A practical guide to connecting fuel use, CO2 emissions, cargo carried, and voyage distance.
Fuel consumption is one of the most important operating figures in shipping. It affects voyage cost, bunker planning, emissions reporting, charter performance, engine operation and the long-term efficiency profile of a vessel. However, fuel consumption by itself does not always explain whether a ship is operating efficiently. A large vessel may burn more fuel than a smaller vessel, but it may also carry much more cargo over the same distance. This is where the Energy Efficiency Operational Indicator, commonly known as EEOI, becomes useful.
EEOI connects fuel consumption with transport work. Instead of only asking how much fuel a ship burned, EEOI asks a better question: how much CO2 was emitted for each tonne of cargo transported over one nautical mile? This makes EEOI a practical operational indicator for comparing voyages, monitoring performance trends and understanding the relationship between fuel consumption and carbon efficiency.
For marine engineers, naval architects, vessel operators and technical departments, EEOI is useful because it is based on real operational data. It reflects how the vessel actually performed in service, not only how it was designed to perform on paper. A ship with a good design index can still show poor operational efficiency if it sails at inefficient speeds, carries low cargo, suffers from hull fouling, operates with excessive auxiliary load or spends too much time in poor operating conditions.
This article explains EEOI from a practical marine engineering point of view. It covers the EEOI formula, the role of fuel consumption, carbon conversion factors, transport work, ballast voyages, slow steaming, auxiliary load, MRV data, CII and common calculation mistakes.
EEOI stands for Energy Efficiency Operational Indicator. It is an operational efficiency indicator used to estimate the amount of CO2 emitted by a ship in relation to the transport work performed. In simple terms, EEOI measures carbon emissions per cargo transport output.
The basic idea is straightforward. A ship consumes fuel. That fuel produces CO2. The ship also transports cargo over a distance. EEOI compares the CO2 emitted with the cargo-distance work completed by the vessel.
The general EEOI formula is:
EEOI = Total CO2 emissions / Transport workFor cargo ships, transport work is usually calculated as:
Transport work = Cargo carried × Distance sailedTherefore, the practical form becomes:
EEOI = CO2 emissions / (Cargo × Distance)EEOI is commonly expressed in grams of CO2 per tonne-nautical mile:
gCO2 / tonne-nmA lower EEOI normally indicates better operational carbon efficiency. It means the vessel emitted less CO2 for each tonne of cargo transported over each nautical mile.
Fuel consumption is essential, but it does not tell the whole story. If one ship burns 20 tonnes per day and another burns 35 tonnes per day, it may seem that the first ship is more efficient. However, this conclusion may be wrong if the second vessel is much larger and carries significantly more cargo.
Consider two vessels sailing the same distance:
Vessel B burns more fuel in absolute terms, but it may have a much lower emission per tonne of cargo transported. From a cargo efficiency point of view, Vessel B may be the better performer.
This is why fuel consumption should be interpreted together with cargo carried and distance travelled. EEOI provides this connection. It turns raw fuel consumption into a transport efficiency indicator.
The detailed EEOI formula can be written as:
EEOI = Σ(Fuel_i × CF_i) / (Cargo × Distance)Where:
If more than one fuel type is used during the voyage, each fuel type should be calculated separately and then added together. For example, if a vessel consumes both HFO and MGO during a voyage, the CO2 from HFO and the CO2 from MGO should be calculated separately using the correct conversion factor for each fuel.
Total CO2 = (HFO consumed × HFO conversion factor) + (MGO consumed × MGO conversion factor)The resulting total CO2 is then divided by the transport work.
EEOI starts with fuel consumption because ship CO2 emissions are directly related to fuel burned. When marine fuel is combusted, the carbon in the fuel combines with oxygen and forms carbon dioxide. This is why one tonne of fuel does not produce only one tonne of CO2. The final CO2 mass is higher because oxygen from the air becomes part of the CO2 molecule.
For example, if a ship burns 100 tonnes of fuel and the fuel has a CO2 conversion factor of 3.114, the CO2 emissions are:
CO2 = 100 × 3.114
CO2 = 311.4 tonnesThis means that accurate fuel measurement is the foundation of accurate EEOI calculation. If the fuel consumption data is wrong, the EEOI result will also be wrong.
Different fuels have different carbon contents, so they use different CO2 conversion factors. A fuel with a higher carbon content will generally produce more CO2 per tonne of fuel burned.
