BSFC Calculator (brake specific fuel consumption, g/kWh & lb/hp·h)

Understanding the BSFC calculator — concept and principle

Brake specific fuel consumption (BSFC) is the mass of fuel an engine burns for each unit of useful work at the crankshaft. It is the standard measure of engine fuel efficiency. This calculator finds BSFC from fuel flow and brake power, and converts it into brake thermal efficiency.

Burning 25 kg of fuel per hour for 100 kW is 250 g/kWh, which means 33.5% of the fuel energy reaches the shaft.

How it is calculated

BSFC = fuel mass flow ÷ brake power (g/kWh).
Volume flows are converted to mass with the fuel density.
1 lb/hp·h = 608.3 g/kWh.
Brake thermal efficiency η = 3,600 ÷ (BSFC in g/kWh × LHV in MJ/kg), because 1 kWh = 3.6 MJ.

The principle

Fuel carries chemical energy, measured by its lower heating value (about 43 MJ/kg for petrol). An engine converts only part of that energy into work at the crankshaft; the rest leaves as heat in the exhaust and coolant and as friction. BSFC expresses this directly: the fewer grams of fuel needed per kilowatt-hour, the more efficient the engine. Thermal efficiency is the same information as a percentage.

Worked example

Default inputs: fuel flow 25 kg/h, brake power 100 kW, LHV 43 MJ/kg.

Fuel mass flow = 25 kg/h; brake power = 100 kW.
BSFC = 25,000 g/h ÷ 100 kW = 250 g/kWh.
In US units = 250 ÷ 608.3 ≈ 0.411 lb/hp·h.
Brake thermal efficiency = 3,600 ÷ (250 × 43) ≈ 33.5%.

Each point is the calculator re-run with only “Brake power at the crankshaft” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Brake power at the crankshaft	BSFC (g/kWh)
25	1,000
50	500
75	333.3
100	250
125	200
150	166.7
200	125

How to read the result

Lower BSFC is better. Petrol engines typically reach about 250–350 g/kWh at their best operating point, diesels about 190–230 g/kWh. 250 g/kWh is a good figure for a petrol engine. BSFC varies strongly with load and speed: engines are least efficient at light load, which is why fuel economy in town is worse.

Common mistakes

Using a volume flow without converting it to mass.
Using wheel power instead of crankshaft (brake) power.
Measuring fuel and power at different operating points.
Using the higher heating value instead of the lower one for efficiency.

Keep in mind: BSFC must be measured at a steady operating point on a calibrated dynamometer for meaningful results.

Frequently asked questions

What is a good BSFC?

Roughly 250 g/kWh or less is good for a petrol engine; modern diesels can reach about 200 g/kWh or less.

How do I convert BSFC to efficiency?

η = 3,600 ÷ (BSFC × LHV), with BSFC in g/kWh and LHV in MJ/kg.

Why is BSFC worse at light load?

Friction and pumping losses stay large while useful output is small.

Sources: Heywood, J. B., Internal Combustion Engine Fundamentals, 2nd ed. McGraw-Hill (2018) · SAE J1349, Engine Power Test Code

Checked 9 Nov 2026. C. Shih, “Internal Combustion Engines” lecture notes, FAMU–FSU College of Engineering: specific fuel consumption sfc = fuel mass flow rate ÷ power. Efficiency relation η = 1/(sfc × Q_LHV) and typical ranges are standard engine textbook values (J. B. Heywood, Internal Combustion Engine Fundamentals). 1 hp = 745.69987 W, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L.

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