Specific Fuel Consumption Calculator

Specific fuel consumption compares fuel mass flow to useful output. Two common forms are used, depending on the propulsion type.

• Power-based SFC (BSFC): \(\text{SFC} = \dot{m}_f / P\)
• Thrust-based SFC (TSFC): \(\text{SFC} = \dot{m}_f / T\)

This tool converts your inputs to base units, then reports popular engineering units:

• Power-based: g/kWh, kg/kWh, and lb/(hp·hr)
• Thrust-based: kg/(N·s), mg/(N·s), and lb/(lbf·hr)

1. Select Calculation mode: power-based for shaft engines, thrust-based for jets.
2. Choose how you will enter fuel data: Mass flow or Volume flow + density.
3. Enter the fuel measurement and the matching power or thrust value.
4. Press Submit to view results above the form.
5. Use Download CSV for spreadsheets or Download PDF for a printable report.

1) What specific fuel consumption represents

Specific fuel consumption (SFC) expresses the fuel required to produce a unit of useful output. For shaft engines, brake specific fuel consumption (BSFC) links fuel mass flow to delivered power. For jets and rockets, thrust specific fuel consumption (TSFC) links fuel mass flow to steady thrust. Because it normalizes fuel rate by output, SFC is widely used for engine maps and comparing operating points.

2) Units you will see in practice

Power-based SFC is commonly reported as g/kWh (SI) or lb/(hp·hr) (imperial). Thrust-based SFC is often lb/(lbf·hr), while some analysis uses kg/(N·s) or mg/(N·s). Do not mix net and gross thrust, and use brake or shaft power (not electrical input power).

3) Typical BSFC ranges

Modern spark-ignition engines often show best BSFC near 230–320 g/kWh, depending on displacement, boosting, and calibration. Well-optimized diesels can reach about 180–230 g/kWh at their best point. At light loads, fixed losses dominate and values can exceed 400 g/kWh. Best BSFC often occurs at moderate rpm and high torque.

4) Typical TSFC ranges

For turbofans, TSFC depends strongly on bypass ratio and flight condition. At cruise, efficient high-bypass turbofans are often around 0.45–0.65 lb/(lbf·hr), while older or lower-bypass designs can be higher. TSFC is usually worse at takeoff because the cycle runs away from its most efficient point.

5) Why operating point and environment matter

SFC is not constant. BSFC changes with rpm and load as friction, pumping losses, and combustion strategy vary. TSFC changes with altitude, Mach number, inlet temperature, and nozzle efficiency. Compare only points with similar conditions.

6) Fuel-flow and density considerations

Mass flow is preferred because density varies with temperature and fuel blends. If you measure volumetric flow, use a density measured or corrected to the same temperature, especially for hot return fuel systems. Also watch for recirculation/return lines and sensor lag. Compute SFC from steady segments, not throttle ramps.

7) How this calculator supports analysis

This calculator accepts mass flow or volume flow plus density, converts to consistent base units, and returns several standard output units. It also reports fuel flow in kg/s, kg/h, and lb/h for quick cross-checks. Use the decimal control to match your instrument precision, then export to CSV for spreadsheets or save a PDF for reports and sign-off records.

8) Turning results into decisions

Lower SFC generally indicates better efficiency, but interpret it with torque, thrust, temperatures, emissions limits, and mission constraints. Use repeated tests, include notes about configuration changes, and avoid over-interpreting small deltas that are within measurement uncertainty. If results look unrealistic, recheck selected units and confirm that power/thrust and fuel flow were captured at the same operating point.

1) What is the difference between BSFC and TSFC?

BSFC relates fuel mass flow to delivered power, typically g/kWh or lb/(hp·hr). TSFC relates fuel mass flow to thrust, commonly lb/(lbf·hr) or kg/(N·s). Choose the form that matches your propulsion output.

2) Why do my BSFC values look too high?

Common causes include using indicated power instead of brake power, mixing fuel-flow and power measurements from different operating points, incorrect unit conversions, or running at low load where fixed losses dominate. Verify steady conditions and input units first.

3) Can I use volumetric fuel flow without density?

Not reliably. Volume flow must be converted to mass flow using density, and density depends on fuel type and temperature. If you only have volume flow, estimate density carefully and note the uncertainty, especially for hot fuel or blended fuels.

4) What decimals should I use for reporting?

Match your measurement precision. If fuel flow and power are only accurate to a few percent, reporting many decimals can be misleading. Two to four decimals is usually enough, while keeping raw measurements and uncertainty notes separately.

5) Is lower SFC always better?

Lower SFC usually means higher efficiency at that point, but it does not capture power density, emissions limits, transient response, or mission constraints. Compare SFC alongside torque, thrust, temperatures, and required operating envelope for a complete assessment.

6) How do I compare engines of different sizes?

SFC normalizes fuel flow by output, so it helps compare efficiency across sizes. Still, compare at similar operating regimes and conditions, and consider the full map. A smaller engine may have different best-efficiency regions and constraints.

7) Why does TSFC change with altitude and speed?

Air density, inlet temperature, compressor operating point, and nozzle expansion all change with altitude and Mach number. Those effects alter how much thrust is produced for a given fuel flow. Always pair TSFC with the flight condition used.