How do you calculate fuel injector size?

A common sizing method is: injector lb/hr = (target horsepower × BSFC) ÷ (number of cylinders × duty cycle fraction). This calculator also converts to cc/min using a gasoline approximation.

What BSFC should I use for injector sizing?

Many naturally aspirated street engines use roughly 0.45–0.55, while boosted or richer-tuned combos are often higher (commonly 0.60–0.80). Choose a value that matches your combo and tuning goals.

What duty cycle should I target?

Most street and performance builds aim to keep maximum injector duty cycle in the 80–90% range to maintain headroom and stable fueling.

Is cc/min conversion exact?

No. cc/min conversion depends on fuel type, injector characterization, and test pressure. This tool uses a common gasoline estimate (cc/min ≈ lb/hr × 10.5) for planning.

EFI / FUEL SYSTEM TOOL

Fuel Injector Size Calculator (lb/hr & cc/min from HP, BSFC & Duty Cycle)

Use this fuel injector size calculator to estimate the injector flow rate you need per cylinder based on target horsepower, BSFC (brake specific fuel consumption), and max injector duty cycle. Results are shown in lb/hr and cc/min, with a suggested common injector size so you can shop confidently for your build.

How to Use the Injector Sizing Calculator

Enter your target crank horsepower.
Enter cylinder count (4/6/8/etc.).
Choose BSFC (NA is usually lower, boosted is higher).
Set duty cycle (most street combos aim for 80–90% max).
Click Calculate Injector Size to get lb/hr and cc/min per injector.

Planning the whole combo? Start here: Calculators & Tools.

BSFC & Duty Cycle Tips (So You Don’t Size It Wrong)

BSFC is your “fuel demand” multiplier

Lower BSFC generally means a more efficient combo. Naturally aspirated street engines are often around 0.45–0.55, while boosted, nitrous, or richer-tuned combos often need more fuel (commonly 0.60–0.80).

Leave headroom

Running injectors at 100% duty cycle removes safety margin. Aiming for 80–90% max duty helps keep tuning stable and gives you room for weather, fuel changes, and future upgrades.

If you’re also sizing airflow, pair this with: Carb CFM Calculator and Engine Displacement Calculator.

Shop Fuel Injectors by Size & Brand

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After you calculate your required lb/hr, pick a common size slightly above your target for safe headroom, then confirm impedance, connector style, fuel type, and fuel pressure requirements.

Popular injector brands

More build tools: Valve Spring Calculator • All Tools

Fuel Injector Sizing FAQ

Why does this calculator use BSFC?

BSFC estimates how much fuel your engine needs per horsepower per hour. It’s a practical way to size injectors when you know target HP but don’t have full airflow/fuel modeling.

Should I size injectors to the exact number?

Usually no. Most builders choose the next common injector size up for headroom and easier tuning, especially for boosted or future-upgrade combos.

How accurate is cc/min conversion?

This tool uses a common gasoline approximation (cc/min ≈ lb/hr × 10.5). Exact conversion depends on fuel type, injector characterization, and test pressure—use this as a solid estimate, then verify with injector data.

Fastime Performance Fuel Injector Size Calculator

Injector size per cylinder based on horsepower, BSFC and duty cycle.

Injector Sizing Result

Per Injector 36.8 lb/hr lb/hr at chosen duty

Per Injector 386 cc/min cc/min (approx)

Suggested Common Size 36 lb/hr Rounded to nearest typical size

Injector lb/hr per cylinder = HP × BSFC ÷ (Cylinders × DutyFraction).
cc/min ≈ lb/hr × 10.5 (gasoline, approximate).