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Fuel Injector Size Calculator

Calculate required injector flow, supported flywheel horsepower, or injector duty cycle from BSFC and injector count. Convert lb/hr and cc/min using an explicit fuel-density assumption. Inputs stay in this browser tab.

Use this for port-injector planning, not direct-injection sizing Results are steady-flow estimates. Confirm injector data at the same fuel and differential pressure, then verify the complete fuel system and calibration with the component manufacturer or a qualified tuner.

Engine and injector values

The unknown field is hidden automatically.

Use realistic gross engine output, not wheel horsepower.

Use measured, fuel-specific data when available.

Total injectors sharing fuel demand at peak power.

Planned maximum on-time, not 100% static flow.

Example gasoline assumption: 0.745 kg/L. Actual density varies by blend and temperature.

Privacy: values are calculated locally. They are not uploaded, stored, or included in analytics events.

Required injector flow

Fuel injector sizing formulas

For evenly shared port injection, required rated flow per injector is:

flow (lb/hr each) = flywheel hp × BSFC ÷ (injector count × duty-cycle fraction)

Rearrange the relationship to estimate either supported power or required duty cycle:

hp = flow × injector count × duty fraction ÷ BSFC
duty fraction = hp × BSFC ÷ (flow × injector count)

BSFC in lb/(hp·h) turns horsepower into total fuel mass per hour. This calculator uses 1 mechanical hp = 0.7456998716 kW and 1 lb/(hp·h) = 608.277387 g/kWh.

To convert mass flow to volume flow using the entered density:

cc/min = lb/hr × 453.59237 ÷ (60 × density in kg/L)

How to size fuel injectors

  1. Start with realistic flywheel power. If only wheel power is known, use a justified drivetrain-loss estimate rather than entering wheel horsepower unchanged.
  2. Use appropriate BSFC. Measured dyno data is best. BSFC must match the fuel, induction, mixture, and operating point.
  3. Count every active injector. For one injector per cylinder, this normally equals cylinder count. Staged systems need system-specific treatment.
  4. Choose a duty-cycle limit. Manufacturer guidance takes priority; 80% to 90% is a common planning range in the cited Holley material.
  5. Round upward and verify. Choose the next available rated size above the calculated requirement, comparing ratings at the same differential pressure and test fuel.

BSFC, duty cycle, density, and pressure

  • BSFC: fuel mass consumed per unit of brake power and time. Lower BSFC means less fuel mass for the same power.
  • Duty cycle: the fraction of available time the injector is commanded open. Static flow ratings describe continuous flow; planning below 100% preserves operating margin.
  • Fuel density: affects only lb/hr ↔ cc/min conversion here. It does not change the mass-flow requirement produced by horsepower and BSFC.
  • Differential pressure: rail pressure minus pressure at the injector outlet. Flow ratings are meaningful only with their stated test pressure and fluid.

Pressure correction: for the same injector and fluid, the idealized relationship is new flow = rated flow × √(new differential pressure ÷ rated differential pressure). Do not raise pressure from this equation alone; pump delivery, regulator behavior, component pressure limits, spray quality, and calibration also change.

Fuel injector sizing FAQs

What is the fuel injector sizing formula?

Multiply flywheel horsepower by BSFC, then divide by injector count and duty cycle as a decimal. The result is required static mass flow for each injector in lb/hr when BSFC is in lb/(hp·h).

What duty cycle should I use?

The cited Holley references describe 80% to 90% as a normal planning range and 90% as a maximum in their general guidance. Use the injector, ECU, engine-speed, and tuner requirements for the actual installation.

What BSFC should I use?

Use measured or engine-builder data when possible. Holley lists broad gasoline examples of 0.50–0.55 lb/(hp·h) for low-to-medium-performance street engines, 0.45–0.50 for performance engines with good cylinder heads, and 0.55–0.65 for supercharged or turbocharged engines. These are starting ranges, not guarantees.

Can I enter wheel horsepower?

The cited formula uses flywheel horsepower. Wheel horsepower is lower because it is measured after drivetrain losses. Convert it with a justified vehicle-specific loss assumption or use a credible flywheel estimate.

Why does cc/min change with fuel density?

cc/min measures volume while lb/hr measures mass. The same mass occupies a different volume when density changes. Injector test fluid, temperature, and published rating conditions therefore matter.

Does boost pressure change injector flow?

Flow depends on differential pressure across the injector. A correctly configured manifold-referenced regulator is intended to maintain that difference as manifold pressure changes. Check rail pressure under load rather than assuming the regulator and pump maintain it.

Can I use this for direct injection?

No. Direct injectors operate within a limited crank-angle window at much higher pressure, and their usable delivery depends on engine speed, injection timing, pump capacity, and manufacturer data.

Are my values tracked?

No. The calculation runs locally in your browser and does not upload, store, or attach input values to analytics events.

Limits and safety disclaimer

  • The estimate assumes all injectors share demand evenly and deliver their rated flow. It does not model injector dead time, nonlinear short pulses, ECU limits, staged injection, cylinder imbalance, or transient enrichment.
  • It does not size the pump, wiring, relay, filter, rail, regulator, lines, fittings, or return system, or predict pressure drop at load.
  • Published flow may use a different pressure, fuel, or test fluid. Volume-flow conversions are only as accurate as the entered density.
  • Fuel compatibility, electrical impedance, connector, spray pattern, physical fit, and calibration data must also match the application.

Safety disclaimer: Pressurized fuel can cause fire, injury, and engine damage. This result is an initial engineering estimate, not installation or tuning approval. Follow component and vehicle manufacturer limits, leak-test the system safely, and have fuel-system changes calibrated and inspected by a qualified professional.

Methodology and sources

Last reviewed: August 2, 2026. The sizing formula, duty-cycle guidance, BSFC examples, and pressure-flow relationship were checked against technical material published by Holley:

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