Sizing fuel system components starts from one relationship: an engine making a given power burns a given mass of fuel per hour. This fuel pump size calculator takes a target power figure and a brake specific fuel consumption value, works out the fuel mass that implies, and converts it into the pump flow and injector flow ratings that would deliver it at the duty cycle and headroom you specify.
Arb Digital publishes this alongside the other vehicle and engine tools our team builds. It is a component sizing aid and nothing more. It calculates what flow a number implies; it does not know your engine, and it cannot tell you whether a build is sound.
Fuelling, ignition timing and knock margin are tuning decisions made on a dynamometer by a qualified tuner with the engine in front of them. This page sizes a component from an assumption you supply. It does not approve a build, validate a target, or substitute for calibration.
What This Fuel Pump Size Calculator Does
It computes total fuel mass flow from power and BSFC, then splits that flow across the number of injectors you specify at the maximum duty cycle you choose, giving a required injector rating in both pounds per hour and cubic centimetres per minute. It converts the total mass flow into a volumetric pump requirement in litres per hour and US gallons per hour using the fuel density you enter, and adds the headroom percentage you set.
Every assumption is a field. There is no built-in table of engines, fuels or pumps, because those figures are properties of specific hardware and belong to the manufacturer who publishes them, not to a calculator.
How to Use It
- Enter the target power figure you are sizing for. Be clear with yourself about whether it is crank or wheel power, because they differ substantially and the fuel flow follows the crank number.
- Enter a BSFC value appropriate to the engine and the operating point. This is the input that carries most of the uncertainty.
- Set the injector count and maximum duty cycle. Sizing to a duty below 100 per cent leaves control margin at the top of the range.
- Set fuel density for the fuel you are actually running. Gasoline, E85 and methanol are meaningfully different.
- Set pump headroom to cover voltage drop, pressure rise and pump ageing, then read the flow requirement.
The Formula: Power, BSFC and Flow
Brake specific fuel consumption is fuel consumption divided by the power produced. The engineering reference Brake Specific Fuel Consumption (BSFC) gives it as BSFC = fuel mass flow ÷ effective engine power, typically expressed in grams per kilowatt hour, where a spark-ignition engine runs near 250 g/kWh and a compression-ignition engine near 200 g/kWh. In imperial workshop units the same quantity is written in pounds per horsepower per hour, and 250 g/kWh corresponds to roughly 0.41 lb/hp/hr. The related concept for gas turbines, thrust specific fuel consumption, is set out on NASA Glenn Research Center's Specific Fuel Consumption page, which defines it as fuel mass burned per hour divided by the thrust produced — the same idea normalised against a different output.
From there the arithmetic is short:
Total fuel flow (lb/hr) = power × BSFC. Per injector (lb/hr) = total flow ÷ (injector count × duty cycle). Per injector (cc/min) = lb/hr × 453.59237 ÷ density in g/cc ÷ 60. Pump flow (L/hr) = total flow × 0.45359237 ÷ density in kg/L, × (1 + headroom).
Worked through with the defaults: 500 hp at 0.50 lb/hp/hr is 250 lb/hr of fuel. Across eight injectors at 80 per cent duty that is 250 ÷ 6.4 = 39.06 lb/hr each, which at 0.74 g/cc is 399 cc/min each. As a volume, 250 lb/hr is 113.4 kg/hr, or 153.2 litres an hour at 0.74 kg/L, and with 20 per cent headroom the pump requirement becomes 183.9 litres an hour — about 48.6 US gallons an hour.
BSFC Is the Input That Decides Everything
Change the power figure by ten per cent and the flow changes by ten per cent. Change BSFC by ten per cent and it does exactly the same thing — but power targets are usually known to within a few per cent while BSFC is frequently guessed at, so in practice BSFC is where the error comes from.
It is not a constant. BSFC varies across the engine's operating map, and the value at peak power is not the value at cruise. It varies with combustion efficiency, with air-fuel ratio, and very strongly with how much a boosted engine is being enriched to control knock and exhaust temperature. A naturally aspirated engine at a modest air-fuel ratio and a heavily boosted engine running rich for thermal margin can differ by a third or more in the figure that matters here.
The only honest source of a BSFC number for a specific engine is measurement: fuel flow and power recorded together on a dynamometer. Everything else is a starting assumption. If you have measured figures, the BSFC calculator derives the value from them — that tool works out BSFC from what an engine actually did, while this page uses a BSFC assumption to size hardware, which is the opposite direction.
