IMO CII rating calculation: reference lines to A–E bands

The Carbon Intensity Indicator (CII) is the IMO's yearly measure of how much CO₂ a ship emits for the transport work it performs — grams of CO₂ per unit of capacity carried one nautical mile. Under regulation 28 of MARPOL Annex VI MEPC.328(76) · Reg. 27, every cargo, ro-ro passenger and cruise ship of 5,000 gross tonnage and above on international voyages has had its attained annual operational CII calculated from the 2023 reporting year onwards, compared with a required value that tightens every year, and rated from A (major superior) to E (inferior). A ship rated D for three consecutive years, or E in any single year, must add a corrective action plan to its SEEMP.

The rating is a four-step chain, and each step has its own IMO guideline: attained CII (G1), the reference line for the ship's type and size (G2), the reduction factor for the year (G3), and the rating boundaries (G4). This page walks the chain in order, ending with the rating example the IMO prints in the G4 guidelines — recomputed every time this site is built.

Step 1 — attained CII

Attained CII is the ship's actual carbon intensity for the calendar year MEPC.352(78) · G1: total CO₂ from the fuel reported under the IMO Data Collection System, divided by capacity and distance sailed.

CII_attained = M × 10⁶C × D_t

M

total CO₂ emitted in the calendar year [t CO₂]

C

ship capacity (DWT, or GT for ro-ro/passenger categories)

D_t

total distance travelled [nm]

The fuel-to-CO₂ conversion uses the IMO conversion factors per fuel type; the distance is the total distance travelled over ground. Before the attained value is computed, the interim G5 guidelines allow specific correction factors and voyage adjustments — for example for ice-class operation or ship-to-ship transfer — to be applied to the underlying fuel or distance figures MEPC.355(78) · G5. They correct the attained value; they do not touch the reference line or the boundaries.

Step 2 — the reference line

The reference line is the fleet-average carbon intensity, in the 2019 baseline year, of ships of the same type and capacity MEPC.353(78) · Table 1. It is a power curve in capacity: two parameters per ship category, a and c, fitted by the IMO and printed in G2.

CII_ref = a × Capacity^−c

a

reference-line coefficient for the ship category (Table 1)

c

reference-line exponent for the ship category (Table 1)

Capacity

capacity as evaluated for the reference line (caps/fixed values per Table 1)

Ship category	Size band	Capacity	a	c
bulk_carrier	gte_279000_dwt	DWT	4745	0.622
bulk_carrier	lt_279000_dwt	DWT	4745	0.622
gas_carrier	gte_65000_dwt	DWT	144050000000	2.071
gas_carrier	lt_65000_dwt	DWT	8104	0.639
tanker	all	DWT	5247	0.610
container_ship	all	DWT	1984	0.489
general_cargo	gte_20000_dwt	DWT	31948	0.792
general_cargo	lt_20000_dwt	DWT	588	0.3885
refrigerated_cargo	all	DWT	4600	0.557
combination_carrier	all	DWT	5119	0.622
lng_carrier	gte_100000_dwt	DWT	9.827	0
lng_carrier	gte_65000_lt_100000_dwt	DWT	144790000000000	2.673
lng_carrier	lt_65000_dwt	DWT	144790000000000	2.673
roro_vehicle_carrier	gte_57700_gt	GT	3627	0.590
roro_vehicle_carrier	gte_30000_lt_57700_gt	GT	3627	0.590
roro_vehicle_carrier	lt_30000_gt	GT	330	0.329
roro_cargo	all	GT	1967	0.485
roro_passenger	all	GT	2023	0.460
roro_passenger_hsc	all	GT	4196	0.460
cruise_passenger	all	GT	930	0.383

Step 3 — required CII

The required CII for a year is the reference line reduced by that year's reduction factor Z, in percent MEPC.400(83) · G3. Z rises each year on the trajectory the IMO adopted for the current phase.