Typical examples used in shipping calculations include:
The exact factor used should match the applicable reporting method, company procedure, regulation or approved calculation guideline. For formal reporting, the conversion factors should not be guessed or casually copied from an unrelated source.
The denominator of EEOI is transport work. For cargo vessels, this is normally calculated by multiplying the cargo mass by the distance sailed.
Transport work = Cargo mass × DistanceIf a ship carries 50,000 tonnes of cargo over 4,000 nautical miles, the transport work is:
Transport work = 50,000 × 4,000
Transport work = 200,000,000 tonne-nmThis number represents the amount of cargo transport completed by the vessel. The EEOI then shows how much CO2 was emitted to perform that work.
Assume a bulk carrier completes a loaded voyage with the following data:
First, calculate CO2 emissions:
CO2 = 1,200 × 3.114
CO2 = 3,736.8 tonnesConvert CO2 tonnes to kilograms or grams if needed. Since EEOI is often expressed in grams of CO2 per tonne-nautical mile:
3,736.8 tonnes CO2 = 3,736,800 kg CO2
3,736,800 kg CO2 = 3,736,800,000 g CO2Next, calculate transport work:
Transport work = 60,000 × 5,000
Transport work = 300,000,000 tonne-nmNow calculate EEOI:
EEOI = 3,736,800,000 / 300,000,000
EEOI = 12.456 gCO2/tonne-nmThe result means the vessel emitted about 12.46 grams of CO2 for each tonne of cargo transported over one nautical mile.
If cargo and distance remain constant, EEOI changes directly with fuel consumption. A 10 percent reduction in fuel consumption normally results in approximately a 10 percent reduction in CO2 emissions and therefore a 10 percent improvement in EEOI.
Using the previous example, if fuel consumption is reduced from 1,200 tonnes to 1,080 tonnes:
CO2 = 1,080 × 3.114
CO2 = 3,363.12 tonnesTransport work remains:
300,000,000 tonne-nmNew EEOI:
EEOI = 3,363,120,000 / 300,000,000
EEOI = 11.2104 gCO2/tonne-nmThe EEOI improves from 12.46 to 11.21 gCO2/tonne-nm. This is why fuel reduction measures are also carbon efficiency measures.
Speed is one of the strongest drivers of ship fuel consumption. For many displacement vessels, required power increases roughly with the cube of speed. This relationship is often simplified as:
P = Pref × (V / Vref)^nWhere:
If speed increases from 12 knots to 13 knots and the exponent is 3:
Power ratio = (13 / 12)^3
Power ratio ≈ 1.27This means the vessel may need about 27 percent more power for only one knot of speed increase. Since fuel consumption is closely related to engine power and operating time, speed decisions can strongly affect EEOI.
Higher speed reduces voyage time, but it also increases hourly fuel consumption. In many cases, the increase in power is large enough that total fuel consumption still rises. This leads to higher CO2 emissions and worse EEOI, unless the operational benefit of the higher speed is necessary for schedule, safety or commercial reasons.
Slow steaming can improve EEOI by reducing fuel consumption and CO2 emissions. When a vessel reduces speed, required propulsive power usually drops significantly. If the vessel carries the same cargo over the same distance, lower fuel consumption leads to lower EEOI.
However, slow steaming should not be treated as a perfect solution in every case. Very low engine loads may reduce engine efficiency, increase maintenance concerns, affect auxiliary operation or create scheduling problems. A vessel may also miss a berth window and spend additional time waiting, which can reduce or even cancel the expected benefit.
The practical question is not simply “what is the slowest speed?” The better question is “what is the most efficient operating profile for the voyage requirement?” This includes distance, cargo, weather, port schedule, engine condition, charter requirements and emissions performance.
EEOI should consider the fuel consumed by the vessel during the relevant voyage or reporting period. This means auxiliary engines and boilers can matter, especially on vessels with high hotel load, cargo heating, reefer containers, pumps, cranes or special cargo systems.
If only main engine fuel is considered, the result may understate actual emissions. For example, a ship may have moderate propulsion fuel consumption but high auxiliary demand during cargo operations or port stay. Depending on the calculation boundary, that fuel may need to be included.
Auxiliary load reduction can therefore improve fuel consumption and EEOI. Practical measures include efficient lighting, optimized HVAC use, proper generator load sharing, avoiding unnecessary pumps and fans, maintaining steam systems and reducing avoidable electrical demand.
Fuel consumption estimates often include a sea margin. Sea margin accounts for real-world effects such as wind, waves, current, hull roughness, propeller condition and operational uncertainty. Calm-water power from a speed-power curve may be too optimistic for an actual voyage.
