How to Calculate Main Engine SFOC: 5 Steps to Daily Fuel

Updated 27 September 2026 · marine engineering, fuel consumption, engine room, SEEMP

To calculate main engine SFOC and daily fuel consumption onboard, you need two readings taken over the same period. SFOC stands for specific fuel oil consumption: the grams of fuel burned for each kilowatt-hour of power the engine produces. The two readings are the mass of fuel the engine burned and the power it delivered. Divide the fuel in grams by the kilowatt-hours produced (kWh, the standard unit for power delivered over time) and you have SFOC in g/kWh. Then multiply SFOC by power and running hours, and convert grams to tonnes, to get daily consumption.

The formula itself is simple. Most wrong answers start earlier than the formula, at the flow meter. That happens when volume is turned into mass using the wrong density or the wrong temperature. It also happens when the power figure belongs to a different hour than the fuel figure. This guide covers the arithmetic first, then spends most of its time on the steps where engine room numbers actually go wrong.

The two formulas you need

Both formulas say the same thing from opposite ends. SFOC is fuel mass divided by energy produced, and daily fuel is SFOC multiplied back by that energy.

SFOC (g/kWh) = fuel consumed (g) / (power (kW) x running hours (h))

Daily fuel (t) = SFOC (g/kWh) x power (kW) x running hours (h) / 1,000,000

The 1,000,000 divisor turns grams into tonnes, nothing more. If your flow reading is per hour, there is a shorter form: SFOC equals fuel mass flow in grams per hour divided by power in kilowatts. MetaCAD's fuel consumption pages and Marine Insight's guide to engineering calculations, both retrieved on 27 September 2026, set out this same relationship.

The one condition that matters most is that both inputs come from the same period. A fuel reading taken over one stretch of time and a power reading from another will give you a number that looks reasonable and means nothing.

How to calculate main engine SFOC step by step

The method has five steps: measure fuel, convert it to mass, get power for the same period, divide, and then scale up to a day. Each step has one thing that tends to go wrong, and those are listed here as well.

Step 1: Record fuel consumed over a defined interval

Take a flow-meter reading at the start and at the end of a fixed interval, and write down the exact times. Marine Insight's calculation guide and a SFOC course handout from a Greek maritime academy, both retrieved in September 2026, recommend reading the meter over at least one hour.

Choose a period where the load is steady. Readings taken while you are manoeuvring, changing speed or changing over fuel tell you about the transition and very little about the engine's real consumption. Check which consumers the meter actually feeds. If the same line also supplies other equipment, your "main engine" figure will include their fuel as well.

Step 2: Convert volume to mass

Most flow meters on board report volume, and SFOC needs mass. The ClassNK SEEMP (Ship Energy Efficiency Management Plan) Part II sample, retrieved on 27 September 2026, uses this conversion:

Fuel mass (t) = meter volume (kL) x density (g/cm³) x volume correction factor

According to that ClassNK sample, the density should come from the bunker delivery note — the paperwork that comes with each fuel delivery, stating its density and quantity — and the fuel quantity is corrected to 15°C. The correction factor is needed because fuel in a heated supply line has expanded. The meter counts hot, expanded litres, and without the correction you would be counting fuel the engine never burned.

A published academic study hosted on EUDL makes the same point more generally: a flow meter only gives an accurate consumption figure when fuel temperature and density are handled correctly. Of all the steps, this one does the most quiet damage, because a result can look perfectly believable and still be off.

Step 3: Get engine power for the same period

You need the power the engine delivered, in kilowatts, over the exact interval you measured fuel. If the ship has a shaft power meter, use its average over that interval.

If all you have is rated power and a load percentage from the control system or the indicator, your result is an estimate. None of the sources behind this guide gives a standard load-factor formula or says how accurate such an estimate is, so label the result as an estimate in your log. Anyone trending it later will then know how much to trust it.

