From 1 January 2023 every ship of 5,000 GT and above engaged in international trade carries an annual operational Carbon Intensity Indicator (CII) rating, A through E. The methodology is set out in MARPOL Annex VI Regulation 28 and IMO MEPC resolutions 336–339(76), with the Z (annual reduction) factor updated by MEPC.355(78).
AER (Annual Efficiency Ratio) is the default attained CII metric:AER = Σ(fuel × Cf) / (DWT × distance)in g CO₂ per DWT·NM. The required CII for the year is the 2019 reference line reduced by the year's Z factor (5% in 2023, ramping by 2% annually). The dd ratio of required to attained then maps to A/B/C/D/E bands.
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What the rating means
A (dd ≥ 1.34) — major superior. Marketing advantage; some charterers offer premium hire.
B (1.21–1.34) — minor superior.
C (1.06–1.21) — moderate. The compliance baseline.
D (0.94–1.06) — minor inferior. Three consecutive D years requires a documented plan of action in SEEMP Part III.
E (< 0.94) — major inferior. A single E year requires a corrective action plan.
Corrective Action Plan (CAP) template
Three consecutive D ratings or a single E rating requires a CAP submitted with the SEEMP Part III to the Recognised Organisation. Structure per MEPC.355(78) resolution:
Root-cause analysis — which drivers pulled the vessel below C (hull condition, port waiting, cargo mix, weather routeing, speed profile, engine tuning)?
Target rating — realistic target (at least C, ideally B) for the next 12-month calendar year.
Measures selected — from the operational + technical lever list below. Reference the specific SEEMP Part III measure numbers.
Monitoring + verification — quarterly review with the DPA; annual verification by the RO.
Master + Chief Engineer sign-off — mandatory endorsement per the SEEMP.
The CAP is a live document; each quarterly review updates the RO record. Non-implementation of a signed CAP is treated as a MARPOL Annex VI compliance failure by charterers + PSC alike.
MEPC.354(78) correction factors
Under MEPC.354(78) certain operational conditions can be excluded from the CII calculation via voyage-adjustment factors:
Weather (fW) — heavy-weather deviation as evidenced by a documented voyage weather log. Reduces the fuel used-attributed to that leg.
Short-voyage (fS) — voyages under a specified minimum distance. Excludes the port-arrival + departure manoeuvring fuel that inflates AER on short trades.
Ice-condition (fi) — voyages inside designated ice regions. Recognises the fuel penalty of navigating through ice.
STS (fSTS) — ship-to-ship transfer operations, particularly relevant for tanker crude-oil transfers.
Correction factors are applied via the DCS reporting; supporting evidence (voyage plans, weather logs, STS records) must be retained for RO verification.
Levers to improve CII
Slow steaming — fuel burn is roughly cubic with speed; small speed reductions yield large CII gains.
Voyage optimisation — weather routeing, just-in-time arrival to avoid waiting at anchor.
Hull and propeller cleaning — biofouling can add 10–15% to fuel burn.
Energy-saving devices — Mewis ducts, pre-swirl stators, propeller boss cap fins.
Fuel switching — LNG (Cf 2.750), methanol, ammonia. Note CII is well-to-tank only; LCA-based metrics are coming.
E rating requires a Corrective Action Plan in the next SEEMP Part III for the following year.
Reference CII (2019 baseline)
0.004
g CO₂ / DWT·NM (Bulk carrier (≥ 279,000 DWT capped))
Required CII for 2026
0.004
= ref × (1 − 11%)
Attained CII (AER)
5.025
= ΣCO₂ / (DWT × distance)
Annual CO₂ emitted
26,784 t
= 8500 t fuel × Cf 3.151
Estimate. Real CII reporting under MARPOL Annex VI Regulation 28 requires correction factors for ice class, cargo cooling, STS transfers, port-time fuel, and other special circumstances per MEPC.355(78). Use this tool for indicative planning only.