Fuel Injector Sizing Calculator
Injector flow rate from engine HP and fuel type.
About this calculator
Sizing fuel injectors comes down to matching how much fuel the engine needs at its target power level to how much fuel each injector can actually deliver. This calculator uses the standard formula: required injector flow (lb/hr) = (target horsepower x BSFC) / (number of injectors x maximum duty cycle). BSFC, brake-specific fuel consumption, is how much fuel (in pounds) the engine burns per horsepower per hour, and it varies by engine type -- roughly 0.45-0.50 for a naturally aspirated gasoline engine, 0.55-0.65 once forced induction is added (a turbo or supercharged engine runs richer for cooling and knock protection), and higher still for ethanol-blended fuels like E85, which need more fuel volume for the same energy.
Duty cycle is the share of each engine cycle an injector spends open; running injectors much past about 80-85% sustained duty cycle risks poor spray control and injector overheating, so the industry convention caps the sizing calculation there rather than at 100%. Because injectors are rated for flow at a specific base fuel pressure (commonly 43.5 PSI for most port-injection systems), the calculator also applies a pressure-correction factor -- flow scales with the square root of the pressure ratio, standard orifice-flow physics -- so a system running higher fuel pressure than the injector's rating needs a smaller rated injector to deliver the same real-world flow, and vice versa.
Inputs
Results
Injector size (cc/min)
361
Injector size (lb/hr)
34.4
How to Use This Calculator
- Enter Target HP, Number of injectors, and BSFC (lb/hp·hr).
- Set Max duty cycle (%) and Fuel pressure (PSI).
- Review Injector size (cc/min) and Injector size (lb/hr).
- Use Total fuel flow (lb/hr) and Fuel consumption (GPH) to inform your decision.
How the result changes with Number of injectors
| Number of injectors | Injector size (cc/min) | Injector size (lb/hr) |
|---|---|---|
| 4 | 722 | 68.8 |
| 6 | 481 | 45.8 |
| 12 | 241 | 22.9 |
| 16 | 180 | 17.2 |
What each input means
- Target HP
- Target wheel horsepower the injectors need to support.
- Number of injectors
- Number of fuel injectors (typically matches cylinder count).
- BSFC (lb/hp·hr)
- Brake-specific fuel consumption. NA gas: 0.45-0.50, turbo gas: 0.55-0.65, E85: 0.65-0.75.
- Max duty cycle (%)
- Maximum safe sustained duty cycle (80% recommended, 85% aggressive).
- Fuel pressure (PSI)
- Operating fuel rail pressure. Most port injection: 43.5 PSI, returnless: 58 PSI.
What each result means
- Injector size (cc/min)
- Minimum injector flow rate needed in cc/min (rated at 43.5 PSI base).
- Injector size (lb/hr)
- Minimum injector flow rate needed in lb/hr.
- Total fuel flow (lb/hr)
- Total fuel demand across all injectors at target HP.
- Fuel consumption (GPH)
- Total fuel consumption in gallons per hour at target HP.
- Duty cycle at target (%)
- Injector duty cycle at target HP with selected injector size.
- Max HP at 100% duty
- Absolute maximum HP these injectors can theoretically deliver.
- Pressure correction
- Flow correction factor for non-standard fuel pressure.
- Injector count
- Number of injectors used in the calculation.
How this is calculated
Worked example, using the default values
- Identify Input Parameters5 parametersTarget HP = 400, Number of injectors = 8, BSFC (lb/hp·hr) = 0.55, Max duty cycle (%) = 80, Fuel pressure (PSI) = 43.5 = 5 input(s) provided
- Calculate Injector size361 = 361
- Calculate Injector size34.4 = 34.4
- Calculate Total fuel flowTotal fuel flow = targetHp * bsfc220 = 220
- Calculate Fuel consumptionFuel consumption = totalFuelFlow / 6.07336.2 = 36.2
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does forced induction change the BSFC value used for sizing?
Turbocharged and supercharged engines commonly run richer air-fuel mixtures than naturally aspirated engines, both to help cool the combustion chamber under boost and to add a margin against knock at higher cylinder pressures. That extra fuel per horsepower raises BSFC into the roughly 0.55-0.65 lb/hp-hr range compared to about 0.45-0.50 for a comparable naturally aspirated engine, which directly increases the required injector flow for the same target power.
Why cap the sizing calculation at 80-85% duty cycle instead of 100%?
Running an injector at or near 100% duty cycle for sustained periods leaves it essentially always open, which degrades spray atomization, can shorten injector life from heat and wear, and leaves no margin for a transient fuel demand spike. Capping the sizing target around 80-85% keeps the injector operating in its well-controlled range while still leaving some headroom above the target power level.
How does raising fuel rail pressure change the injector size I need?
An injector's actual flow rate increases with fuel pressure, following the square root of the pressure ratio -- standard orifice-flow behavior. Running the fuel system at higher pressure than the injector's base rating pressure means each injector flows more than its printed rating, so a smaller rated injector can be used to hit the same real fuel-delivery target, and this calculator's pressure-correction factor accounts for that.
What does 'Max HP at 100% duty' actually represent?
It's the theoretical power ceiling the selected injector size could support if pushed to a full 100% duty cycle, calculated by scaling up from the target duty cycle used for sizing. It is not a recommended operating point -- running injectors at 100% duty sustained is the condition the duty-cycle cap exists to avoid -- so treat it as a theoretical headroom figure rather than a usable power target.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
More in Automotive & Motorcycles.