Estimate a vessel's attained annual operational Carbon Intensity Indicator (CII) from fuel consumption, vessel capacity and distance travelled. This tool converts each fuel quantity into CO₂ emissions, compares the resulting intensity with the applicable required CII, and assigns an indicative A–E rating using IMO ship-type reference lines and rating boundaries. Choose Annual CII for a complete calendar year, or Monthly / YTD Tracking for operational monitoring from cumulative monthly records. Review the calculation details to see the factors, capacity rules and arithmetic used, and explore how unchanged performance would compare with required CII levels through 2030. Results are unadjusted estimates, not verified regulatory ratings.
This calculator is an independent engineering tool developed by NauticalSolver using publicly available IMO CII calculation guidelines and regulatory information. It is not an official IMO calculator, classification society tool, Flag Administration tool, verifier, or compliance certificate.
Results are provided for educational, engineering estimation and operational monitoring purposes only. Official CII calculations, ratings, reporting and compliance assessments must be verified using the applicable MARPOL Annex VI requirements, current IMO resolutions, Flag Administration requirements and authorized verification procedures.
NauticalSolver is not affiliated with or endorsed by the International Maritime Organization (IMO).
The IMO CII framework generally applies to applicable ships of 5,000 GT and above, subject to MARPOL Annex VI. Entering a ship type and capacity does not establish a vessel's legal applicability. DWT is not GT.
This is a mathematical comparison at unchanged capacity and distance, not a prescribed IMO fuel- or emissions-reduction measure. Required CII and the C/D boundary are distinct: a vessel above Required CII can still receive rating C.
Standard, unadjusted CII; G5 corrections and voyage adjustments are not applied. Comparison with required CII is not an official compliance determination. For consistency with the three-decimal presentation specified in MEPC.348(78), this calculator performs rating comparisons using the displayed three-decimal CII values. Where a value is exactly equal to a rating boundary, the better adjacent rating is assigned in accordance with the verification guidance. Meeting required CII and the C/D rating boundary are different comparisons.
| Month | Distance (nm) | Fuel (t) | CO₂ (t) | Monthly CII Estimate | YTD CII |
|---|
Future ratings assume the current attained annual or YTD carbon intensity and vessel particulars remain unchanged. They are projections, not verified future ratings.
| Year | Reduction Factor | Required CII | Rating if Current Attained CII Remained Unchanged |
|---|
| Period | Fuel | Consumption (t) | CF (tCO₂/t fuel) | CO₂ (t) |
|---|
Regulatory tables checked 28 September 2026; verification, fuel-factor and biofuel guidance checked 29 September 2026. This tool uses the coefficients as printed in G2, including the distinct LNG coefficients below and above 65,000 DWT. For consistency with the three-decimal presentation specified in MEPC.348(78), this calculator performs rating comparisons using the displayed three-decimal CII values. Where a value is exactly equal to a rating boundary, the better adjacent rating is assigned in accordance with the verification guidance.
The Carbon Intensity Indicator (CII) is an IMO operational efficiency measure used to assess how much carbon dioxide a ship emits in relation to its transport capacity and distance travelled. Under MARPOL Annex VI, applicable ships calculate an attained annual operational CII and compare it with a required CII for the relevant year.
CII is based on actual ship operation. Fuel consumption, fuel type and distance travelled during the assessment period directly affect the attained value. The ship's type and capacity determine the reference line used to calculate the required CII.
The NauticalSolver CII Calculator calculates an estimated attained CII, required CII and A–E rating from vessel particulars, distance travelled and fuel consumption. It can be used for annual calculations or for monthly and year-to-date operational monitoring.
The calculator follows the standard IMO CII calculation structure and does not apply the optional voyage adjustments and correction factors contained in the CII Guidelines G5.
CII stands for Carbon Intensity Indicator. It measures the operational carbon intensity of a ship over a defined period.
In its standard form, the attained CII is calculated from:
Attained CII = CO₂ Emissions / Transport Work
For the supply-based CII calculation used for individual ships, transport work is represented by the ship's capacity multiplied by the distance travelled:
Transport Work = Capacity × Distance Travelled
This gives:
Attained CII = CO₂ Emissions / (Capacity × Distance Travelled)
The final result is normally expressed as:
Deadweight tonnage is used for ship types including tankers, bulk carriers, container ships, gas carriers, LNG carriers, general cargo ships, refrigerated cargo carriers and combination carriers.
Gross tonnage is used for the applicable ro-ro and passenger ship categories defined in the IMO CII guidelines.
CII is based on CO₂ emissions rather than fuel mass alone.
For each fuel type, the amount consumed is multiplied by its carbon conversion factor:
CO₂ = Fuel Consumption × CF
where CF is the mass conversion factor between the fuel consumed and the resulting mass of CO₂.
Different fuels therefore produce different calculated CO₂ emissions for the same mass of fuel.
