CII reduction factors: the Z factor trajectory
The annual carbon intensity reduction factors, the 2019 reference line, and the MEPC.400(83) amendment of 11 April 2025 that set the values for 2027 to 2030.
The CII reduction factor, conventionally written Z, is the percentage by which a ship’s required annual operational carbon intensity is tightened relative to its reference line. It is set in Table 1 of the CII Reduction Factors Guidelines (G3), and it is the single lever that decides how much harder compliance becomes each year.
The reference line is the median attained operational carbon intensity of a defined group of ships in 2019, not 2008. The factors are 5 percent for 2023, 7 for 2024, 9 for 2025 and 11 for 2026. The values for 2027 to 2030 were left blank when the guidelines were adopted in 2021 and were filled in by resolution MEPC.400(83), adopted 11 April 2025: 13.625 percent for 2027, 16.250 for 2028, 18.875 for 2029 and 21.500 for 2030.
The step from 11 to 13.625 percent between 2026 and 2027 is the largest single-year tightening in the series, and it is now fixed rather than pending, which changes the forward economics of every operational efficiency measure. The full article will cover the reference line derivation, the amendment history and the interaction with the Phase 2 review. See what is CII and slow steaming .