CII Year-on-Year Improvement Calculator

This tool reads the year-on-year change in a ship’s attained CII and sets it against the one fact that decides whether the rating moves: the required line tightens every reporting year. A ship can cut nothing, report the same gCO2 per capacity-mile two years running, and still slide a band, because the rating is a ratio and the denominator shrank. Enter two consecutive attained figures and the calculator returns the percent change and a trend flag; the sections below turn that single number into a rating call by comparing it against the reduction factor Z for the year pair.

How to use this calculator

1. Enter the attained CII for the earlier year (the field labelled Attained CII 2024) from that year’s verified Data Collection System reporting, the DCS verifier’s statement, or the CII attained calculator.
2. Enter the attained CII for the later year (the field labelled Attained CII 2025) on the same gCO2 per capacity-mile basis and the same capacity definition, deadweight or gross tonnage, so the two years are comparable.
3. Read the year-on-year change in percent. A negative value means attained intensity fell year on year; a positive value means it rose.
4. Read the trend flag: the tool marks the change improving below minus one percent, deteriorating above plus one percent, and flat in the band between.
5. Take that change to the tightening-line table below and read it against the required-line movement for your year pair. The trend flag is about the attained number alone; the rating call needs both.

The two input fields carry calendar-year labels, but the arithmetic is year-agnostic: it is the change between any earlier attained value and any later one. Use whichever consecutive pair you are reporting.

Formula, assumptions, and limits

Formula

The calculator returns the fractional year-on-year change in attained CII, expressed as a percent:

A negative $\Delta$ is a reduction in carbon intensity, the direction the regulation rewards. The trend flag mirrors the component logic exactly: improving when $\Delta < -1$, deteriorating when $\Delta > 1$, flat in the one-percent dead band around zero.

The rating, though, does not turn on $\Delta$ alone. The A to E grade is the ratio $r = \text{CII}_{attained} / \text{CII}_{required}$, and the required value moves each year:

So holding the rating steady means the attained figure must fall at least as fast as the required line. The condition to not lose ground from year $y$ to year $y+1$ is:

The right side is the required line’s own year-on-year factor. A ship whose attained falls slower than that ratio sees $r$ climb and the letter erode, even with $\Delta$ negative.

Derivation

The rating ratio in year $y$ is $r_y = \text{CII}_y / [\,a \cdot \text{Capacity}^{-c} \cdot (1 - Z_y)\,]$. The reference line $a \cdot \text{Capacity}^{-c}$ is fixed for a given ship, so dividing $r_{y+1}$ by $r_y$ cancels it and leaves the attained ratio divided by the required ratio. Setting $r_{y+1} \le r_y$ (no band loss) and rearranging gives the condition above. The whole question of whether intensity work “counts” reduces to one comparison: did attained fall by more than the line did.

Assumptions

• The two attained values use the same capacity basis and the same scope of voyages, so the change is real and not an artifact of a changed denominator.
• Both attained figures come from verified DCS data on the same fuel-to-CO2 conversion factors (the tank-to-wake Cf values in MEPC.364(79)).
• The trend flag’s one-percent dead band is a display convenience in the tool, not a regulatory threshold; the rating is decided by the ratio, not by this band.
• The required-line comparison below assumes the ship stays in the same ship-type category, so the reference coefficients $a$ and $c$ and the rating boundaries do not change between the two years.

Worked example

Take a bulk carrier whose required CII in 2025 is 5.551 gCO2/dwt-nm. Its attained CII that year is 5.8, so the ratio is $5.8 / 5.551 = 1.045$. The bulk-carrier C and D boundary (d3) is 1.06, so 1.045 is a C, with thin margin.

The operator changes nothing and reports 5.8 again in 2026. The year-on-year change this tool returns is zero: $\Delta = (5.8 - 5.8)/5.8 = 0.0$ percent, flagged flat. But the required line moved. The reduction factor went from $Z_{2025} = 0.09$ to $Z_{2026} = 0.11$, so the same ship’s required CII fell from 5.551 to $5.551 \times (0.89/0.91) = 5.429$ gCO2/dwt-nm, a 2.2 percent tightening. The 2026 ratio is now $5.8 / 5.429 = 1.068$. That sits above 1.06, so the rating is a D. The letter slipped from C to D on a flat attained CII.

To hold the C, the ship needed attained at or below $1.06 \times 5.429 = 5.755$ gCO2/dwt-nm, a cut of 0.045 from 5.8, which is 0.78 percent. So a year-on-year change of about minus 0.78 percent (a $\Delta$ the trend flag would still call flat, inside its one-percent band) was the floor just to hold position. Anything above that, including the zero this operator reported, costs a band. Reproduce by hand: 0.09 and 0.11 are the published Z values, 5.551 times 0.89 over 0.91 is 5.429, and 5.8 over 5.429 is 1.068, which is above the bulk-carrier d3 of 1.06.

