For a ship that already knows its attained CII for the reporting year (from the CII Attained calculator, the EU MRV report, the IMO DCS submission, or noon-report aggregation), this calculator computes the Required CII for the chosen year and ship type, the Attained/Required ratio, the rating, and the headroom or deficit in absolute, percentage and fuel-tonnes-equivalent terms.
The calculator is the headline tool for charter party CII verification, in-year compliance tracking, and operational decision support: it answers “where do I stand vs the rating threshold?” without requiring re-entry of fuel data.
Ratio and rating
$$ r = \frac{\text{CII}\text{attained}}{\text{CII}\text{required}} $$
The rating is determined by comparing $r$ against the four boundaries $d_1 < d_2 < d_3 < d_4$ from MEPC.339(76):
| Range | Rating |
|---|---|
| $r \leq d_1$ | A (superior) |
| $d_1 < r \leq d_2$ | B (advanced) |
| $d_2 < r \leq d_3$ | C (compliant) |
| $d_3 < r \leq d_4$ | D (low minor) |
| $r > d_4$ | E (inferior) |
The d-vector values vary slightly by ship type. For bulk carriers the boundaries are typically $d_1 = 0.86, d_2 = 0.94, d_3 = 1.06, d_4 = 1.18$. For tankers $d_1 = 0.82, d_2 = 0.93, d_3 = 1.08, d_4 = 1.28$. The exact values per ship type are tabulated in MEPC.339(76).
Required CII
$$ \text{CII}_\text{required} = a \cdot \text{Capacity}^{-c} \cdot (1 - Z) $$
The reference line $a \cdot \text{Capacity}^{-c}$ is from MEPC.337(76) Table 1; the annual reduction factor $Z$ from MEPC.338(76) is 5% (2023), 7% (2024), 9% (2025), 11% (2026).
Headroom and deficit
The headroom or deficit is calculated relative to the C/D boundary at $\text{CII}_\text{required} \times d_3$:
- Compliant (A/B/C ratings): the headroom is the percentage by which the attained CII is below the C/D boundary. Higher headroom means more margin against unexpected operational variations or regulatory tightening.
- Non-compliant (D/E ratings): the deficit is the percentage by which the attained CII exceeds the C/D boundary. The deficit must be eliminated through corrective measures.
$$ \text{Headroom (%)} = \frac{\text{CII}\text{required} \cdot d_3 - \text{CII}\text{attained}}{\text{CII}_\text{required} \cdot d_3} \times 100 \quad \text{(if compliant)} $$
$$ \text{Deficit (%)} = \frac{\text{CII}\text{attained} - \text{CII}\text{required} \cdot d_3}{\text{CII}_\text{required} \cdot d_3} \times 100 \quad \text{(if non-compliant)} $$
Distance to next-better and next-worse band
The calculator also reports the absolute CII change required to enter the next-better band (e.g. C → B) or to drop to the next-worse band (e.g. C → D). This is operationally useful for in-year planning: if the vessel is well into its band, modest measures may move it to the next better band (capturing reputational benefit); if it is near the boundary, marginal additional emissions could drop it to a worse band.
Fuel-tonnes-equivalent
For owners with a known annual fuel burn $F_\text{annual}$, the headroom or deficit is converted to a fuel-tonnes-equivalent: the additional or saved fuel mass per year that corresponds to the gap from the C/D boundary, evaluated at the dominant fuel’s $C_f$ factor:
$$ F_\text{equivalent} = \frac{|\text{CII}\text{boundary} - \text{CII}\text{attained}|}{\text{CII}\text{attained}} \cdot F\text{annual} $$
This expresses the regulatory margin in operationally meaningful terms: tonnes of fuel that can be saved (if compliant) or that must be saved (if non-compliant).
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $\text{CII}_\text{attained}$ | Attained Carbon Intensity Indicator | g CO₂/(cap·nm) | input |
| $\text{CII}_\text{required}$ | Required Carbon Intensity Indicator | g CO₂/(cap·nm) | computed |
| $r$ | Attained / Required ratio | - | computed |
| $a$, $c$ | Reference-line coefficients | - | MEPC.337(76) Table 1 |
| $Z$ | Annual reduction factor | fraction | MEPC.338(76) |
| $Capacity$ | DWT (cargo) or GT (ro-pax/cruise) | t or - | MEPC.337(76) |
| $d_1, d_2, d_3, d_4$ | Rating boundary multipliers | - | MEPC.339(76) Table |
| $F_\text{annual}$ | Total annual fuel burn | t | input |
| $F_\text{equivalent}$ | Fuel mass equivalent of the headroom / deficit | t | computed |
Worked example
A 82,000 DWT Kamsarmax bulk carrier in 2025 with attained CII of 5.20 g CO₂/(dwt·nm) and annual fuel burn of 8,500 t VLSFO.
Required CII calculation:
- Reference line for bulk carriers (MEPC.337(76)): $a = 4.745, c = 0.622$.
- $\text{CII}_\text{ref} = 4.745 \times 82000^{-0.622} = 4.81$ g CO₂/(dwt·nm).
- $\text{CII}_\text{required}(2025) = 4.81 \times (1 - 0.09) = 4.38$ g CO₂/(dwt·nm).
Ratio and rating:
- $r = 5.20 / 4.38 = 1.187$.
- Bulk carrier d-vector: $d_1 = 0.86, d_2 = 0.94, d_3 = 1.06, d_4 = 1.18$.
- $r$ is between $d_3$ and $d_4$, so rating is D (low minor).
Deficit:
- C/D boundary = $4.38 \times 1.06 = 4.64$ g CO₂/(dwt·nm).
- Deficit = $(5.20 - 4.64) / 4.64 = 12.0%$.
Fuel-tonnes-equivalent (at VLSFO Cf = 3.151):
- $F_\text{equivalent} = (5.20 - 4.64) / 5.20 \times 8500 = 915$ t/yr that must be saved.
- This corresponds to approximately 11% reduction in annual fuel consumption to return to the C rating threshold.
The owner can use this to plan corrective measures: combinations of slow steaming, hull cleaning, weather routing, and possibly biofuel blending to achieve the required 11% fuel reduction.
Practical use cases
- Charter party CII verification: a charterer can quickly confirm a vessel’s CII status and the headroom against rating thresholds.
- In-year compliance tracking: an owner can monitor the cumulative attained CII against the year-end target as the year progresses.
- Operational decision support: deciding whether to take a longer voyage at higher speed or accept a delay to maintain CII compliance.
- Corrective action planning: quantifying the magnitude of the corrective measures required to return to compliance.
- Resale market valuation: vessels with high headroom command premium charter rates and resale prices.
Sources
- IMO Resolution MEPC.336(76) - 2021 Guidelines on operational CII.
- IMO Resolution MEPC.337(76) - Reference lines.
- IMO Resolution MEPC.338(76) - Reduction factors.
- IMO Resolution MEPC.339(76) - Rating boundaries.