Why Injectors Are Sized Below Full Duty
An injector at 100 per cent duty cycle is held open continuously and has no capacity left. That matters for two reasons. The first is control: a fuel system running at the very top of its range cannot respond to a transient demand, a hot restart or a drop in fuel pressure. The second is linearity — injectors behave predictably across a middle band of pulse widths and less predictably at the extremes, so a calibration built at the top of the range is fragile.
Sizing to a maximum duty in the region of 80 per cent is the common workshop practice, and this tool defaults there. What number is right for a particular injector and calibration is a decision for whoever is tuning the engine, which is why it is an editable field rather than a fixed assumption.
A Pump Rating Is Not a Single Number Either
Fuel pumps are rated at a specified pressure and a specified voltage, and both matter enormously. Raise the fuel pressure and flow falls, sometimes steeply — a pump quoted at 43 psi will move noticeably less at 60 psi, and a boosted engine on a rising-rate regulator sees exactly that at full load. Drop the supply voltage and flow falls again, which is why wiring gauge, relay quality and earth path are part of fuel system design rather than an afterthought.
That is what the headroom field is for. It is not a fudge factor for optimism, it is coverage for the difference between a datasheet condition and the condition your car is actually in on a hot day at full boost with a battery that is not quite keeping up. Read the pump manufacturer's own flow-versus-pressure curve at the voltage you will really see, rather than the single headline number on the box.
What This Calculation Does Not Cover
Plenty. Line and filter sizing, which limits flow independently of the pump. Return system design and regulator behaviour. Fuel temperature, which rises in a recirculating system and changes density. Lift pump and surge tank arrangements. Injector dead time and the way it interacts with pulse width at high duty. Compatibility of seals and pump internals with ethanol and methanol. Any of these can be the actual constraint in a system whose pump and injectors are nominally large enough.
It also does not touch the parts of a build that decide whether the power target is reachable at all. Displacement and compression are covered by the engine displacement calculator and the engine compression ratio calculator, mixture by the air fuel ratio calculator, and the power arithmetic itself by the horsepower calculator and the boost horsepower calculator. None of them, and none of this, replaces a calibration session.
Arb Digital builds free calculators that state their assumptions and cite their sources instead of hiding both. Browse the library, or get in touch about a set for your audience.
Browse Free Tools Get in TouchCommon Mistakes to Avoid
- Using a wheel power figure as the target — fuel flow follows crank power, and the difference is not small.
- Carrying a naturally aspirated BSFC into a boosted build — enrichment for knock and thermal margin raises fuel consumption per horsepower substantially.
- Reading a pump's headline flow as its flow at your pressure — flow falls with rising fuel pressure and with falling voltage.
- Sizing injectors at 100 per cent duty — a system with no capacity left has no transient response and no margin.
- Leaving fuel density at the gasoline value while running E85 — the density and the required mass flow are both different, and the error compounds.
Related Free Tools From Arb Digital
Derive a measured figure with the BSFC calculator, work the power side with the horsepower calculator and the boost horsepower calculator, and cover the engine basics with the engine displacement calculator, the engine compression ratio calculator and the air fuel ratio calculator. More sit in the free online tools hub.
Frequently Asked Questions
Multiply the target power by a brake specific fuel consumption figure in pounds per horsepower per hour. That gives fuel mass flow in pounds per hour, which converts to a volume once you apply the density of the fuel you are running.
That depends on the engine and the operating point, and the only reliable source is measurement on a dynamometer. Published figures for spark-ignition engines cluster around 250 grams per kilowatt hour, which is roughly 0.41 pounds per horsepower per hour, with boosted engines running higher because of enrichment.
Because an injector held fully open has no capacity left for transients, hot restarts or a pressure drop, and injectors behave less predictably at the extremes of their pulse width range. The exact figure is a calibration decision, which is why it is an editable field here.
No. Pump flow falls as fuel pressure rises and as supply voltage drops. A rating is quoted at a specific pressure and voltage, so read the manufacturer's flow-versus-pressure curve at the conditions your system will actually see.
Because it carries less energy per unit mass, so more fuel mass is needed for the same power, and its density differs too. Both the required mass flow and the mass-to-volume conversion change, which is why fuel density is an input on this page.
No. It produces a flow requirement from your assumptions. Matching that to a specific pump means reading the manufacturer's published flow curve at your fuel pressure and system voltage, and considering lines, filters, regulator and wiring alongside it.
Oversizing brings its own consequences — more heat into the fuel, more current draw and more return flow — and none of that is settled by arithmetic. Fuel system design and calibration decisions belong with a qualified tuner working on the engine.
This page is a component sizing aid for general information only. It does not constitute tuning guidance, does not approve any build or power target, and cannot account for your fuel system's lines, regulator, wiring or thermal behaviour. Fuelling and knock margin are decisions made on a dynamometer by a qualified tuner.
