The CII rating system is a mandatory carbon intensity measurement framework introduced by the International Maritime Organization (IMO) that assigns vessels an annual performance grade from A to E based on how efficiently they use fuel relative to the cargo they carry and the distance they travel. It applies to most ocean-going vessels of 5,000 gross tonnage and above, and it directly determines whether a vessel can continue operating without regulatory intervention. The sections below break down how the score is calculated, what each rating means, and what operators can do to stay on the right side of the scale.
How is a vessel’s CII score actually calculated?
A vessel’s CII score is calculated by dividing its total annual CO₂ emissions by a capacity and distance measure, producing a carbon intensity ratio expressed in grams of CO₂ per cargo-carrying capacity per nautical mile. That raw figure is then compared against a reference line set by the IMO for that vessel type and size, which determines where the vessel falls on the A-to-E rating scale.
In practical terms, CO₂ emissions are derived from fuel consumption across all fuel types used during the year, converted using standard emission factors. The denominator combines the vessel’s deadweight tonnage (or gross tonnage for certain ship types) with the total nautical miles sailed. The result is the attained CII value.
Crucially, the reference lines tighten every year. The IMO has set annual reduction factors that make the required CII value progressively stricter through 2030 and beyond. A vessel that achieved a C rating in 2024 may find itself rated D or E in 2026 using the exact same operational profile, simply because the benchmark has moved. This trajectory means operators cannot treat a passing grade as a fixed achievement.
What do the CII rating categories A to E mean in practice?
The five CII rating categories indicate how a vessel’s carbon intensity compares to the IMO reference value for its type. An A rating means the vessel significantly outperforms the required standard, while a B rating indicates moderate outperformance. A C rating represents compliance within an acceptable band around the reference line. A D rating signals minor underperformance, and an E rating indicates significant underperformance requiring mandatory corrective action.
From a commercial standpoint, the distinction between ratings carries real weight. Charterers, cargo owners, and financiers increasingly scrutinize CII grades when making contracting decisions. A vessel with a sustained A or B rating signals operational efficiency and lower fuel costs, which can translate into preferential charter rates or access to environmentally conscious cargo owners. Conversely, a C rating, while technically compliant, may still disadvantage a vessel in a competitive market where counterparties prefer higher performers.
The D and E categories trigger formal obligations under IMO regulations, which are covered in more detail below. It is worth noting that the rating is recalculated annually, so a poor year does not permanently define a vessel, but a pattern of low ratings does accumulate regulatory and reputational pressure.
Which vessels and ship types does the CII rating apply to?
The CII rating system applies to ocean-going vessels of 5,000 gross tonnage and above that fall under MARPOL Annex VI. This includes bulk carriers, tankers, container ships, general cargo ships, gas carriers, cruise ships, ro-ro vessels, and refrigerated cargo carriers, among others. The specific reference lines and capacity metrics differ by ship type to reflect the nature of each sector’s operations.
Inland waterway vessels, small coastal craft, and vessels below the 5,000 GT threshold are currently outside the IMO’s CII framework. However, operators in inland shipping should be aware that regional regulatory frameworks, particularly within the EU, are developing their own carbon intensity and reporting requirements that may affect smaller vessel categories in the coming years. The distinction matters for tanker operators who move between ocean-going and inland routes, since compliance obligations may differ depending on the segment of the journey and the vessel’s registration.
Vessels that spend significant time at anchor, in port, or waiting for cargo are still subject to CII measurement during those periods, which has direct implications for how port time and operational delays affect the annual score.
What happens if your vessel gets a D or E rating?
If a vessel receives a D rating for three consecutive years, or an E rating in any single year, the ship operator is required to submit a corrective action plan to the flag state. This plan must set out concrete steps to achieve a C rating or better within a defined timeframe. Failure to produce an acceptable plan can result in the vessel being detained, denied port access, or refused certification renewal.
Beyond the regulatory requirement, a D or E rating carries commercial consequences. Port state control authorities in major jurisdictions use CII ratings as a risk indicator when selecting vessels for inspection. Insurers and lenders are increasingly factoring environmental performance into risk assessments and financing conditions. Some major charterers have already introduced contractual clauses that allow them to terminate or renegotiate contracts if a vessel’s CII rating falls below a specified threshold.
The corrective action plan itself is not a light administrative exercise. It requires documented evidence of operational changes, technical upgrades, or route optimizations that will credibly move the vessel’s carbon intensity into compliance. Operators who find themselves in D or E territory benefit from acting early, before the formal trigger is reached, because the options available are broader and less costly when addressed proactively.
How do degassing and idle port time affect your CII score?
Degassing operations and idle port time directly affect a vessel’s CII score because fuel burned during these periods contributes to the annual CO₂ total without generating cargo-carrying distance. The CII formula rewards efficient cargo movement, so any time a vessel consumes fuel while stationary or conducting non-transport operations increases the carbon intensity figure without improving the denominator.