Edge cases and limits

• If either attained value is zero or missing, the change is undefined and the calculator returns no result. A genuine zero attained CII does not occur for a trading ship.
• Near a band boundary the tool’s flat flag is misleading: a change inside the dead band can still cross a d-boundary, as the worked example shows, because the dead band is fixed at one percent while the required-line step is 2.2 percent in 2026 and 2.625 percentage points per year from 2027.
• A change in trade pattern that alters the capacity or the scope of reported voyages breaks the year-on-year comparison; the two figures are then not measuring the same thing.
• The tool measures attained drift only. For the full rating, ratio, and corrective-action call, use the CII rating calculator; to project a multi-year glide path against the line, use the CII corrective-trajectory calculator.

Regulatory basis

CII has been in force since 1 January 2023 under MARPOL Annex VI Reg.28, introduced by amendment MEPC.328(76). The reduction factors for 2023 to 2026 are MEPC.338(76); the factors for 2027 to 2030 are MEPC.400(83), adopted 11 April 2025 at MEPC 83. The rating boundaries are the current G4 d-vectors per MEPC.354(78), the 2022 amendment to the original MEPC.339(76).

Common errors

1. Reading a negative change as automatic improvement. A 1.5 percent reduction in 2026 still loses ground, because the required line tightened 2.2 percent that year. The rating fell relative to the line even as the raw number dropped.
2. Trusting the flat flag near a boundary. The one-percent dead band is a display threshold, not a rating rule. A ship near d3 can slip a band on a change the tool calls flat, as the worked example shows.
3. Comparing across different capacity bases. Mixing a deadweight-based attained value with a gross-tonnage-based one, or changing the scope of reported voyages, makes the year-on-year change meaningless.
4. Assuming the 2026 step size carries forward. The 2025-to-2026 tightening is 2.2 percent of the required value; from 2027 the annual step is 2.625 percentage points of the reference line, a larger and compounding cut.
5. Treating a single D as a trigger. The year-on-year slip to D is a warning, not yet an obligation; it is three consecutive D years, or one E, that triggers the SEEMP Part III corrective plan.

The required-line tightening by year

The required CII is the reference line reduced by the year’s factor Z. These are the published fractions; the third column is the year-on-year tightening, the amount attained must fall just to hold the same ratio.

The 2027-to-2030 phase is the part most operators misstate. It is not a flat two percent per year. Each step is 2.625 percentage points of the reference line, reaching 21.5 percent below the line by 2030. Expressed as a cut on the prior year’s required value, that is roughly 3 percent each year, and it compounds against an already-reduced base, so the operational reduction needed to hold a letter grows over the window.

When to use this tool versus the corrective-trajectory calculator

This calculator answers a narrow question: between two consecutive years, did attained CII rise or fall, and by how much. It is the quickest read on a single year’s operational result. It does not project, and it does not return the rating; it returns a percent and a trend flag.

The CII corrective-trajectory calculator answers the forward question: given today’s attained CII and the tightening factors out to 2030, what glide path of annual reductions keeps the ship at a target letter. Use this year-on-year tool to score the year that just closed; use the trajectory tool to plan the years ahead. For the standing-grade question, what letter the ship is right now and how much margin it holds, the CII rating calculator reads the ratio against the d-vector, and the attained-versus-required calculator shows the same gap as the line moves under a held-steady attained value.

Limitations

The tool does not model the cause of a year-on-year change. A negative $\Delta$ can come from genuine efficiency work (hull cleaning, slower steaming, weather routing) or from a softer market that put the ship on shorter, fuller laden legs; the number is the same and the cause is the operator’s to read. It does not weight the change against the required line on its own; that comparison is in the table above, run by hand against the year pair. It assumes both attained values are verified DCS figures on a consistent basis, and it does not check that assumption. It returns no rating and no corrective-action call. For the full grade and the SEEMP Part III trigger, the work belongs to the CII rating calculator and the CII corrective action plan framework; for the concept itself, see what is CII.

Frequently asked questions

Why does my CII rating get worse with no operational change?

The rating is the ratio of attained CII to required CII, and the required CII falls every year as the reduction factor Z rises. Hold attained flat and the ratio climbs toward worse bands because the denominator shrank. From 2025 to 2026 the factor moves from 9 percent to 11 percent below the reference line, a 2.2 percent cut in the required value, so a ship on the C and D boundary in 2025 can be a D in 2026 with identical fuel and distance. The bar rises annually by design under MARPOL Annex VI Reg.28.

How much must CII improve each year to hold a C?

Attained CII must fall at least as fast as the required line for that year pair, because the rating depends on the ratio of the two. From 2025 to 2026 the required line tightens 2.2 percent, so a ship sitting mid-band needs roughly that much reduction to hold its place; a ship near the C and D boundary needs only enough to keep the ratio at or below the d3 boundary, often less than one percent. From 2026 to 2027 the annual step jumps to 2.625 percentage points of the reference line under MEPC.400(83), so the required reduction grows.

What is a good year-on-year CII improvement?

A reduction that beats the required-line tightening for that year, so the ratio falls and the ship gains margin instead of losing it. For 2025 to 2026 that means an attained reduction larger than 2.2 percent; for the 2027 to 2030 window, larger than about 3 percent per year as the 2.625-point step compounds against an already-reduced base. Anything smaller than the line's movement is a relative slide even when the raw attained number went down.