IMO CII explained: reference lines, ratings, reductions
IMO CII for compliance teams: attained and required CII, reference lines, reduction factors and A–E rating boundaries, cited to MEPC.353(78) and MEPC.354(78).
The Carbon Intensity Indicator sits in regulation 28 of MARPOL Annex VI as revised in 2021. Every ship of 5 000 gross tonnage and above on international voyages has had its attained annual operational CII calculated from the 2023 reporting year, and is rated A to E against a required value that tightens each year. MEPC.328(76) · Reg. 27
The reference lines come from the G2 guidelines, the yearly reduction factors from MEPC.400(83) and the rating boundaries from the G4 guidelines; the G5 guidelines set the correction factors and voyage adjustments a ship may apply before it is rated. MEPC.353(78) · Table 1MEPC.400(83) · G3MEPC.354(78) · Table 1MEPC.355(78) · G5
A ship rated D for three consecutive years, or E once, must develop a plan of corrective actions to reach its required CII. The articles below give the rating arithmetic end to end and the corrections that change the attained value. MEPC.328(76) · Reg. 27
IMO CII
Carbon Intensity Indicator reference lines, reduction factors and the A–E rating boundaries from MEPC.353(78) and MEPC.354(78). · MEPC.353(78) · MEPC.354(78)
Calculators
Carbonlogy platform
Run this calculation across your fleet
The worked examples on these pages are computed by the same engine that powers the platform — per ship, per voyage, every day.