If a vessel requires 5,000 kW in calm water but operates with a 15 percent sea margin, the adjusted propulsion power becomes:
Adjusted power = 5,000 × 1.15
Adjusted power = 5,750 kWThis higher power increases fuel consumption and therefore CO2 emissions. If cargo and distance remain unchanged, EEOI becomes worse.
Hull fouling and propeller roughness can create the same effect. The vessel may need more power to maintain the same speed. More power means more fuel, more CO2 and higher EEOI. This is why hull cleaning, propeller polishing and performance monitoring are practical EEOI improvement measures.
Ballast voyages are one of the most common sources of confusion in EEOI discussions. A ballast voyage still consumes fuel and produces CO2, but the vessel is not carrying cargo. If cargo is zero, the transport work for that voyage becomes zero.
Transport work = Cargo × Distance
Transport work = 0 × Distance
Transport work = 0This means a pure ballast voyage does not produce a normal cargo-based voyage EEOI value. Mathematically, dividing emissions by zero transport work is not useful.
However, ballast fuel consumption is not irrelevant. Ballast voyages still affect the overall operational efficiency of the ship or trade pattern when assessed over a longer period. A vessel that frequently sails ballast will consume fuel and emit CO2 without producing cargo transport work during those legs. This can worsen the broader efficiency picture.
For this reason, companies may assess EEOI over a complete round voyage, a series of voyages or an annual operating period rather than only one isolated loaded leg. The treatment of ballast voyages should be clearly defined in the calculation method.
For some trades, it may be useful to calculate EEOI for loaded voyages only. For other trades, a round-voyage view may be more realistic because ballast legs are a normal part of the operation. The chosen method should match the purpose of the analysis.
A loaded-voyage EEOI focuses on the efficiency of cargo movement. It answers the question: how efficiently did the vessel move cargo from load port to discharge port?
A round-voyage or period-based EEOI gives a wider operational picture. It can show the effect of ballast legs, repositioning, waiting, port stays and cargo availability. This may be more useful for fleet management and commercial analysis.
The most important point is consistency. If one month is calculated using loaded voyages only and the next month includes ballast legs differently, the trend may become misleading.
EEOI and CII are related, but they are not the same thing. EEOI is an operational indicator that can be used for voyage analysis, internal monitoring and performance improvement. CII, or Carbon Intensity Indicator, is a regulatory carbon intensity rating system for applicable ships.
EEOI usually uses actual cargo transported in the denominator. CII uses a ship capacity-based approach depending on ship type and regulatory methodology. This means EEOI may be more sensitive to actual cargo carried, while CII is used for annual regulatory rating.
A simplified comparison:
| Item | EEOI | CII |
|---|---|---|
| Main purpose | Operational efficiency analysis | Regulatory carbon intensity rating |
| Typical period | Voyage, round voyage or selected period | Annual |
| Denominator | Actual transport work | Capacity-distance basis depending on method |
| Use case | Performance monitoring and optimisation | Compliance and rating |
Both indicators are connected to fuel consumption and CO2 emissions, but they answer different questions.
EEOI should also not be confused with EEXI. EEXI is related to the technical energy efficiency of an existing ship. It is closer to a design or technical index. EEOI is based on real operation.
A vessel may have an acceptable EEXI but still show poor EEOI if it operates inefficiently. For example, poor weather routing, high speed, low cargo utilization, hull fouling or excessive auxiliary load can all increase real fuel consumption and worsen EEOI.
In simple terms:
EEOI calculation requires data that many vessels already collect for reporting and performance monitoring. Typical required data includes fuel consumption, fuel type, distance sailed, cargo carried and voyage details.
EU MRV and IMO DCS frameworks already require structured collection of fuel and voyage-related data for many ships. This makes EEOI easier to calculate because the necessary data may already exist in noon reports, bunker records, voyage reports, cargo documents and emissions reporting systems.
For a practical EEOI workflow, the operator should collect:
Once these values are available, EEOI can be calculated consistently for each voyage or reporting period.
EEOI looks simple, but mistakes are common. The most frequent problems are related to inconsistent data boundaries, wrong fuel factors, incorrect cargo values and unclear treatment of ballast legs.
EEOI should normally use actual cargo transported, not design deadweight or summer deadweight. If a ship is only partly loaded, using full deadweight will make the EEOI look better than reality.
If a ship burns more than one fuel type, each fuel type should use its own conversion factor. Adding all fuel together and applying one generic factor can create an incorrect CO2 value.
