How CII (Carbon Intensity Indicator) is calculated — short, practical guide
Quick definition: CII is the ship’s annual CO₂ emitted per unit of transport work (grams CO₂ per tonne-nautical mile). It’s used by IMO to give each ship a yearly rating (A–E).
Formula (simplified)
Attained CII (g CO₂ / t·nm) =
Annual CO₂ emissions (g) ÷ (Annual distance sailed (nm) × Ship capacity)
Where:
- Annual CO₂ emissions = sum over fuels of (annual fuel consumption (t) × CO₂ emission factor (t CO₂/t fuel)) then convert to grams.
- Ship capacity = DWT (deadweight) for most cargo ships, or GT/other metric depending on ship type — check IMO guidance for which capacity to use for your ship type.
Units: CO₂ in grams, capacity in tonnes, distance in nautical miles, result in g CO₂ / t·nm.
Step-by-step calculation
- Gather fuel data for the calendar year: consumption by fuel type (tonnes).
- Apply CO₂ emission factors for each fuel type (t CO₂ per t fuel), sum to get total t CO₂ for the year and convert to grams (1 tCO₂ = 1,000,000 g).
- Compute annual transport work = total distance sailed in the year (nm) × capacity (t). Use the IMO-prescribed capacity type for your ship.
- Divide total annual CO₂ (g) by annual transport work (t·nm) → that’s your attained CII (g CO₂ / t·nm).
- Apply any allowed correction/adjustment factors (voyage adjustments, technical/operational corrections) if eligible — these are defined in the IMO guidelines and may change over time.
Worked example (digit-by-digit)
Assume:
- Annual fuel = 2,500 t heavy fuel oil (HFO)
- CO₂ emission factor for HFO = 3.114 t CO₂ / t fuel (example factor)
- Annual distance = 20,000 nm
- Capacity = 50,000 t
Calculations:
- Annual CO₂ (t) = 2,500 × 3.114 = 7,785 t CO₂.
- Convert to grams: 7,785 t × 1,000,000 = 7,785,000,000 g CO₂.
- Transport work = 20,000 nm × 50,000 t = 1,000,000,000 t·nm.
- CII = 7,785,000,000 ÷ 1,000,000,000 = 7.785 g CO₂ / t·nm.
What the number means / rating
The attained CII is compared to IMO reference/reference-lines for your ship type & size and converted to a rating A (best) → E (worst). Ships must report the attained annual CII (typically within 3 months after year end) and keep records; corrective obligations apply if the rating is poor. See IMO resolutions and class guidance for exact reference values and reporting timelines.
Practical tips to improve CII
- Slow steaming / speed optimization and just-in-time arrivals reduce fuel burn.
- Hull/propeller maintenance, optimized trim and weather routing.
- Fuel switching to lower-carbon fuels or use of abatement technologies (where available).
- Better cargo utilization (higher cargo per voyage) increases transport work and helps CII.
Each ship’s attained CII (actual performance) is compared with the required CII (IMO benchmark for its type & size).
The ratio determines the rating grade:
| Rating | Meaning | Performance level vs Required CII | Action required |
|---|---|---|---|
| A | Major superior performance | ≥ 10% better than required | Excellent efficiency; no action required. |
| B | Minor superior performance | 5–10% better | Good efficiency; maintain operations. |
| C | Compliant performance | Within ±5% of required | Acceptable; continue monitoring. |
| D | Minor inferior performance | 5–10% worse | Needs improvement; plan required if two consecutive D ratings. |
| E | Inferior performance | >10% worse | Non-compliant; immediate corrective action plan mandatory. |
(“Better” = lower CO₂ per transport work.)
🧮 Example of Rating Interpretation
Let’s say a 50,000 DWT bulk carrier has:
- Required CII = 7.8 g CO₂ / t·nm
- Attained CII = 7.0 g CO₂ / t·nm
Then performance = (7.8 − 7.0) / 7.8 = 10.3% better → Rating A
🗓️ Rating Period and Reporting
- Applies from: 1 January 2023 under MARPOL Annex VI Regulation 28.
- Reporting: Annually via the IMO DCS (Data Collection System).
- Verification: Class society or flag verifies and issues a Statement of Compliance (SoC).
⚠️ Actions for Poor Ratings
- Two consecutive D’s or one E:
→ Ship must submit a Corrective Action Plan (CAP) as part of SEEMP Part III, explaining how to improve to C or better.
→ Ship must submit a Corrective Action Plan (CAP) as part of SEEMP Part III, explaining how to improve to C or better.
🚢 Typical Reference Ship Types
Different reference lines and correction factors exist for:
- Bulk carriers
- Tankers
- Container ships
- Gas carriers
- Ro-Ro and Ro-Pax ships
- General cargo ships
- LNG carriers, etc.
Each has a size-based reference CII (from IMO MEPC. 336(76)).
