Enter the desired engine horsepower, number of injectors, injector duty cycle, and brake specific fuel consumption into the calculator to determine the injector size needed for your engine.

Injector Size Formula

The following formula is used to calculate the injector size:

IS = (HP * BSFC) / (NI * (DC / 100))


  • IS is the injector size (lb/hr)
  • HP is the desired engine horsepower
  • BSFC is the brake specific fuel consumption (lb/HP-hr)
  • NI is the number of injectors
  • DC is the injector duty cycle (%)

To calculate the injector size, multiply the desired engine horsepower by the brake specific fuel consumption, then divide by the product of the number of injectors and the injector duty cycle (as a percentage).

What is Injector Size?

Injector size refers to the flow rate of fuel injectors in an engine, typically measured in pounds per hour (lb/hr). It determines how much fuel can be delivered to an engine’s combustion chamber and is critical for achieving the desired horsepower while maintaining proper air-fuel ratios for efficiency and performance.

How to Calculate Injector Size?

The following steps outline how to calculate the Injector Size.

  1. First, determine the desired engine horsepower (HP).
  2. Next, determine the brake specific fuel consumption (BSFC) in lb/HP-hr.
  3. Next, determine the number of injectors (NI).
  4. Next, determine the injector duty cycle (DC) as a percentage.
  5. Next, gather the formula from above = IS = (HP * BSFC) / (NI * (DC / 100)).
  6. Finally, calculate the Injector Size (IS) in lb/hr.
  7. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Desired engine horsepower (HP) = 400 HP

Brake specific fuel consumption (BSFC) = 0.5 lb/HP-hr

Number of injectors (NI) = 8

Injector duty cycle (DC) = 80%