fouling and increased resistance can raise the energy required to maintain speed.

Data consistency: Differences between noon reports, bunker records, flowmeter readings and voyage records can make it harder to establish a reliable annual calculation.

Changing performance requirements: Annual reduction factors become more demanding over time, so a vessel’s historical rating should not be treated as a guarantee of future performance.

How Can Shipowners Improve Their CII Rating?

Improving CII requires a combination of operational action and reliable reporting.

1. Monitor fuel consumption regularly. Review fuel use by vessel, voyage and fuel type rather than waiting until year-end.
2. Improve voyage efficiency. Evaluate speed, routing, weather conditions and avoidable waiting time.
3. Maintain hull and propulsion systems. Schedule appropriate cleaning and maintenance to reduce unnecessary resistance.
4. Reconcile operational records. Compare fuel, distance and voyage data across source documents and resolve discrepancies promptly.
5. Track performance throughout the year. Use periodic reviews to identify deterioration early and evaluate the likely year-end outcome.
6. Document corrective actions. Maintain clear evidence of operational changes, data checks and improvement measures.
7. Coordinate compliance and technical teams. Ensure vessel operators, technical managers and reporting teams work from consistent records.

Not every measure will deliver the same result for every ship. Improvement plans should reflect the vessel’s operating profile, technical condition, commercial commitments and applicable regulatory requirements.

Why Accurate IMO DCS Data Matters for CII

The IMO Data Collection System (DCS) provides annual fuel-consumption data that supports the CII calculation. Errors or gaps in operational records can complicate verification and reduce confidence in the resulting rating.

A reliable process should connect fuel records, distance travelled, voyage information and supporting documentation. When discrepancies appear, teams should investigate them while the underlying records and operational context are still available.

This is particularly important for fleets managing multiple vessels, reporting periods and data sources. Standardised checks and documented reconciliation help teams identify exceptions before they become last-minute reporting problems.

How Sternbow Supports Maritime Compliance

Sternbow helps shipowners and managers organise maritime compliance workflows around vessel data, documentation, validation, reconciliation and reporting.

By bringing operational records and compliance checks into a more structured process, teams can identify missing information, investigate inconsistent figures and prepare more reliable reporting evidence.

For fleets managing CII alongside EU MRV, EU ETS and FuelEU Maritime obligations, connecting related data workflows can reduce fragmented manual work and improve visibility across reporting activities.

Explore our maritime compliance software guide and our EU ETS and FuelEU Maritime 2026 guide for related requirements and workflows.

CII Compliance Checklist for 2026

Before finalising a vessel’s annual CII calculation, confirm that:

• The vessel’s applicability and ship category have been checked.
• Annual fuel consumption records are complete and reconciled.
• Fuel types and applicable CO₂ conversion factors are correct.
• Distance and capacity inputs follow the relevant methodology.
• Applicable correction factors and adjustments have been reviewed.
• The correct 2026 required CII and rating boundaries have been applied.
• Supporting documents are available for verification.
• The result and any required corrective actions are documented.

Conclusion

Calculating a ship’s CII rating involves more than dividing emissions by distance. Shipowners need reliable fuel and voyage records, the correct capacity measure, the required CII for the reporting year and the applicable IMO rating boundaries.

In 2026, consistent data validation and year-round performance monitoring can help maritime teams identify issues earlier, prepare for verification and make more informed operational decisions.

Need a more structured approach to maritime compliance? Talk to Sternbow about your fleet’s reporting workflow.

Official References

• IMO: EEXI and CII — Frequently Asked Questions
• IMO: Improving the Energy Efficiency of Ships
• IMO GreenVoyage2050: Energy Efficiency Technologies Information Portal

Regulatory note: This article is an educational overview, not a substitute for the applicable MARPOL Annex VI requirements, current IMO guidelines or vessel-specific verification advice. Confirm the applicable calculation method and requirements before filing.