Injector size calculator

Tell it the power you are targeting and how many cylinders share the work, and this tool returns the flow each injector must deliver — in both lb/hr and cc/min — at a safe maximum duty cycle. Every assumption (BSFC, duty cycle, the E85 factor) is stated on the page so you can sanity-check the answer instead of trusting it blindly.

Target horsepower (crank)

Number of cylinders / injectors

Aspiration

Fuel

Max duty cycle (%)

Pick the next injector size at or above the result — and check it is rated at your fuel pressure.

The formula

Required flow per injector (lb/hr) = (HP × BSFC) ÷ (cylinders × max duty cycle)

BSFC (brake specific fuel consumption) is how many pounds of fuel an engine burns per horsepower-hour: about 0.50 for a naturally aspirated gasoline engine and 0.60 for a boosted one, which runs richer for cooling. Duty cycle is the share of time the injector may stay open; 80% leaves headroom for transients and ageing, which is why it is the default here. E85 carries less energy per litre, so the required volume goes up by a factor of about 1.35.

Worked example (pre-filled above): a 400 hp naturally aspirated V8 on gasoline at 80% duty needs (400 × 0.50) ÷ (8 × 0.80) = 31.25 lb/hr per injector, which is 328 cc/min. A common 32 lb/hr (336 cc/min) injector is the nearest sensible pick.

Sizing judgement calls

Do not size at 100% duty. An injector held fully open cannot meter fuel and overheats; 80% is the standard safe ceiling, 85% at the very most. Check your real number with the duty cycle calculator.
Bigger is not free. A hugely oversized injector idles poorly because the ECU cannot command a short enough pulse. Stay within roughly 20% above the calculated need.
Already own injectors? Run the reverse maths with the max horsepower calculator, and make sure the pump can feed the set — see the fuel pump flow calculator.

Sized them? Now service them

New injectors still need fresh seals and filters. The Universal EV1 Rebuild Kit — 200-Injector Service Tray + Removal Tool, $72.49 covers 200 injectors with matched O-rings, pintle caps and micro basket filters, and the Micro-Filter Basket Removal Tool, from $14.99, pulls the old basket filter without damage.

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Shop the 200-injector kit — $72.49 Get the tool — from $14.99

Not sure which kit your injectors take? See kits by injector part number or the fitment guide.

More injector calculators

Injector flow converter — cc/min ↔ lb/hr.
Injector size calculator — injector size for a target horsepower.
Max horsepower from your injectors — the reverse calculation.
Flow rate at a different fuel pressure — the square-root rule.
Injector duty cycle calculator — pulse width and RPM to duty %.
Fuel pump flow requirement — LPH your engine actually needs.

Last updated October 20, 2026 · Partsapiens, 5325 Alton Pkwy, Suite C300, Irvine, CA 92604 · partsapiens@gmail.com