Step 4: Calculate SFOC

Divide fuel mass by kilowatt-hours. Keep grams on top and kilowatt-hours on the bottom, and the answer comes out in g/kWh without any extra conversion.

Then compare it with the engine's shop test or sea trial figure at a similar load. A figure from the ship in service will usually differ from the test bed figure, since the test bed runs on reference fuel under controlled conditions. One academic study, hosted on EUDL, applies an 85% operational correction factor to test bed SFOC within its own method, but the study does not show that this factor applies to every engine or ship. Your own trend over several months is a better reference.

Step 5: Scale to daily consumption using actual running hours

Multiply SFOC by power and by the hours the engine actually ran, then divide to get tonnes. Use real running hours from the log. A ship that stopped for part of the day did not burn a full day of fuel.

When the load changes during the day, split the day into periods and add them up:

Daily fuel (t) = sum of (SFOC_i x power_i x hours_i) / 1,000,000

The sources don't set a standard number of load bands. The practical choice is to use the periods your engine log already records, such as full sea speed, reduced speed and manoeuvring, and to use the SFOC that matches each load.

Daily fuel consumption onboard: a worked example

MetaCAD's worked example, retrieved on 27 September 2026, uses 170 g/kWh at 10,000 kW over 24 hours, which comes to 40.8 tonnes per day. In the same MetaCAD example, running that engine for 18 hours gives 30.6 tonnes.

170 x 10,000 x 24 / 1,000,000 = 40.8 t
170 x 10,000 x 18 / 1,000,000 = 30.6 t

Both figures are outputs of the formula and did not come from a real ship. They show the method and do not tell you what any particular engine burns. To check your own figure, compare it with the drop in tank contents over the same period. If the calculated tonnes and the tank change disagree by more than your soundings can explain, go back to Step 2 before you start doubting the engine.

What the calculation includes and what it leaves out

The result covers the main engine only, as long as your power and SFOC inputs refer to the main engine alone. According to MetaCAD's calculator pages, retrieved on 27 September 2026, generators, boilers and other auxiliary consumers have to be added separately to get total vessel consumption.

MetaCAD's own result text lists weather, hull fouling, current, engine tuning, fuel properties and hotel loads as reasons real fuel use differs from the formula. A formula only accounts for these if they are already reflected in the power and SFOC you enter. Fuel density, lower calorific value and grade also change the conversion between volume, mass and energy, which matters when you compare figures across different bunkers.

Noon reports deserve their own warning. A 2026 review posted on arXiv describes noon report data as manually entered and coarse, which makes it useful for long voyages but weak for short voyages or short-term prediction. A noon report works well as a daily record, and it is a poor tool for chasing a small SFOC change.

Calculator tools compared

Several free web tools and reference documents do this arithmetic or define the method. The table below summarises what each publisher says its tool does. None of us here has used these tools on board, so treat it as a map of what exists.

None of these pages stated a price or plan limit when retrieved on 27 September 2026, so check each one before you rely on it. All of them need a connection. For the same arithmetic without the satellite link, our guide to offline calculator apps for marine engineers covers tools that keep working when the ship's internet drops. The wider roundup of best apps for marine engineers is where to look for logbook and reference apps that go alongside them.

Pros and cons of each method

Each way of getting the fuel figure trades effort for accuracy. Here is how the three common routes compare.

Formula calculators

• Fast with no hardware: you only need power, SFOC and hours, which makes them handy for planning a passage.
• Only as good as the SFOC you enter: feed in a shop test value and you get a shop test answer, which your engine in service will rarely match.
• Blind to conditions: according to MetaCAD's own text, weather, fouling and tuning are not accounted for unless your inputs already include them.

Flow meter plus power reading

• Measured, with no assumptions: this is the only route here that gives you the engine's actual SFOC.
• Sensitive to conversion errors: a wrong density or a missing temperature correction shifts every figure after it.
• Needs matching periods: fuel and power must cover exactly the same interval, and that takes discipline at every reading.

