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Boost to horsepower calculator

A quick estimate of what boost adds to a naturally aspirated engine. Pressure ratio does most of the work, and an efficiency allowance accounts for heat and drive losses.

Enter your numbers
Engine and boost
Lower for no intercooler or a belt-driven blower, higher for a well-intercooled turbo.

Estimated horsepower

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Pressure ratio
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With no losses
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Gain over naturally aspirated
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Air pressure at your altitude
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Fuel and timing come first

Boost raises cylinder pressure, so the fuel system and ignition timing have to keep up. The injector and fuel pump calculators size the fuel side for your target.

How the estimate works

Pressure ratio = (boost + atmospheric pressure) ÷ atmospheric pressure
Estimated hp = naturally aspirated hp × pressure ratio × efficiency allowance

Boost packs more air into each cylinder. At sea level, 14.7 psi of boost doubles the air pressure, so an engine would ideally make about twice its naturally aspirated power. Real engines fall short because compressed air gets hot and less dense, and superchargers take power to drive.

The efficiency allowance is a rough correction for those losses, not a measured value. At altitude, atmospheric pressure is lower, so the same gauge boost produces a higher pressure ratio but the naturally aspirated baseline is also lower.

Frequently asked questions

Why isn't boost horsepower a straightforward doubling of pressure?

Real-world losses — heat from compression, drivetrain and pumping losses, and how efficiently the engine actually uses the extra air — mean the power gain never fully matches the theoretical pressure increase, which is exactly what the efficiency allowance in this calculator accounts for.

What efficiency allowance should I use?

It varies by setup — a well-tuned, intercooled system runs more efficiently than a basic setup with more heat and pressure loss, so treat the default as a reasonable planning estimate and adjust it based on real dyno data from similar builds if you have it.

Does this work the same for a supercharger as a turbo?

The underlying pressure-ratio math is the same for any way of adding boost, though a turbo and a supercharger have different real-world efficiency characteristics and parasitic losses — the efficiency allowance is where that real difference shows up in the estimate.

More Fuel & Forced Induction calculators

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Read the guide

Turbo and Supercharger Basics: Boost, Pressure Ratio and Reliability

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