For example, commonly used IMO conversion factors include:
| Fuel Type | CF, tCO₂/t fuel |
|---|---|
| Diesel / Gas Oil | 3.206 |
| Light Fuel Oil | 3.151 |
| Heavy Fuel Oil | 3.114 |
| LPG – Propane | 3.000 |
| LPG – Butane | 3.030 |
| Ethane | 2.927 |
| LNG | 2.750 |
| Methanol | 1.375 |
| Ethanol | 1.913 |
When more than one fuel is used, CO₂ emissions are calculated separately for each fuel and then added together.
The calculator therefore allows multiple fuel entries rather than assuming that the vessel operated on a single fuel throughout the assessment period.
The attained CII represents the calculated operational carbon intensity of the vessel from the entered fuel consumption, capacity and distance data.
The required CII is the target value applicable to the vessel for a particular year.
IMO first defines a ship-type-specific 2019 reference line:
CIIref = a × Capacity⁻ᶜ
The coefficients a and c depend on ship type and, for some ship types, vessel size.
The required CII for a particular year is then calculated by applying the applicable reduction factor:
Required CII = CIIref × (1 − Z / 100)
where Z is the reduction factor relative to the 2019 reference line.
The currently adopted reduction factors through 2030 are:
| Year | Reduction from 2019 Reference |
|---|---|
| 2023 | 5.000% |
| 2024 | 7.000% |
| 2025 | 9.000% |
| 2026 | 11.000% |
| 2027 | 13.625% |
| 2028 | 16.250% |
| 2029 | 18.875% |
| 2030 | 21.500% |
This means that the required CII becomes progressively lower over time.
A vessel maintaining exactly the same operational carbon intensity may therefore receive a different rating in a later year because the required CII and the corresponding rating boundaries have changed.
The attained CII is compared with four rating boundaries calculated from the required CII.
The resulting operational carbon intensity rating is:
A, B, C, D or E
where A represents the lowest carbon intensity band and E the highest.
The rating boundaries are not identical for every type of ship. IMO publishes different boundary vectors for bulk carriers, tankers, container ships, gas carriers, LNG carriers, general cargo ships, ro-ro ships and the other covered categories.
For this reason, a CII rating cannot be determined correctly using one universal percentage range for every vessel.
The calculator first determines the required CII for the selected ship type, capacity and assessment year. It then applies the applicable ship-type rating vectors and compares the attained CII with the resulting A/B, B/C, C/D and D/E boundaries.
A vessel does not have to achieve exactly the required CII to receive a C rating. The required CII lies within the C rating band, while the actual limits of that band are determined by the applicable IMO rating vectors.
Consider a tanker with the following annual data:
| Ship type: | Tanker |
|---|---|
| Capacity: | 50,000 DWT |
| Assessment year: | 2026 |
| Distance travelled: | 50,000 nm |
| Heavy Fuel Oil consumption: | 4,600 t |
| Diesel / Gas Oil consumption: | 200 t |
First, the CO₂ emissions from each fuel are calculated.
Heavy Fuel Oil:
4,600 × 3.114 = 14,324.4 tCO₂
Diesel / Gas Oil:
200 × 3.206 = 641.2 tCO₂
Total annual CO₂ emissions are therefore:
14,324.4 + 641.2 = 14,965.6 tCO₂
The attained CII is:
14,965.6 × 1,000,000 / (50,000 × 50,000)
which gives approximately:
5.986 gCO₂/dwt·nm
For a tanker, the IMO 2019 reference-line parameters are:
a = 5247
c = 0.610
The 2019 reference CII for a 50,000 DWT tanker is therefore approximately:
7.137 gCO₂/dwt·nm
For 2026, the reduction factor is 11%.
The required CII becomes:
7.137 × 0.89 = 6.352 gCO₂/dwt·nm
For tankers, the IMO rating-boundary multipliers are:
0.82 / 0.93 / 1.08 / 1.28
This produces approximate boundaries of:
With an attained CII of 5.986 gCO₂/dwt·nm, the tanker falls within the C rating band.
This example also shows why comparing the attained CII only with the required CII is not enough to determine the A–E rating. The ship-type-specific rating boundaries must also be applied.
Annual mode is intended for a complete calendar-year calculation.
The user enters the vessel type and capacity together with the total distance travelled and fuel consumed during the year.
The calculator then determines:
Where the attained CII is above the required value, the calculator can also estimate the reduction in CO₂ emissions that would be required to reach the required CII while keeping the entered capacity and distance unchanged.
This is a mathematical comparison and should not be interpreted as a prescribed operational measure. A vessel may change its carbon intensity through changes in fuel consumption, fuel type, operating profile, technical efficiency or other measures.
The formal IMO CII is an annual operational indicator based on calendar-year data.
Monthly calculations are therefore provided by NauticalSolver as an operational monitoring tool rather than as an official monthly IMO rating.
Monthly mode calculates an estimated CII from the fuel consumption and distance recorded during each month.
The Year-to-Date value is calculated from the cumulative totals:
YTD CII = Total YTD CO₂ / (Capacity × Total YTD Distance)
The calculator does not calculate YTD CII by taking the arithmetic average of individual monthly CII values.
This distinction matters because monthly sailing distance and fuel consumption can vary considerably. A month in which a vessel travels 1,000 nm should not have the same weighting in the annual result as a month in which it travels 6,000 nm.
