BSFC presets: NA gasoline ~0.50, FI gasoline ~0.60, E85 FI ~0.70.
Size for ~80–85% max duty — not 100% — so you keep pulse control and heat margin.
Undersized injectors max out before the engine makes the power you planned; oversized ones can still work if the ECU can control short pulses. This free injector sizing calculator turns a horsepower target, fuel BSFC, injector count, and duty-cycle ceiling into a recommended lb/hr flow (with approximate cc/min for shopping).
Defaults assume a 500 hp forced-induction gasoline build with eight injectors, BSFC 0.60, and 80% duty — about 46.9 lb/hr (~492 cc/min). Swap the BSFC preset for NA pump gas or E85 on boost, toggle hp/kW to match your dyno sheet, and optionally enter your current injectors to see margin.
Pair this with the horsepower and torque calculators when you are working from dyno numbers, and with engine displacement when the short-block size is still in flux. Confirm pump volume and rail pressure separately — this tool sizes injectors, not the whole fuel system.
Sizing examples at 80% duty, eight injectors
BSFC is the lever people skip. The same 500 hp on E85 wants more fuel than a pump-gas turbo car, which wants more than a naturally aspirated mill. Round up to a common injector size and leave duty headroom — do not shop the exact lb/hr as if it were a catalog SKU.
Required injector flow at 80 percent duty
Target
BSFC
Required (lb/hr each)
400 hp NA gasoline
0.50
31.3
500 hp FI gasoline
0.60
46.9
600 hp E85 FI
0.70
65.6
lb/hr = (HP × BSFC) ÷ (injectors × duty). cc/min ≈ lb/hr × 10.5 for gasoline-density shopping only. Confirm flow at your rail pressure.
How to use the injector sizing calculator
Enter target horsepower. Use crank or wheel HP you actually plan to support — not a hopeful dyno number you have not built for yet.
Set injector count and BSFC. Match cylinder/injector count. Use a BSFC preset (NA gas, FI gas, or E85 FI) or enter a custom value.
Choose max duty cycle. Leave headroom — 80–85% is typical. Avoid sizing for 100% duty.
Read required flow. The fuel bay shows lb/hr and approximate cc/min. Optionally enter your current injectors to see margin.
Injector sizing FAQ
Required flow (lb/hr) = (horsepower × BSFC) ÷ (number of injectors × max duty cycle). Pick a BSFC that matches your fuel and boost, leave headroom at ~80–85% duty, then round up to a common injector size.
Typical starting points: naturally aspirated gasoline ~0.50 lb/hp·hr, forced-induction gasoline ~0.60, and E85 on boost ~0.70. Richer targets, poor VE, or methanol need higher BSFC — when unsure, size up.
At 100% the injector never closes, so you lose pulse control, heat the coil, and have no margin for heat soak or a bit more power. Most tuners size for 80–85% max duty at peak load.
cc/min ≈ lb/hr × 10.5 is a common gasoline-density approximation. Actual density and manufacturer ratings vary — treat cc/min as a shopping guide and confirm the vendor’s rated flow at your fuel pressure.
No. This sizes static flow for a power target. Higher rail pressure raises delivered flow; low impedance, short pulse, and voltage still matter at idle. Use the result as a sizing floor, then verify with your ECU and pump capacity.