Fuel pump sizing from target power
A pump has to supply more than the engine's bare fuel consumption — voltage sag, line losses and pressure drop all eat into rated flow before it reaches the rail. This works out the bare requirement and a sized-with-margin figure separately.
| Target power | Required flow (LPH) | Required flow (GPH) |
|---|---|---|
| 200 hp | 85 LPH | 22.4 GPH |
| 300 hp | 127 LPH | 33.5 GPH |
| 400 hp | 169 LPH | 44.7 GPH |
| 500 hp | 212 LPH | 55.9 GPH |
| 700 hp | 296 LPH | 78.3 GPH |
| 1000 hp | 423 LPH | 111.8 GPH |
Also check the injectors can flow this much fuel individually — the injector calculator uses the same BSFC figure.
Getting a number you can act on
- 01Set the target power figure
Use the peak power the fuel system needs to support, not an average or typical figure — the pump has to cover the worst case, which is wide-open throttle at redline.
- 02Match BSFC to how the engine will actually be run
A more aggressive tune burns fuel less efficiently per unit of power, which raises BSFC and, with it, the flow the pump has to supply.
- 03Keep a genuine safety margin
25% is a common default. Battery voltage sag under load, line and filter restriction, and a pump's output naturally declining somewhat with age and wear all eat into rated flow before it reaches the rail.
- 04Compare the result against a specific pump's rated flow
Pump manufacturers rate flow at a specific voltage and pressure — check that the comparison is apples to apples before assuming a pump is adequate.
What the calculator is actually doing
Nothing here is proprietary. If you would rather check it by hand, or explain it to someone at a counter, these are the same expressions the tool evaluates.
flow (lb/hr) = target hp × BSFC × safety factorThe same fuel-consumption relationship the injector calculator uses, with a margin built in.
LPH = (lb/hr ÷ 6.15 lb per US gallon) × 3.785 litres per gallonGasoline's typical density, converted through US gallons into the litres-per-hour figure pumps are usually rated in.
Why a margin matters more here than almost anywhere else in the fuel system
A fuel pump is the one component in the fuel delivery chain that everything downstream depends on entirely — an undersized injector starves one cylinder under extreme conditions, but an undersized pump starves the whole engine, and it does so exactly when demand is highest.
Voltage matters more than people expect. A fuel pump's flow output is roughly proportional to the voltage it receives, and a wiring harness under load — particularly a stock harness feeding an aftermarket pump beyond its original design current — can sag several tenths of a volt at wide-open throttle. That sag translates directly into reduced flow exactly when the engine is asking for the most fuel.
The 25% margin used as a default here is a starting point rather than a rule. A build with a fresh, properly relayed wiring harness and a pump rated with real headroom over the target can run closer to the bare minimum; a build inheriting an older harness, or targeting the edge of what a given pump can do, benefits from more margin rather than less.
Fuel Pump Size Calculator FAQ
How do I size a fuel pump for my horsepower target?+
Multiply target horsepower by BSFC (typically 0.50-0.60 for gasoline) and a safety factor (commonly 1.25), then convert from lb/hr to litres per hour through gasoline's density.
Why does the pump need more capacity than the injectors need?+
It doesn't need more than the injectors in isolation — it needs a margin over the bare fuel-consumption figure, because voltage sag, line restriction and pump wear all reduce real-world output below the rated figure.
What safety margin should I use for pump sizing?+
25% is a common default. A fresher, properly wired fuel system can run closer to the bare minimum; an older harness or a build near the edge of a pump's capability benefits from more.
Does voltage affect fuel pump flow?+
Significantly — flow output is roughly proportional to supply voltage, and voltage sag under load can meaningfully reduce real flow below a pump's rated figure at full voltage.
Standards and references behind these figures
The arithmetic on this page is fixed, but the boundaries and conventions around it come from published standards and manufacturer guidance. These are the documents they come from, so you can check them rather than take them on trust.
01Dyno correction factors — SAE J1349 against STDExplains the J1349 reference conditions of 77°F, 0% humidity and 29.234 in-Hg, and why STD-corrected figures read higher.↗02Ford Performance — dynamometer testing and engine performance tech tipsManufacturer guidance on dyno correction and how quoted power figures are arrived at.↗A starting point, not a tune. Fuel system sizing depends on assumptions — BSFC, target duty cycle, fuel pressure — that vary by engine and by how hard it is actually driven. Use these figures to shortlist hardware, then verify the actual mixture with wideband data before trusting the car to it.