Step-by-step methodology behind the IMO Carbon Intensity Indicator: MEPC.354(78) formula, ship-type reference lines, annual reduction factors, A–E band boundaries, and the corrective action plan trigger.
Track CII trajectory across your entire fleet automatically — per voyage, projected year-end, D/E early warning.
Start Free Trial →A mandatory operational efficiency metric — measures how the ship actually performs each year, not just how it was designed.
The IMO Carbon Intensity Indicator (CII) falls under MARPOL Annex VI Regulation 28. From 1 January 2023, every ship of 5,000 GT and above on international voyages must calculate its annual CII, compare it to a ship-type-specific reference, and report the resulting A–E rating alongside its IMO DCS submission.
Unlike EEXI which is a technical (design-based) metric verified once, CII is operational. A ship that carries less cargo, sails at higher speeds, or spends more time in heavy weather will have a worse CII than a sister vessel run more efficiently — even with identical hulls.
Three consecutive D-ratings or one E-rating triggers a Corrective Action Plan (CAP) — a class-approved plan the operator must incorporate in SEEMP Part III and use to return to C or better within one reporting cycle.
The IMO methodology reduces to four calculations. Every calc app runs the same math.
Sum fuel mass × emission factor across all fuel types consumed during the year. Cf values from MARPOL Annex VI Regulation 26 Appendix IX: HFO 3.114, VLSFO 3.150, MGO 3.206, LNG 2.750, LPG 3.000.
Attained CII = CO₂ / (Capacity × Distance). Capacity is DWT for most ships, Gross Tonnage for passenger/RoPax. Distance is nautical miles underway only (drift, anchor, berth excluded).
Reference CII = a × Capacity^(-c) using ship-type coefficients from MEPC.338(76). Required CII = Reference × (1 − reduction%). Reduction ramps annually via MEPC.354(78) — 5% in 2023 up to 15% in 2028.
The ratio (Attained ÷ Required) is compared to four boundaries d1, d2, d3, d4. A = ratio ≤ d1 (major superior), C = required minimum, E = ratio > d4 (major inferior).
Reference CII = a × DWT^(−c). Different ship types have different curves reflecting technology-dependent baseline efficiency.
| Ship Type | Coefficient a | Exponent c | Capacity |
|---|---|---|---|
| Bulk carrier | 4,745 | 0.622 | DWT |
| Tanker | 5,247 | 0.610 | DWT |
| Container ship | 1,984 | 0.489 | DWT |
| Gas carrier (≥65k DWT) | 14.4 × 10⁹ | 2.071 | DWT |
| LNG carrier (≥100k DWT) | 9.827 | 0.000 (constant) | DWT |
| LNG carrier (65-100k) | 14.5 × 10¹² | 2.673 | DWT |
| General cargo (≥20k) | 31,948 | 0.792 | DWT |
| Refrigerated cargo | 4,600 | 0.550 | DWT |
| Ro-ro cargo | 1,967 | 0.485 | DWT |
| Ro-ro passenger | 2,023 | 0.381 | GT |
| Cruise passenger | 930 | 0.383 | GT |
Reduction applied to the reference line each year. IMO agreed 2023-2026 rates; post-2026 rates being reviewed under the 2028 revision.
The A–E bands are set by four ratios d1, d2, d3, d4. C is the minimum required for compliance.
| Ship Type | d1 (A/B) | d2 (B/C) | d3 (C/D) | d4 (D/E) |
|---|---|---|---|---|
| Bulk carrier | 0.86 | 0.94 | 1.06 | 1.18 |
| Tanker | 0.82 | 0.93 | 1.08 | 1.28 |
| Container ship | 0.83 | 0.94 | 1.07 | 1.19 |
| LNG carrier | 0.89 | 0.98 | 1.06 | 1.13 |
| General cargo | 0.83 | 0.94 | 1.06 | 1.19 |
| Refrigerated cargo | 0.78 | 0.91 | 1.07 | 1.20 |
| Ro-ro cargo | 0.66 | 0.90 | 1.11 | 1.37 |
| Ro-ro passenger | 0.72 | 0.90 | 1.12 | 1.41 |
| Cruise passenger | 0.87 | 0.95 | 1.06 | 1.16 |
Identify the specific driver — hull fouling, ballast passages, wrong trade lane, aged engines, cargo utilisation, weather-routing gaps.
Operational (slow steaming, weather routing, hull cleaning) and/or technical (retrofits, EPL, air lubrication). Must show quantifiable CII improvement.
The Ship Energy Efficiency Management Plan Part III (operational compliance plan) must include the CAP. This becomes the ship's binding action plan.
The revised SEEMP + CAP is verified by an authorised third-party organisation or the flag state. Verification is required before the next IAPP renewal.
Ship must implement the plan and demonstrate at least a C rating in the next reporting period. Failure blocks IAPP renewal — effectively grounding the vessel.
Three related IMO efficiency measures that easily get confused. Here's how they fit together.
| Dimension | CII | EEXI | EEDI |
|---|---|---|---|
| Type | Operational | Technical (existing ships) | Technical (new ships) |
| Applies to | Ships ≥5,000 GT from 2023 | Ships ≥400 GT from 2023 | Ships built ≥2013 |
| Frequency | Annual | Once at survey | Once at delivery |
| Metric | Rating A-E | Pass/fail vs required | Pass/fail vs required |
| Base regulation | MARPOL Reg. 28 | MARPOL Reg. 23 | MARPOL Reg. 21 |
| Data source | Actual voyage data (DCS) | Sea trial / design | Sea trial / design |
| Compliance lever | Operational (speed, cargo, hull) | Engineering (EPL, retrofit) | Design |
| Non-compliance | CAP + IAPP-block risk | IAPP blocked | Cannot deliver ship |
Ranked by leverage per dollar of investment.
| Lever | Typical CII improvement | Cost profile |
|---|---|---|
| Slow steaming (−10% speed) | 15-25% | Operational — trades voyage time |
| Weather routing optimisation | 2-5% | Software subscription |
| Hull cleaning + prop polish (annual) | 3-8% | $30-80k per event |
| Trim optimisation | 1-3% | Software + procedures |
| Port stay minimisation | 2-5% | Coordination — cheap |
| Cargo utilisation (higher DWT filled) | 5-15% | Commercial |
| Air lubrication (retrofit) | 5-8% | $1-3M capex |
| Wind-assist (rotor sails) | 5-15% | $1-5M capex |
| Fuel switch to LNG | ~12% (Cf 3.114→2.750) | $25-50M retrofit |
| Waste heat recovery | 3-8% | $5-15M retrofit |