WHAT IS CII AND HOW IS IT CALCULATED

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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).

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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.

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Step-by-step calculation

1. Gather fuel data for the calendar year: consumption by fuel type (tonnes).
2. 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).
3. Compute annual transport work = total distance sailed in the year (nm) × capacity (t). Use the IMO-prescribed capacity type for your ship.
4. Divide total annual CO₂ (g) by annual transport work (t·nm) → that’s your attained CII (g CO₂ / t·nm).
5. 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.

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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.

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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.

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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.)

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🧮 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

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🗓️ 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).

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⚠️ 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.

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🚢 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)).