CII Attained vs Required Calculator: Rating and Headroom

Formula, assumptions, and limits

The comparison runs the attained CII against the required figure for the year and reads the ratio as a rating, then measures the gap to the compliance boundary. To compute it:

CII_attained - the year’s operational carbon intensity, gCO₂ per capacity-mile. CII_required - the reference line reduced by the year’s reduction factor. d3 - the C-to-D rating boundary for the ship type, from MEPC.339(76); CII_C/D is the carbon intensity at which the ship drops out of compliance. The ratio places the ship on the A to E scale; the headroom (positive) or deficit (negative) measures the distance to the C/D line.

The page distinguishes two things charter and operations desks routinely conflate: the rating and the margin. The rating is a letter, a step function that hides how close the ship is to the next band; the headroom is the continuous distance, which is what actually predicts next year’s letter. A ship two-tenths of a percent below the C/D boundary and one five percent below are both rated C, but one is a corrective-plan case waiting for the line to tighten and the other is not, and only the headroom tells them apart.

The fuel-tonne-equivalent is the page’s translation layer. A carbon intensity gap in gCO₂ per capacity-mile is hard to act on; the same gap expressed as the tonnes of fuel it represents over the year, at the dominant fuel’s carbon factor, is a bunker number an operator can plan against, slow steaming, trim, hull cleaning, or a cleaner blend. The limit on it is the single-fuel assumption: a ship burning a genuine mix needs the dominant-fuel approximation read as indicative, and the attained CII itself, built from the full bunker list, remains the authoritative figure.

How to use this calculator

1. Select the ship type and enter the capacity (DWT or GT).
2. Select the reporting year.
3. Enter the attained CII.
4. Optionally add the annual fuel burn and dominant fuel for the fuel-tonne-equivalent.
5. Read the ratio, the rating, the headroom or deficit, the adjacent-band distances, and the year-erosion chart.

The erosion the rating hides

A CII rating is a snapshot, and the most expensive mistake in CII planning is treating it as durable. Because the required line tightens every year, the honest question is never “what is the rating now” but “what is the rating across the next three years on this trade,” because the three-consecutive-D trigger and the charter-party warranties both run on the trajectory. The chart on this page holds the attained CII fixed and steps the required line through the reporting years, so the year the ship crosses from C to D is a visible fact for the entered vessel rather than a surprise in an annual statement. A thin-headroom C that becomes a D in 2026 is the standard shape, and seeing it is the difference between budgeting a corrective plan a year ahead and discovering one after the verifier’s letter arrives.

Common errors

1. Reading the rating without the headroom. Two ships rated C can sit at opposite ends of the band; the headroom, not the letter, is what predicts next year.
2. Entering cargo tonnage as capacity. As on the required page, the capacity must be the certificate figure; the wrong basis moves the required line and the rating with it.
3. Trusting the fuel-tonne-equivalent for a real fuel mix. It assumes a single dominant fuel; a genuine blend needs the figure read as indicative against the full attained calculation.
4. Quoting a ratio in a clause without the ship type. The same ratio grades differently by type; a charter-party figure is ambiguous unless the d-vector is named.

CII in the charter party

CII moved into commercial contracts faster than into any enforcement action, because a poor rating is the owner’s asset risk and the charterer’s operational doing at the same time. A time charterer controls speed, routing, and waiting time, the very levers that set attained CII, while the owner holds the rating that follows the ship to its next fixture and its financing. BIMCO’s CII clause for time charter parties answers the split by sharing the data and the decisions: the parties agree a target rating, exchange the operational figures that build the attained CII, and allocate who bears the cost of the choices that move it. This calculator is the verification layer under such a clause: it turns the year’s attained figure into the ratio and rating the clause is written around, and the headroom into the early warning a charterer needs before an instruction tips the ship into a worse band. A clause that names a rating without a shared way to compute it invites the dispute the shared module here is built to avoid.

About This CII Attained vs Required Calculator

This is the working CII page: the one a charter desk opens to verify a warranty, an operator opens to track in-year margin, and a technical manager opens to size a corrective plan. It takes the attained CII and the ship particulars and returns the required figure, the ratio, the A to E rating, the headroom or deficit in carbon-intensity, percentage, and fuel-tonne terms, and the distances to the adjacent bands.

The arithmetic is the shared MEPC.336/337/338/339 module, so the required figure, the boundaries, and the rating match the required and rating calculators exactly. The headroom and the fuel-tonne-equivalent are this page’s contribution beyond the letter: the continuous margin that predicts next year, and the bunker quantity that turns the margin into an action.

Where the attained figure is built from the year’s fuel and distance, the CII attained calculator computes it; where the rating’s annual erosion is the planning question, this page’s chart and the required calculator’s trajectory show the line moving under the ship.

Further reading

• What is CII?
• BIMCO CII clauses
• CII corrective action plan
• CII attained calculator
• CII rating calculator

Frequently asked questions

What does CII headroom mean?

Headroom is how far a compliant ship sits below the C-to-D boundary: the margin, in carbon intensity, before the next operational year of the same performance would slip it out of compliance. A ship with thin headroom is a C today and a likely D once the required line tightens, even with no change in operation.

How is the attained-to-required ratio used?

The ratio of attained CII to required CII is the number the MEPC.339(76) rating boundaries grade. Below the type's d1 boundary is A, and the bands run up to d4, above which is E. The ratio, not the raw attained figure, is what charter-party CII clauses and rating statements quote.

What is the fuel-tonne-equivalent of a CII deficit?

The carbon-intensity gap to the C-to-D boundary, translated back into the tonnes of fuel that gap represents over the year at the dominant fuel's carbon factor. It turns an abstract gCO₂-per-mile shortfall into the bunker quantity a ship must cut, or the margin it holds, which is the figure operations actually plan against.

Can a ship's rating change without its operation changing?

Yes, and it routinely does. The required CII tightens every year under MEPC.338(76), so a ship holding its attained CII steady moves toward worse bands as the line descends beneath it. A 2025 C with thin headroom is a 2026 D on identical trading, which is exactly what the year-erosion chart on this page shows.