Noon reports

• Already exist: every ship produces them, so you have a long daily history at no extra cost.
• Coarse and manual: the 2026 arXiv review describes manual-entry noise and day-long granularity as built-in weaknesses.
• Weak on short voyages: according to the same review, noon report data is not useful for short-term consumption prediction.

Who should not rely on a formula calculator

Anyone settling a charter party performance claim should use measured data, and MetaCAD's own calculator text says the same. A formula output will not hold up against a charterer's surveyor.

Ships on short coastal runs are a second case. Noon report data is already weak for short voyages, and a formula estimate built on top of it adds more error.

The third case is SEEMP reporting. Your company's approved SEEMP method decides how consumption gets measured and converted. The ClassNK sample shows what such a method looks like, but the version that counts is the one in your own ship's plan.

Common mistakes that skew the result

Most bad SFOC figures come from a handful of repeat errors. The first is mixing periods, such as a four-hour fuel reading paired with a power figure read off the screen at one moment. The second is taking density from a lab report or a guess when the bunker delivery note figure is available. The third is skipping the temperature correction, which makes consumption look higher than it really is.

Another common one is including the manoeuvring hours in the SFOC period. Low and changing load pushes specific consumption up, and a single averaged figure then makes the engine look worse than it performs at sea speed. The last is reporting the main engine figure as ship consumption when generators and the boiler are burning fuel too.

For the wider set of tools a senior engineer keeps on the laptop, see our list of digital tools every chief engineer should know.

Verdict

The formula takes a minute to learn. The accuracy comes from the measurement discipline around it: a steady-load interval, fuel and power from the same period, density from the bunker delivery note and a proper temperature correction. Use a web or offline calculator to check your division, use flow-meter data for anything that goes into a claim or a report, and treat noon reports as the daily trend line. Trend your own SFOC over months and ignore any single reading that disagrees with the tank.

FAQ

What is a normal SFOC for a ship's main engine?

None of the sources behind this guide gives a fleet-wide SFOC benchmark, and figures vary with engine type, load and fuel. The useful reference is your own engine's shop test or sea trial record at a similar load. Watch your own trend over time and investigate any lasting drift.

Why is my calculated SFOC higher than the shop test figure?

In service the engine burns commercial fuel, works against fouling and weather, and drifts out of tune, and the test bed has none of those conditions. Conversion errors in density or temperature can also inflate the number. One academic paper, hosted on EUDL, applies an 85% operational correction factor to test bed figures in its own method, but it does not claim the factor fits every engine.

Do I need a flow meter to calculate SFOC onboard?

You need a reliable fuel mass for a defined period, and the IMO uncertainty analysis document describes flow meters as the standard way to get one. Tank sounding differences can give a rough figure over a long period, but they are too coarse for a short SFOC check. Without a meter, label your result as an estimate.

How do I calculate daily fuel consumption when the load changes during the day?

Split the day into periods at roughly constant load, work out SFOC multiplied by power multiplied by hours for each period, and add the results. Then divide the total by a million to convert grams to tonnes. Use the periods your engine log already records so the figures can be checked later.

Does the daily figure include generators and boilers?

It does not, as long as the power and SFOC you used belong to the main engine alone. Generators, boilers and other auxiliary consumers have to be calculated separately and added to get the ship's total consumption.

Related reading

• Our roundup of the best apps for marine engineers covers the wider toolkit for engine room paperwork.
• The guide to offline calculator apps for marine engineers looks at fuel and power calculators that work without a connection.
• The list of digital tools every chief engineer should know covers the software side of running an engine department.

About the author

Sea Current Tech is written by a working marine engineer who fills in engine logs and noon reports as part of the job, and who knows how one wrong density can throw off a whole month of figures. We have not used the calculator tools named in this guide ourselves. Every statement about them comes from the publisher's own page or from a third-party study, and the list at the end of this page says which is which. That is how this guide keeps the method for how to calculate main engine SFOC and daily fuel consumption onboard something you can check line by line.