Estimate the fuel pump flow in L/h and gal/h your target horsepower needs, or the horsepower a pump supports, using typical BSFC values for each fuel.
Fuel Pump Sizing Formula
W = HP * BSFC * (1 + SM/100)
GPH = W / D
LPH = GPH * 3.785411784
- W is the required fuel flow in pounds per hour (lb/hr)
- HP is the target crank horsepower
- BSFC is the brake specific fuel consumption in lb/hp/hr
- SM is the adjustable flow allowance in percent (blank uses an assumed 20%)
- D is the fuel density in pounds per US gallon
- GPH is the required flow in US gallons per hour
- LPH is the required flow in liters per hour
The calculator multiplies crank horsepower by BSFC to estimate engine fuel demand in lb/h, multiplies that demand by 1 + SM/100, then divides by fuel density to obtain US gal/h and converts to L/h. The flow allowance adds headroom; it does not set or guarantee a pump duty cycle. Wheel horsepower is converted using HP = WHP / (1 – loss/100). The blank drivetrain-loss setting assumes 15%; use a measured value if available. Open Fuel and operating assumptions to supply BSFC or density, change the allowance, and enter pressure assumptions. For diesel, BSFC is a required main input.
The reverse mode converts tested pump flow to lb/h, divides by 1 + SM/100, then divides by BSFC to estimate supported crank horsepower. A 20% allowance means dividing pump mass flow by 1.20, not subtracting 20%. Enter flow measured for the actual fuel, maximum operating pressure and voltage at the pump under load. The displayed base-plus-boost pressure is a comparison point for a 1:1 boost-referenced regulator; it does not recalculate pump flow or apply to every fuel system.
BSFC, Fuel Density, and Real-World Pump Output
BSFC varies with fuel, engine design, tune and operating point. Blank gasoline, E85 and methanol values use the upper end of the naturally aspirated, forced-induction and nitrous guidance in Aeromotive technical bulletin TB-501. These are estimates; enter measured BSFC when available. Density defaults below are also approximate and can be overridden for the actual fuel blend and temperature.
| Fuel | Default BSFC, naturally aspirated | Default BSFC, forced induction / nitrous | Approximate density (lb/US gal) |
|---|---|---|---|
| Gasoline | 0.50 | 0.75 / 0.60 | 6.0 |
| E85 | 0.70 | 0.95 / 0.80 | 6.59 |
| Methanol | 1.10 | 2.00 / 1.30 | 6.63 |
| Diesel | Supplied BSFC required | Supplied BSFC required | 7.05 |
Pump rating conditions vary by model. Use the manufacturer’s flow-versus-pressure data for the actual fuel and voltage available at the pump under load; neither a free-flow figure nor a class name establishes usable flow. With a 1:1 boost-referenced regulator, gauge rail pressure rises by one psi for each psi of boost. Fixed-pressure and other regulator arrangements require their actual operating pressure. The calculator does not model line or filter pressure losses, voltage changes or a pump’s performance curve.
| Operating condition | Manufacturer data to check |
|---|---|
| Fuel and blend | Compatibility and flow data for the actual fuel |
| Maximum operating pressure | Flow at the system’s required pressure, including applicable system losses |
| Voltage at the pump under load | Matching tested flow data and the pump’s permitted voltage range |
| Pump and controller | Model-specific pressure, current, speed and operating limits |
| Complete fuel system | Manufacturer requirements for filters, lines, regulator and return capacity |
A pump advertised as 340 L/h may deliver a different flow at your required pressure and supply voltage. Compare the calculated demand with the specific manufacturer curve; the advertised flow class alone does not establish supported horsepower or headroom.
Fuel Pump Sizing Example
Example 1: Estimate flow for 500 crank horsepower on turbocharged gasoline, with 15 psi boost and 43.5 psi base pressure in a 1:1 boost-referenced system. In Fuel and operating assumptions, explicitly enter BSFC 0.60, density 6.0 lb/US gal and flow allowance 20%; the blank turbo-gasoline BSFC preset is 0.75. The calculation is W = 500 * 0.60 * 1.20 = 360 lb/h; 360 / 6.0 = 60 US gal/h; 60 * 3.785411784 = 227.1247 L/h. The pressure comparison point is 58.5 psi. Confirm that a specific pump delivers at least the calculated flow at the actual fuel, operating pressure and voltage; an advertised 255 or 340 L/h class does not prove that.
Example 2: Suppose a pump is verified to deliver 340 L/h of E85 at the required operating pressure and voltage. Select the horsepower-supported mode and, in Fuel and operating assumptions, explicitly enter BSFC 0.80, density 6.59 lb/US gal and flow allowance 20%; the blank turbo-E85 BSFC preset is 0.95. Then 340 / 3.785411784 = 89.8185 US gal/h, and 89.8185 * 6.59 is about 591.9039 lb/h. Dividing unrounded mass flow by 1.20 gives 493.2533 lb/h; dividing by 0.80 gives about 616.5666 crank horsepower. This estimate depends on the entered BSFC and density and on the tested pump-flow conditions.
Fuel Pump Sizing FAQ
Should I size the pump from crank or wheel horsepower?
The BSFC relationship uses crankshaft power. If you enter wheel horsepower, the calculator divides it by 1 minus the supplied drivetrain loss as a fraction; it does not simply add that percentage. For example, 400 wheel horsepower with an assumed 15% loss gives 400 / 0.85 = about 470.6 crank horsepower. Drivetrain loss varies; the blank 15% value is an assumption rather than a measurement.
Why does my pump support less power than its LPH rating suggests?
A flow rating may refer to different fuel, pressure or voltage conditions from those in your installation. Check the pump manufacturer’s curve at the actual maximum operating pressure and voltage measured at the pump under load. For a 1:1 boost-referenced regulator only, the displayed comparison pressure is base plus boost. Use the actual pressure for other systems. Supply measured BSFC and density when available, and choose the flow allowance to suit the engine builder’s and system manufacturer’s requirements; this calculator does not diagnose wiring or installation faults.
Can a fuel pump be too big?
A higher advertised pump-flow rating does not by itself establish that the pump suits the system. Confirm fuel compatibility, pressure and flow limits, controller operation and the regulator or return path’s ability to handle the flow using the relevant manufacturer instructions. The calculator estimates demand or supported power under its stated assumptions; it does not select a pump size class, controller, wiring arrangement or staged system.
