CII Required Calculator: Annual Target Carbon Intensity

Formula, assumptions, and limits

The required CII is the reference line for the ship’s type and size, reduced by the year’s factor. To compute it, evaluate the reference line and apply the reduction:

a, c - the ship-type reference-line coefficients from MEPC.337(76), fitted to 2019 fleet data. Capacity - deadweight for most cargo ships, gross tonnage for passenger, ro-pax, ro-ro vehicle carriers, and cruise ships. Z - the annual reduction factor: 0.05 (2023), 0.07 (2024), 0.09 (2025), 0.11 (2026) under MEPC.338(76), then 0.13625, 0.16250, 0.18875, 0.21500 for 2027-2030 under MEPC.400(83).

Two features of the formula drive most of the confusion around it. The negative exponent means the required CII per capacity-mile falls as the ship grows: bigger ships are held to a lower number, because moving more cargo over the same distance is intrinsically more carbon-efficient per tonne-mile, and the reference line bakes that in. And the capacity basis is not interchangeable: a passenger ship’s required CII is built on gross tonnage while a bulker’s is built on deadweight, so feeding the wrong tonnage produces a number that looks plausible and is wrong by the ratio of the two.

The reduction schedule is now settled to 2030. MEPC.338(76) specified the factors through 2026 (5 to 11 percent), and MEPC.400(83), adopted at MEPC 83 in April 2025, set the strengthened phase: 13.625 percent in 2027 rising in 2.625-point steps to 21.5 percent by 2030. The calculator carries the full adopted schedule, so a required CII for any year from 2023 to 2030 is a fixed target, not a projection; only the post-2030 trajectory remains open.

How to use this calculator

1. Select the ship type for its reference coefficients.
2. Enter the capacity on the correct basis (DWT or GT).
3. Select the reporting year for the reduction factor.
4. Read the reference line and the required CII; the chart shows the required value falling from 2023 to 2026 as Z rises.

Why the line moves under the ship

The required CII descends every year by design, and the calculator’s chart draws that descent for the entered ship so the planning consequence is visible rather than abstract. A bulk carrier required to attain 5.0 gCO₂ per dwt-mile in 2023 is held to roughly 4.7 by 2026 and 4.1 by 2030 on the same hull and the same reference line, a 17 percent tightening that no operational change earns back: the ship must run cleaner every year merely to hold its rating. This is why CII is an operational treadmill and why fleet plans model the required line forward rather than the current year alone; a vessel comfortably compliant today can be a corrective-plan case in two years with an unchanged trade, and the required-CII trajectory is where that shows up first.

Common errors

1. Wrong capacity basis. Deadweight for cargo ships, gross tonnage for passenger and ro-pax types; swapping them produces a plausible wrong number.
2. Cargo tonnage instead of certificate capacity. The required line uses the ship’s rated capacity, not the tonnage a voyage actually carried.
3. Projecting past 2030. The factors are adopted through 2030 (MEPC.400(83)); a required CII for 2031 or later is a projection against a phase the IMO has not yet set.
4. Comparing required figures across sizes as if fixed. The negative exponent makes a larger ship’s required CII lower per capacity-mile; the figures are not interchangeable between sister ships of different deadweight.

Capacity is the certificate figure, not the cargo carried

The capacity in the formula is a fixed ship particular, not the cargo actually loaded on a voyage. For most cargo ships it is the deadweight from the ship’s certificates, the maximum the hull is rated to carry, and for passenger, ro-pax, ro-ro vehicle, and cruise ships it is the gross tonnage. Neither moves with how full the ship sailed: a bulker that ran half-laden all year has the same required CII as one that ran full, because the required line is a property of the ship, not the trade. This is also why CII rewards being busy: the attained figure falls as the ship moves more cargo over more miles on the same fuel, while the required figure stays fixed on certificate capacity, so a well-utilized ship beats its required line more easily than an idle one of the same class. The capacity input here is the certificate value; feeding a voyage’s actual cargo tonnage produces a required CII far too lenient to be useful.

What the required line is the denominator of

Every CII rating is a ratio, and the required CII is its denominator: get this number wrong and the rating, the headroom, and the charter-party warranty built on it are all wrong by the same factor. That is why the required line earns a page of its own rather than living inside the rating calculator, and why the capacity basis and the year’s reduction factor, the two things most often fed in wrong, get the scrutiny here that a combined tool would bury.

The arithmetic is the shared CII module the rating and attained-versus-required calculators also use, so a required figure read here matches the one those pages compute internally. The chart’s contribution is the year dimension: a single required CII is a number, but the descent from 2023 to 2026 is the regulation’s actual mechanism, and seeing the slope is what separates a one-year compliance check from a fleet plan.

For the attained side of the comparison, the CII attained calculator builds the figure from fuel and distance; to place an attained value against this target and read the grade, the attained-versus-required calculator does both at once.

Further reading

• What is CII?
• SEEMP I, II and III
• CII corrective action plan
• CII rating calculator
• CII attained vs required calculator

Frequently asked questions

What is Required CII?

The annual carbon-intensity target a ship must meet to be rated C or better: the MEPC.337(76) reference line for its type and size, reduced by the MEPC.338(76) annual reduction factor. It is the denominator every CII rating is measured against, in grams of CO₂ per capacity-mile.

How is the reference line calculated?

Reference line = a times capacity raised to the power minus c, with a and c set per ship type in MEPC.337(76) from 2019 fleet data. Capacity is deadweight for most cargo ships and gross tonnage for passenger, ro-pax, ro-ro vehicle, and cruise ships, so the two cannot be interchanged.

What are the CII reduction factors by year?

Under MEPC.338(76) the reference line is reduced by 5 percent in 2023, 7 percent in 2024, 9 percent in 2025, and 11 percent in 2026. MEPC.400(83), adopted in April 2025, set the strengthened phase: 13.625 percent in 2027 rising in 2.625-point steps to 21.5 percent by 2030. Factors beyond 2030 are not yet adopted.

Is Required CII the same for every ship of a type?

No. The reference line falls with capacity through the negative exponent, so a larger ship of the same type has a lower required CII per capacity-mile, reflecting the efficiency of scale. Two bulk carriers a year apart in deadweight have different required figures, which is why the capacity input must be the ship's own.