The resulting YTD rating is therefore shown as an indicative operational rating against the selected year's CII boundaries.
The official annual result remains dependent on the complete applicable reporting period and the required verification process.
CII depends mainly on the relationship between CO₂ emissions and the ship's capacity-distance denominator.
Operational changes that affect fuel consumption can therefore affect the attained CII. These may include vessel speed, weather, hull and propeller condition, engine performance, waiting time, routing, loading pattern and auxiliary energy demand.
Fuel type also affects the calculated CO₂ mass because different fuels use different carbon conversion factors.
Distance travelled forms part of the CII denominator. This means two periods with similar fuel consumption but significantly different sailing distances may produce different carbon-intensity values.
CII should therefore be interpreted together with the ship's actual operating profile rather than as a fuel-consumption figure alone.
CII and EEXI address different aspects of ship efficiency.
CII is an operational measure. It changes according to the vessel's actual fuel consumption and distance travelled during the assessment period.
EEXI is a technical energy-efficiency measure based on the ship's design and technical characteristics.
A ship can therefore meet its applicable EEXI requirement while still receiving a poor operational CII rating, or vice versa.
The two calculations should not be substituted for one another.
Under the IMO CII framework, a ship rated D for three consecutive years or E for one year is required to develop a corrective action plan. The plan should achieve at least a C rating in the calendar year following its adoption and ultimately achieve the required annual operational CII.
The corrective action is addressed through the Ship Energy Efficiency Management Plan (SEEMP).
For this reason, monitoring the CII during the year can be useful even though the statutory CII calculation itself is annual. A YTD calculation can indicate whether the vessel is moving towards a different rating band before the reporting year has ended.
IMO Resolution MEPC.355(78), commonly referred to as the CII Guidelines G5, contains correction factors and voyage adjustments that may apply to specific operations and vessel types.
These include provisions relating to certain voyage exclusions, ice conditions, cargo-related energy consumption and other operational factors.
The current NauticalSolver CII Calculator does not apply G5 correction factors or voyage adjustments.
The result should therefore be treated as a standard, unadjusted CII calculation based on the entered fuel consumption, distance, capacity and IMO reference data.
Where G5 adjustments are applicable to a vessel, the official attained CII may differ from the unadjusted result shown by this calculator.
The calculator can be used for preliminary calculations and operational monitoring by ship operators, technical departments, marine engineers, naval architects, energy-efficiency teams and students working with IMO carbon-intensity calculations.
Typical uses include:
It is not a replacement for Flag Administration, Recognized Organization, classification society or authorized verifier calculations.
Related NauticalSolver resources: EEOI Calculator, MRV CO₂ Recorder and EEOI and fuel consumption.
The calculation methodology and regulatory values used by this calculator are based on publicly available IMO material, including:
MARPOL Annex VI, Regulation 28
Operational carbon intensity requirements.
Resolution MEPC.352(78) — CII Guidelines, G1
2022 Guidelines on operational carbon intensity indicators and the calculation methods.
Resolution MEPC.412(84)
Amendments to the 2022 Guidelines on operational carbon intensity indicators and the calculation methods.
Resolution MEPC.353(78) — CII Reference Lines Guidelines, G2
Defines the ship-type-specific 2019 reference lines and the coefficients used to calculate reference CII.
Resolution MEPC.338(76), as amended by MEPC.400(83) — CII Reduction Factors Guidelines, G3
Defines the annual reduction factors applied to the 2019 reference lines, including the adopted factors through 2030.
Resolution MEPC.354(78) — CII Rating Guidelines, G4
Defines the A–E operational carbon-intensity rating method and ship-type-specific rating boundaries.
Resolution MEPC.348(78), as amended by MEPC.389(81) — Verification Guidelines
Section 5.7 of MEPC.348(78) addresses three-decimal presentation of attained CII, required CII and rating boundaries. Equality with a rating boundary receives the better adjacent rating.
Resolution MEPC.364(79), as amended by MEPC.410(84)
Section 2.2.1 provides the standard fuel carbon conversion factors referenced by G1, including Ethane at 2.927 tCO₂/t fuel.
MEPC.1/Circ.905/Rev.1 — Revised Interim Biofuel Guidance
Applicable from 1 January 2027 to biofuels and biofuel blends under MARPOL Annex VI regulations 26, 27 and 28. Use the applicable certification and supporting documentation to determine the custom CF; this calculator does not assess certification or calculate lifecycle emissions.
Resolution MEPC.355(78) — CII Guidelines, G5
Contains correction factors and voyage adjustments for CII calculations. These adjustments are not included in the current version of this calculator.
Users carrying out statutory, contractual or verified calculations should refer to the latest applicable MARPOL Annex VI requirements, IMO resolutions, Flag Administration instructions and verification requirements.
The NauticalSolver CII Calculator is an independent calculation tool developed from publicly available IMO regulations, resolutions and calculation guidance.
It is not an official IMO calculator and NauticalSolver is not affiliated with or endorsed by the International Maritime Organization.
Results are intended for engineering calculation, study and operational monitoring. They should not be used as a substitute for an official verified CII assessment.