Estimate engine SFOC (g/kWh) at any load, calculate absolute fuel consumption, CO₂ emissions, and fuel cost — for HFO, MDO, MGO and LNG.
| Load (%) | SFOC (g/kWh) | Δ MCR (%) | CO₂ (g/kWh) |
|---|---|---|---|
| Plot the curve to populate table | |||
Specific Fuel Oil Consumption (SFOC) is the single most important efficiency metric for marine diesel engines. It quantifies exactly how much fuel mass an engine consumes to produce one kilowatt-hour of useful mechanical work, expressed in grams per kilowatt-hour (g/kWh). Understanding, monitoring, and optimising SFOC is central to fuel management, voyage planning, emissions reporting, and regulatory compliance on every modern vessel.
SFOC represents the thermal efficiency of a marine engine in practical, measurable terms. A lower SFOC value means the engine extracts more mechanical energy from each kilogram of fuel, reducing operating costs and greenhouse gas emissions simultaneously. For a large container vessel or bulk carrier running a 15,000 kW main engine, even a 1 g/kWh improvement in SFOC translates to hundreds of tonnes of fuel saved over a year of operation — a saving worth hundreds of thousands of US dollars at current bunker prices.
Engine manufacturers publish guaranteed SFOC values for their engines at 100% Maximum Continuous Rating (MCR), measured under ISO 3046-1 reference conditions (25 °C air temperature, 100 kPa charge-air pressure, 25 °C cooling water temperature). In real-world operation, the actual SFOC deviates from this reference based on load, ambient conditions, engine wear, fouling, and tuning state.
The basic definition of SFOC is straightforward:
In practice, deriving SFOC from measurements requires accurate fuel flow metering and power measurement. This calculator uses a parametric model commonly applied for part-load estimation:
Where L is engine load expressed as a percentage of MCR, SFOC100 is the reference SFOC at 100% MCR, and a and b are adjustable coefficients that shape the curve to match a specific engine's behaviour. The default coefficients (a = 0.25, b = 0.75) are suitable for most slow-speed two-stroke and medium-speed four-stroke marine diesel engines. They can be adjusted in the Advanced section to match your engine's actual shop-trial data.
Engine SFOC is not constant across the load range. The relationship between load and SFOC has a characteristic U-shape or hockey-stick shape, with several distinct regions:
Understanding where your engine sits on its SFOC curve at any given voyage speed is essential for slow steaming analysis, speed optimisation, and voyage fuel estimation. Plotting the full curve, as this calculator provides, makes these trade-offs immediately visible.
The fuel type used on board directly affects both the SFOC value measured and the resulting CO₂ emissions. This calculator supports the most common marine fuels:
| Fuel Type | Abbrev. | LHV (MJ/kg) | Carbon Factor (tCO₂/t fuel) | Density (kg/m³) | Sulphur Limit |
|---|---|---|---|---|---|
| Heavy Fuel Oil 380 cSt | HFO | 40.20 | 3.114 | 991 | 0.50% (outside ECA) |
| Very Low Sulphur Fuel Oil | VLSFO | 40.60 | 3.151 | 920 | ≤0.50% |
| Marine Diesel Oil | MDO | 42.70 | 3.206 | 870 | ≤0.10% (ECA) |
| Marine Gas Oil | MGO | 42.70 | 3.206 | 840 | ≤0.10% (ECA) |
| Liquefied Natural Gas | LNG | 50.00 | 2.750 | 450 | Near-zero sulphur |
| Methanol | MeOH | 19.90 | 1.375 | 791 | Near-zero sulphur |
Note that the Lower Heating Value (LHV) of the fuel affects the theoretical minimum SFOC achievable. An engine burning LNG will show a numerically lower SFOC than the same engine burning HFO for the same thermal efficiency, because LNG has a higher energy content per kilogram. The CO₂ emission factor is used to convert fuel consumption to equivalent CO₂ mass for emissions reporting under IMO DCS, EU MRV, and CII calculations.
Since 2023, IMO requires all vessels of 5,000 GT and above to calculate and report their annual Carbon Intensity Indicator (CII) rating — a measure of how efficiently the ship transports cargo relative to CO₂ emitted. SFOC is a direct input to CII calculation: a vessel running its engines at lower SFOC (closer to the optimal load band) will accumulate fewer CO₂ grams per cargo-tonne-mile, improving its CII rating.
This calculator outputs CO₂ emission intensity in g/kWh, which can be directly used in CII analysis and Energy Efficiency Existing Ship Index (EEXI) technical file preparation. The CO₂ output per hour is calculated as:
Slow steaming — intentionally operating vessels below their design speed to save fuel — became widespread after the 2008 fuel price spike and has remained a key fuel management strategy. However, slow steaming involves complex trade-offs related to SFOC.
Ship speed is approximately related to engine power by a cubic relationship: halving speed reduces required power to roughly one-eighth of full-speed power. This dramatic reduction in absolute fuel consumption is the primary driver of slow steaming economics. However, operating at very low load also causes SFOC to rise, partially offsetting the savings. The net effect depends on how steeply the engine's SFOC curve rises at low load.
By plotting the full SFOC curve with this calculator and examining the table of SFOC values across the load range, operators can identify the optimal slow steaming load — the point at which the product of engine power and SFOC (representing absolute fuel consumption per unit time) is minimised while still meeting schedule requirements.
Two fundamentally different SFOC measurements appear in marine engineering practice:
In service, actual SFOC typically increases further due to propeller fouling, engine wear, and non-ideal ambient conditions. Regular performance monitoring comparing current SFOC to the baseline sea trial figure is the standard method for detecting engine degradation and quantifying hull and propeller cleaning benefits.
Accurate onboard SFOC measurement requires both power measurement and fuel flow measurement over the same time period:
A steady upward trend in SFOC (corrected for load and ambient conditions) is one of the clearest indicators of engine deterioration. Common causes of SFOC increases in service include:
Performance monitoring systems typically alarm on a 2–3% sustained deviation in corrected SFOC as an early warning trigger for maintenance investigation. This calculator's delta output (Δ vs 100% MCR) helps contextualise measured deviations against expected part-load behaviour.
This calculator provides engineering estimates based on a parametric SFOC model. It is intended for planning, training, and preliminary analysis purposes. Actual SFOC depends on engine condition, fuel quality (LHV, density, viscosity), ambient conditions, and tuning state. For contractual fuel consumption guarantees, IMO EEXI/CII compliance calculations, or NOx Technical File preparation, always refer to certified engine test data, the engine builder's documentation, and applicable IMO guidelines. The carbon conversion factors used follow IMO MARPOL Annex VI guidelines (Resolution MEPC.364(79)).