How to use this calculator
Enter the mass of fuel consumed per unit time, then the engine brake power measured during that same steady operating condition. Keep RPM, load, fuel and measurement conditions with your comparison notes. If power is unavailable but torque and RPM are known at that point, calculate power first using the related horsepower and torque tool.
This is a positive-output engine operating-point calculation. It excludes idle with zero brake power, overrun, fuel cut and motoring. Use fuel actually consumed, not pump delivery that includes returned fuel, and crankshaft brake power rather than wheel power. It does not estimate an engine map, injector size, road MPG or a thermal-efficiency percentage.
The formula
A worked example
At an illustrative 25 kg/h and 100 kW, BSFC = 25 × 1,000 ÷ 100 = 250 g/kWh, or approximately 0.41 lb/(hp·h). The engine delivers 4 kWh of brake energy per kilogram consumed at that point. Doubling both flow and power leaves all three ratios unchanged. Using 50 lb/h and 100 mechanical hp instead gives exactly 0.50 lb/(hp·h).
Fuel flow at the same brake power
Original hypothetical examples at 100 kW. These are arithmetic illustrations, not typical values or tuning targets for any engine.
| Fuel mass flow (kg/h) | Brake power (kW) | BSFC (g/kWh) | Brake energy (kWh/kg) |
|---|---|---|---|
| 20 | 100 | 200 | 5.00 |
| 25 | 100 | 250 | 4.00 |
| 30 | 100 | 300 | 3.33 |
Common questions
What does a lower BSFC mean?
At the compared operating point, less fuel mass is consumed per unit of brake energy delivered. Compare the same fuel and measurement boundary. A mass-based comparison across different fuels does not account for their different energy content.
Can I enter liters per hour or gallons per hour?
No. Volume flow must first be converted to mass flow using the density appropriate to the actual fuel and conditions. This tool deliberately makes no gasoline or diesel density assumption. Do not enter a volume rate in a mass-flow field.
Can I use the engine’s advertised maximum horsepower?
Only when the fuel-flow measurement was made at that same power. Combining cruise fuel flow with rated peak power would understate BSFC. Engine speed and load identify the operating point; one calculated value is not a full BSFC map.
Is brake energy per kilogram the thermal efficiency?
No. It is an output-energy-to-fuel-mass ratio. A thermal-efficiency percentage also needs the fuel heating value on a specified basis. Fuel energy content is not an input here, so this calculator cannot determine that percentage.
How do I combine two operating periods?
Divide total fuel mass by total brake energy, not the simple average of the two BSFC numbers. For example, one hour at 20 kg/h and 100 kW plus one hour at 15 kg/h and 50 kW consumes 35 kg and delivers 150 kWh: 233.33 g/kWh overall. Averaging 200 and 300 would incorrectly give 250 g/kWh. This example assumes constant positive power in each period.
Why reject zero power?
Dividing fuel flow by zero power cannot produce a finite BSFC. Idle fuel use can be recorded as mass per hour, but it cannot be compared as a finite brake-energy-specific value at zero output. This tool also requires positive fuel flow for its inverse energy-per-mass result.
Does this tell me the most economical road speed?
No. Vehicle gearing, road load, drivetrain losses and the engine’s operating map would also be needed. For actual trip fuel economy, measure distance and fuel used with the fuel log guide.
Sources & calculation method
MathWorks Generic Engine documentation defines BSFC as fuel consumption rate divided by output power and describes tables indexed by engine operating conditions. This tool evaluates that ratio directly; it does not use the documentation’s example engine map.
NIST conversion factors provide unit-conversion references. The implementation distinguishes mechanical horsepower from metric horsepower and uses exact defining constants for pound mass, standard gravity and length. Examples on this page are original calculations, not measured engine tests.
Calculate power from matched torque and speed with the Horsepower & Torque Calculator. Compare torque per displacement with the BMEP Calculator, or measure road fuel use with the fuel economy guide.
References accessed September 26, 2026. Sources do not endorse this site or provide independent professional review.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.