For tanker operators, this is a particularly relevant issue. Degassing a tank prior to accepting a new cargo type is a necessary operational step, but how and where it is carried out can have measurable consequences for annual carbon intensity. Open-air degassing, beyond its regulatory and environmental concerns, often involves extended waiting periods at anchor or in port, during which auxiliary engines continue to run. This idle fuel consumption accumulates in the annual CII calculation.
Using shore-based or mobile degassing solutions that process emissions efficiently and reduce the time a vessel spends waiting in port can help limit the fuel burn attributed to non-productive operational periods. Solutions such as mobile containerized degassing units, which can be deployed directly at the berth, reduce the time between cargo discharge and the next loading, keeping port time and associated fuel consumption to a minimum. This operational efficiency has a secondary benefit: it supports a better CII outcome by reducing the fuel consumed outside of productive cargo voyages.
Operators planning their annual voyages should account for port call frequency, expected degassing requirements, and anchorage time when projecting their end-of-year CII position. These factors are often underestimated in early-stage planning but can shift a vessel from a C to a D rating when accumulated across a full operating year.
What can vessel operators do to improve their CII rating?
Vessel operators can improve their CII rating through a combination of speed optimization, fuel efficiency measures, route planning, technical upgrades, and reducing non-productive fuel consumption in port. Because the CII score is a ratio of emissions to cargo-distance, improvements can be achieved either by reducing the numerator (less fuel burned) or by increasing the denominator (more cargo moved per nautical mile).
Operational measures that reduce fuel consumption
Slow steaming remains one of the most effective short-term levers available to operators. Fuel consumption increases disproportionately with speed, so even modest reductions in average voyage speed can produce meaningful carbon intensity improvements. Voyage optimization software that accounts for weather routing, current patterns, and port scheduling can further reduce fuel burn without compromising delivery timelines.
Reducing idle time in port and at anchor is equally important. Every hour a vessel burns fuel without generating cargo-carrying nautical miles worsens the CII ratio. Coordinating arrival times with terminal availability, minimizing anchorage waiting periods, and accelerating turnaround during cargo operations all contribute to a cleaner annual figure.
Technical and commercial strategies for sustained improvement
On the technical side, hull cleaning and propeller polishing can recover efficiency losses caused by biofouling, which accumulates continuously and can significantly increase fuel consumption if left unaddressed. Engine tuning and waste heat recovery systems offer additional efficiency gains for operators with the capital to invest in upgrades.
Commercially, operators can improve their CII position by prioritizing longer voyages with full cargo loads over short, partially loaded trips, since the distance and capacity components of the denominator benefit from sustained, high-utilization voyages. Charter agreements that allow flexibility in speed and routing give operators more room to manage their carbon intensity in real time rather than discovering a problem at year-end.
Monitoring CII performance on a rolling basis throughout the year, rather than waiting for the annual report, gives operators the ability to course-correct before a D or E outcome becomes unavoidable. Several fleet management platforms now offer real-time CII tracking, making it practical to flag underperformance early and respond with targeted operational adjustments.
How SuperFlox supports your CII rating performance
For tanker operators looking to protect their CII rating, degassing is one of the most controllable variables in the annual score — and it is where SuperFlox delivers a direct, measurable impact. SuperFlox provides mobile, containerized degassing units that can be deployed at the berth, eliminating the extended anchorage waiting periods that silently drain a vessel’s carbon intensity budget. Here is what that means in practice:
- Faster turnaround at berth: SuperFlox units process tank vapors efficiently on-site, reducing the time between cargo discharge and the next loading and cutting the idle fuel burn that accumulates in your CII calculation.
- Reduced auxiliary engine runtime: By shortening port stays, SuperFlox limits the hours your auxiliary engines run without generating any cargo-carrying distance — one of the most common hidden drags on annual carbon intensity.
- Regulatory and environmental compliance: SuperFlox degassing solutions capture and treat vapor emissions rather than venting them, supporting compliance with both IMO regulations and increasingly strict port authority requirements.
- Flexible deployment: Units can be mobilized to a wide range of terminals and berths, meaning operators are not dependent on fixed shore-based infrastructure that may not be available at every port of call.
- Proactive CII planning support: Because degassing requirements are predictable in advance, integrating SuperFlox into voyage planning allows operators to account for port time accurately and avoid end-of-year surprises that push a vessel from a C into a D rating.
If your fleet includes tankers that undergo regular degassing operations, the CII impact of how those operations are managed is too significant to leave unaddressed. Contact SuperFlox to discuss how mobile degassing solutions can be integrated into your operational planning and help keep your vessels on the right side of the rating scale.