Work out what size fuel injectors your engine needs. Enter your horsepower goal, engine type (BSFC), number of cylinders, maximum duty cycle and fuel, and get the minimum injector flow in lb/hr and cc/min at its rated pressure, the nearest common size, total fuel flow and the horsepower each injector size can support.
Last updated:
Power target
hp
%
Typical 10–15% manual RWD, 15–20% automatic or AWD.
Engine & fuel
lb/hp·hr
Brake specific fuel consumption on gasoline.
%
80% is a safe target; above ~85–90% injectors may not close reliably.
Fuel pressure
psi
Test pressure on the flow sheet; 43.5 psi = 3 bar is common.
psi
psi
Enter your values to see the result.
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How to use the Fuel Injector Size Calculator
Enter your target horsepower and whether it is crank or wheel horsepower.
Pick the engine type to set a typical BSFC, or enter your own, and choose the fuel.
Select the number of cylinders, injectors per cylinder and the maximum duty cycle you want to run (80% is a safe default).
Enter the fuel pressures: the injector's rated test pressure, your base pressure and peak boost.
Read the minimum injector size and choose the nearest common size at or above it from the table.
Fuel injector sizing formula
Total fuel (lb/hr) = crank horsepower × BSFC
Injector size (lb/hr) = total fuel ÷ (number of injectors × maximum duty cycle)
Example: a turbo 4-cylinder making 500 whp on E85 with 15% drivetrain loss: 500 ÷ 0.85 = 588 crank hp; BSFC 0.60 × 1.40 = 0.84; 588 × 0.84 = 494 lb/hr total; ÷ (4 × 0.80) = 154 lb/hr per injector, about 1,620 cc/min.
Horsepower supported by common injector sizes
Injector (at rated pressure)
cc/min
4-cyl NA gas (0.50)
4-cyl turbo gas (0.60)
4-cyl turbo E85
8-cyl turbo gas
24 lb/hr
250
154
128
91
256
30 lb/hr
315
192
160
114
320
36 lb/hr
380
230
192
137
384
42 lb/hr
440
269
224
160
448
52 lb/hr
550
333
277
198
555
60 lb/hr
630
384
320
229
640
72 lb/hr
750
461
384
274
768
80 lb/hr
840
512
427
305
853
95 lb/hr
1000
608
507
362
1,013
120 lb/hr
1250
768
640
457
1,280
Crank horsepower at 80% duty cycle and the rated fuel pressure. Double the 4-cylinder figures for a V8 with the same BSFC, or multiply by 1.5 for a 6-cylinder.
Tips for choosing injectors
Size for the fuel you might run. If E85 or a flex-fuel kit is possible later, size for E85 now.
Do not oversize wildly. Very large injectors can make idle and low-speed drivability harder to tune because of short pulse widths.
Check the impedance and connector. High-impedance (saturated) injectors suit most factory ECUs; low-impedance (peak-and-hold) ones need a compatible driver.
Match the fuel system. Pump flow falls as pressure rises, so check the pump's flow chart at base pressure plus boost.
Use real injector data. Flow at your pressure and dead times (offsets) at different voltages are essential for an accurate tune.
Frequently asked questions
How do you calculate fuel injector size?
Injector flow (lb/hr) = horsepower × BSFC ÷ (number of injectors × maximum duty cycle). For 400 crank horsepower on a turbocharged gasoline 6-cylinder with BSFC 0.60 and 80% duty: 400 × 0.60 ÷ (6 × 0.80) = 50 lb/hr per injector, about 525 cc/min.
What is BSFC?
Brake specific fuel consumption is the mass of fuel an engine burns per horsepower per hour (lb/hp·hr). Typical gasoline values are about 0.45–0.50 for efficient naturally aspirated engines, 0.55–0.60 for supercharged and 0.60–0.65 for turbocharged engines, which run richer and hotter under boost. Lower BSFC means fewer pounds of fuel per horsepower.
How do I convert lb/hr to cc/min?
The common industry conversion is 1 lb/hr ≈ 10.5 cc/min (based on a gasoline-like test fluid density of about 0.72 g/cc). So a 42 lb/hr injector is about 440 cc/min, and a 1,000 cc/min injector is about 95 lb/hr. The exact factor depends on the fuel's density.
How much bigger do injectors need to be for E85?
E85 needs roughly 30–45% more fuel by volume than gasoline for the same power because ethanol contains less energy and runs at a richer stoichiometric ratio (about 9.8:1 vs 14.7:1). This calculator uses a 1.40 multiplier for E85, 1.12 for E30 and 2.2 for methanol. Flex-fuel builds should be sized for the ethanol case.
What duty cycle should I size injectors for?
Most tuners size for a maximum of about 80% duty cycle at peak power. That leaves headroom for hot fuel, voltage changes and future power, and keeps the injector out of the range near 100% where it may not fully close between events. Some modern injectors are rated for higher duty, but 80–85% is a sensible target.
How does fuel pressure change injector flow?
Flow rises with the square root of the pressure across the injector: new flow = rated flow × √(new pressure ÷ rated pressure). A 42 lb/hr injector rated at 43.5 psi flows about 48.5 lb/hr at 58 psi. On boosted engines a 1:1 rising-rate regulator keeps the pressure difference constant; without one, boost subtracts from the effective pressure.
Is the calculated size all I need to change?
No. Larger injectors need new injector data (dead times and flow) in the engine computer, and the fuel pump, lines and regulator must supply the total flow at your pressure. Have the engine tuned on a dyno or with wideband data after any injector change.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.