CII calculation starts with annual fuel consumption. Fuel is converted into CO2 emissions, then divided by annual transport work. The result is the attained annual CII. That value is compared with the required annual CII and rating boundaries for the vessel type, size and reporting year.
The calculation is simple in concept but sensitive to data quality. Incorrect fuel mapping, missing distance, wrong capacity basis, unsubmitted noon reports or duplicate reports can change the rating.
Attained CII is the vessel's actual annual carbon intensity based on verified fuel consumption and transport work for the reporting year.
What is required CII?
Required CII is the target value calculated from IMO reference lines and annual reduction factors for the vessel type and reporting year.
What is transport work?
Transport work is generally capacity multiplied by distance travelled. For many cargo ships, capacity is deadweight. Some ship categories use gross tonnage based metrics.
Which data errors are most dangerous?
Wrong fuel type, wrong CO2 factor, missing distance, duplicate noon reports, incorrect vessel capacity and mixing port/waiting fuel without proper review are common high-impact errors.
Technical Explanation
For most cargo vessels, the calculation follows this logic:
1. Sum annual fuel consumption by fuel type.
2. Convert each fuel quantity to CO2 using the relevant conversion factor.
3. Sum total annual CO2 emissions.
4. Determine annual distance travelled.
5. Determine applicable capacity basis.
6. Calculate attained CII.
7. Calculate required CII for vessel type, capacity and year.
8. Compare attained versus required and assign rating.
Checklists
• Fuel quantities reconciled by grade
• CO2 factors applied by fuel type
• Distance checked for gaps and anomalies
• Capacity and vessel type confirmed
• Year reduction factor confirmed
• Rating calculation independently reviewed
Practical Case Studies
If a vessel produces 30,000 tonnes of CO2, has capacity of 50,000 DWT and sails 40,000 nautical miles, transport work is 2,000,000,000 dwt-nm. The attained CII is then calculated using CO2 in grams divided by dwt-nm.
KPI Monitoring
• Missing noon reports
• Fuel variance between noon and bunker records
• Distance variance by voyage
• CO2 by fuel grade
• Attained CII versus required CII trend
Formula
CO2 emissions = fuel consumed x CO2 conversion factor
Transport work = capacity x distance travelled
Attained CII = CO2 emissions in grams / transport work
Required CII = reference line x (1 - reduction factor)
Common Mistakes
• Using tonnes of CO2 without converting to grams where the formula expects grams.
• Using calendar distance instead of verified distance travelled.
• Applying the wrong vessel type reference line.
• Using design deadweight for a ship type that requires a different capacity basis.
• Ignoring missing noon report days.
