Injector Size Calculator

Work out injector size instantly with clear inputs, formula shown and shareable results.

About Injector Size Calculator

Injector sizing works from brake specific fuel consumption: total fuel flow is power times BSFC, divided by cylinders and by the maximum duty cycle you will allow. Keeping duty at or below 85% leaves margin for injector response and prevents the pintle from staying open.

Formula

Injector sizing

lb/h per injector = power x BSFC / cylinders / duty cycle; cc/min = lb/h x 10.5

Worked example

With the default values below, Injector Size Calculator returns Injector size required of 510. Change any input above and every figure here is recalculated in your browser.

Inputs used

Target power: 300 hp
Cylinders: 4 cylinders
Brake specific fuel consumption: 0.55 lb/hp/h
Maximum injector duty cycle: 85 %

Results

Injector size required: 510 cc/min
Injector flow: 48.53 lb/h
Total fuel flow: 165.00 lb/h
Headroom at 100% duty: 15.0% %

Step by step

1Values used
2Injector sizing
3Injector size required= 510 cc/min
4Injector flow= 48.53 lb/h
5Total fuel flow= 165.00 lb/h
6Headroom at 100% duty= 15.0 %

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What BSFC should I use?

About 0.45-0.50 lb/hp/h for a naturally aspirated petrol engine and 0.55-0.65 for boosted, since forced induction engines run richer.

Why cap duty cycle at 85%?

Above that the injector has little closed time, flow becomes non-linear and heat soak affects it. Static (100% open) operation loses all metering control.