CII_required = CII_ref × (1 − Z/100)

Z

reduction factor for the year (5% in 2023 rising to 21.5% in 2030) [%]

Year	Z (%)
2023	5
2024	7
2025	9
2026	11
2027	13.625
2028	16.250
2029	18.875
2030	21.500

Step 4 — the rating

A rating is a comparison of attained against required. The G4 guidelines define four boundaries — superior, lower, upper and inferior — each a fixed multiple of the required CII MEPC.354(78) · Table 1. The multiples are the exponentiated d vectors printed per ship category; a ship at or below the superior boundary is rated A, between superior and lower B, between lower and upper C, between upper and inferior D, and above the inferior boundary E.

boundary_i = d_i × CII_required

d_i

rating boundary vector d₁…d₄ for the ship category (printed exp form)

Ship category	Size band	d₁	d₂	d₃	d₄
bulk_carrier	all	0.86	0.94	1.06	1.18
gas_carrier	gte_65000_dwt	0.81	0.91	1.12	1.44
gas_carrier	lt_65000_dwt	0.85	0.95	1.06	1.25
tanker	all	0.82	0.93	1.08	1.28
container_ship	all	0.83	0.94	1.07	1.19
general_cargo	all	0.83	0.94	1.06	1.19
refrigerated_cargo	all	0.78	0.91	1.07	1.20
combination_carrier	all	0.87	0.96	1.06	1.14
lng_carrier	gte_100000_dwt	0.89	0.98	1.06	1.13
lng_carrier	lt_100000_dwt	0.78	0.92	1.10	1.37
roro_vehicle_carrier	all	0.86	0.94	1.06	1.16
roro_cargo	all	0.76	0.89	1.08	1.27
roro_passenger	all	0.76	0.92	1.14	1.30
cruise_passenger	all	0.87	0.95	1.06	1.16

Last reviewed 27 August 2026 · Carbonlogy Compliance Team

The example the IMO prints

G4 illustrates the boundaries with one bulk carrier. Every number below is the resolution's own; the exp(d) vector is resolved from Carbonlogy's coefficient package and the rating is recomputed through the product's rating function each time this site is built — if either drifted from the printed digits, the build would fail and this page would not ship.

Worked example

CII rating boundaries — the MEPC.354(78) example

✓ Engine-verifiedregulator-printed example · recomputed automatically · updated 2026-08-28

The G4 rating guidelines print one worked example: a bulk carrier whose required annual operational CII in a given year is 10 gCO₂/(dwt·nmile). Table 1 gives the bulk-carrier dd vectors (after exponential transformation); Eq. (3) scales them by the required CII to get the four rating boundaries. An attained CII of 9 sits between the superior and lower boundaries, so the ship is rated B.

Required annual operational CII

10 gCO₂/(dwt·nmile)

Attained annual operational CII

9 gCO₂/(dwt·nmile)

exp(d₁) — bulk carrier

0.86

exp(d₂) — bulk carrier

0.94

exp(d₃) — bulk carrier

1.06

exp(d₄) — bulk carrier

1.18

1. 1
   Superior boundary (A/B)0.86 × 10 = 8.6 gCO₂/(dwt·nmile)
2. 2
   Lower boundary (B/C)0.94 × 10 = 9.4 gCO₂/(dwt·nmile)
3. 3
   Upper boundary (C/D)1.06 × 10 = 10.6 gCO₂/(dwt·nmile)
4. 4
   Inferior boundary (D/E)1.18 × 10 = 11.8 gCO₂/(dwt·nmile)

Result · Rating (8.6 < attained 9 ≤ 9.4)B

How Carbonlogy applies it

Every ship in a Carbonlogy fleet carries its attained CII, reference line, required CII and rating for the current year, computed from the same reference line, reduction factor and boundary tables shown on this page and from the ship's reported fuel and distance. Ship types for which the IMO publishes no reference line are shown as "CII not applicable" rather than rated against a guessed curve, and the rating gauge marks the four boundaries so a ship's distance to the next band is visible at a glance. FuelEU Maritime measures something different — greenhouse-gas intensity per unit of energy — and the two are compared in the related article below.