Environment and regulation

The calculation period for annual operational CII and what happens on transfer (MARPOL Annex VI Regulation 28)

CII covers a full calendar year. When the flag or company changes during that year, the data collection and reporting periods need separate review. Regulations 27 and 28 of the 2021 revised MARPOL Annex VI set different obligations.

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The calculation period runs 1 January to 31 December

shall calculate the attained annual operational CII over a 12-month period from 1 January to 31 December for the preceding calendar year, using the data collected in accordance with regulation 27 of this Annex, taking into account the guidelines to be developed by the Organization.

It applies to ships of 5,000 gross tonnage and above falling into one of the categories the Annex lists. The period is the 12-month calendar year and the input is the data collected under regulation 27. The calculation runs from the end of calendar year 2023 and every year after.

The reporting deadline is three months after the year ends

2 Within three months after the end of each calendar year, the ship shall report to its Administration, or any organization duly authorized by it, the attained annual operational CII via electronic communication and using a standardized format to be developed by the Organization.

The recipient is the flag Administration or an organization duly authorized by it. The method is electronic communication in a standardized format.

A change of flag or company does not split the year

3 Notwithstanding 1 and 2 of this regulation, in the event of any transfer of a ship addressed in regulations 27.4, 27.5 or 27.6 completed after 1 January 2023, a ship shall, after the end of the calendar year in which the transfer takes place, calculate and report the attained annual operational CII for the full 12-month period from 1 January to 31 December in the calendar year during which the transfer took place, in accordance with regulations 28.1 and 28.2, for verification in accordance with regulation 6.6 of this Annex, taking into account guidelines to be developed by the Organization.

This is where it parts from regulation 27. Regulation 27 has the period split on a transfer. On a change of Administration the ship reports the aggregated data for the losing Administration’s portion on the day the transfer completes or as close to it as practical; on a change of company it reports the portion corresponding to that company. Where both happen at once, the Administration rule governs.

Regulation 28 does the opposite. For the calendar year in which the transfer happened, it calculates and reports the full 12-month period. Ratings are not to be built out of fragments.

Regulation	How it sees the year a transfer falls in
Regulation 27 (fuel oil consumption data)	Period split by Administration and by company, each aggregated and reported
Regulation 28 (operational CII)	The whole 1 January to 31 December calculated and reported as one

And the same paragraph adds a line: nothing in the regulation relieves any ship of its reporting obligations under regulation 27 or under regulation 28. Counting the year whole does not replace reporting it split.

The required CII comes from a reference value and a reduction factor

Regulation 28 paragraph 4 defines the required annual operational CII by applying an annual reduction factor to a reference value. The reduction factor is the value that keeps improvement running inside a given rating level, and it and the reference value are set by guidelines developed by the Organization. The provision carries the shape of the formula; the numbers are in the guidelines.

The rating splits five ways around the required CII

6 The attained annual operational CII shall be documented and verified against the required annual operational CII to determine operational carbon intensity rating A, B, C, D or E, indicating a major superior, minor superior, moderate, minor inferior, or inferior performance level, either by the Administration or by any organization duly authorized by it, taking into account the guidelines developed by the Organization. The middle point of rating level C shall be the value equivalent to the required annual operational CII set out in paragraph 4 of this regulation.

Two places to read. First, the procedure is documented and verified. It does not end at the operator’s own calculation; the Administration or an authorized organization verifies. Second, the middle point of rating C equals the required CII. Hitting the required CII exactly puts the ship in the centre of C.

Three consecutive D ratings or an E rating leads to a plan of corrective actions carried into the SEEMP, and Administrations, port authorities and other stakeholders are encouraged to provide incentives to ships rated A or B.

Distinguish the review date from future amendments

Checked on 2026-09-28. The earlier text’s 1 January 2026 review deadline is not an expiry date for CII. MEPC.407(84) changes the review clause, with entry into force scheduled for 2027-09-01 subject to acceptance. Distinguish current guidance from future Convention amendments.

Worth confirming

The passages above come from page 44 of the 2021 revised MARPOL Annex VI (MEPC.328(76)). The reduction factor, the reference value and the rating boundaries come from guidelines developed by the Organization, so the text alone does not produce numbers. Whether that ship is in a regulation 28 category, whether a change of flag or company fell inside this year, and whether the twelve months of data survive without a gap are where the actual judgement starts.

Calculate an annual CII for a fictional bulk carrier

This is a learning example created by Bellbook. Assume a bulk carrier operating throughout 2026: 50,000 DWT, total annual distance 60,000 nm and fossil diesel/gas oil consumption 4,800 t. There are no other fuels, biofuels, correction factors or voyage adjustments. This is not a verified ship result.

MEPC.352(78) divides total fuel-derived CO2 by capacity times distance. MEPC.412(84) retains ship-type DWT or GT rather than actual cargo carried, and includes distance both under way and not under way. This bulk carrier uses DWT.

The diesel/gas oil factor of 3.206 t-CO2/t-fuel appears in the appendix to MEPC.1/Circ.925. Use the applicable bulk-carrier reference line in MEPC.353(78) and the 2026 reduction factor in MEPC.400(83).

CO2 = 4,800 × 3.206 = 15,388.8 t = 15,388,800,000 g
W = 50,000 × 60,000 = 3,000,000,000 dwt·nm
CII = 15,388,800,000 / 3,000,000,000 = 5.1296 gCO2/(dwt·nm)
CIIref = 4745 × 50,000^(-0.622) ≈ 5.668614
CIIrequired(2026) = (1 − 11/100) × CIIref ≈ 5.045066

Multiplying required CII by the bulk-carrier boundary factors 0.86, 0.94, 1.06 and 1.18 from MEPC.354(78) gives approximately 4.338757, 4.742362, 5.347770 and 5.953178. The example’s 5.1296 falls between 4.742362 and 5.347770, so the example rating is C. Only displayed values are rounded; retain precision through the actual comparison.

Reproduce the result from fuel and distance records

The CII calculation example CSV splits fictional records into the first and second halves of the year. Add fuel 3,000 + 1,800 = 4,800 t and distance 40,000 + 20,000 = 60,000 nm first. Do not average period CIIs or ratings to obtain the annual result.

At a company transfer, collect the previous company’s period records and reports, conversion factors, gap and overlap checks, and adjustment evidence so the annual calculation can be reproduced. The CSV is not an official submission form. For an actual calculation, confirm ship type, capacity and fuels, and assess corrections and voyage adjustments under MEPC.355(78). If distance or the denominator is missing, investigate the records instead of inventing a divisor.

Apply the factor for each planning year

MEPC.400(83) sets reductions against the 2019 reference line: 11% for 2026, 13.625% for 2027, 16.250% for 2028, 18.875% for 2029 and 21.500% for 2030. These are not successive reductions from last year’s performance. Use the corresponding year’s factor in SEEMP planning.

You can consult the cited source documents

These links lead to the official publishers of the documents discussed here. Match the edition and provisions cited in the article, and check subsequent amendments and flag-state implementation before applying them.

Bellbook takes the fuel and distance behind the annual CII day by day

Bellbook retains fuel and distance in noon reports and voyage records. The company must reconcile gaps, duplicates and transfer records, then calculate and obtain verification of annual CII using the applicable methodology.

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