Auxiliary fuel can be significant. If the calculation boundary includes total voyage fuel, auxiliary engines and boilers should not be ignored.
Distance should be defined consistently. Some calculations may use distance over ground, log distance, laden voyage distance or reported MRV distance. Mixing methods can distort trends.
Ballast voyages must be handled carefully. A pure ballast leg does not have normal cargo transport work, but ballast fuel still affects broader operational efficiency.
EEOI is useful, but direct comparison between very different vessel types can be misleading. A tanker, bulk carrier, container ship and ferry may have very different operating patterns and cargo definitions.
Improving EEOI usually means reducing CO2 emissions for the same transport work, increasing transport work without increasing emissions proportionally, or both. Since CO2 emissions are driven by fuel consumption, most EEOI improvements are also fuel-saving measures.
Optimising speed is one of the most effective ways to reduce fuel consumption. Avoiding unnecessary high-speed operation can significantly reduce power demand and CO2 emissions.
Hull fouling increases resistance. Increased resistance requires more power, more fuel and higher emissions. Regular hull performance monitoring helps identify when cleaning is economically and environmentally justified.
A rough or damaged propeller reduces propulsive efficiency. Propeller polishing can reduce required power and improve fuel performance.
Operating at an efficient trim can reduce resistance. The effect varies by ship type, speed and loading condition, but trim optimisation can be a practical low-cost improvement.
Weather routing can reduce fuel consumption by avoiding severe wind, waves and adverse currents. The shortest route is not always the most fuel-efficient route.
Poor combustion, turbocharger fouling, charge air cooler issues and incorrect engine settings can increase fuel consumption. Regular engine monitoring supports better performance.
Reducing unnecessary electrical and steam demand lowers fuel consumption. Generator load management, pump operation and hotel load control can all contribute.
For the same voyage and fuel consumption, carrying more cargo improves transport work and lowers EEOI. Commercial and operational planning therefore affect the indicator.
EEOI is useful, but it is not perfect. It is strongly affected by the trading pattern, cargo availability, weather, port delays, ballast ratio and voyage profile. A vessel may show a worse EEOI because it had low cargo utilisation, not because the machinery performed badly.
EEOI should therefore be used as an indicator, not as a final judgement without context. It is strongest when used to compare similar voyages, monitor trends for the same vessel, evaluate the effect of operational changes and support internal performance discussions.
For example, if a vessel’s EEOI gradually worsens over several similar voyages, this may indicate increased resistance, fouling, engine performance issues or operational changes. But if the vessel changes trade, cargo type or route, the EEOI may change for reasons unrelated to technical performance.
A practical EEOI calculation workflow can be simple:
The calculation should be repeatable. Another person using the same data and method should be able to reach the same result.
Assume a vessel completes a voyage with the following data:
Calculate CO2 from HFO:
HFO CO2 = 420 × 3.114
HFO CO2 = 1,307.88 tonnesCalculate CO2 from MGO:
MGO CO2 = 35 × 3.206
MGO CO2 = 112.21 tonnesTotal CO2:
Total CO2 = 1,307.88 + 112.21
Total CO2 = 1,420.09 tonnesTransport work:
Transport work = 35,000 × 2,800
Transport work = 98,000,000 tonne-nmConvert CO2 to grams:
1,420.09 tonnes CO2 = 1,420,090,000 grams CO2Calculate EEOI:
EEOI = 1,420,090,000 / 98,000,000
EEOI = 14.49 gCO2/tonne-nmThis result can then be compared with previous voyages of the same vessel, similar loaded conditions or internal company performance targets.
Fuel consumption is the foundation of ship emissions analysis, but fuel consumption alone does not show the full operational efficiency of a vessel. EEOI improves the picture by connecting fuel burned, CO2 emitted, cargo carried and distance travelled.
A vessel with high fuel consumption may still be efficient if it carries a large amount of cargo over a long distance. A vessel with low daily fuel consumption may still have poor EEOI if it carries little cargo or performs many ballast voyages. This is why EEOI is useful for understanding real operational performance.
For practical use, EEOI should be calculated with clear boundaries, correct fuel conversion factors, accurate cargo data and consistent distance measurement. It should be used as a trend and performance indicator, not as an isolated number without context.
When combined with fuel consumption analysis, MRV data, voyage records and operational monitoring, EEOI becomes a valuable tool for improving efficiency, reducing emissions and making better technical and commercial decisions in ship operation.