This boost horsepower calculator estimates how much power a turbocharger or supercharger adds to a naturally aspirated engine, based on boost pressure in PSI. Enter your baseline NA horsepower and target boost to see the projected crank horsepower — a fast way to plan a turbo or blower build before committing to parts.
Boost Horsepower Formula
Forced induction makes power by cramming more air into the cylinders than the atmosphere alone can. Since horsepower scales almost directly with air mass burned, the pressure ratio — absolute manifold pressure divided by atmospheric pressure — is what multiplies your baseline power. At 14.7 PSI of boost the engine sees roughly twice atmospheric pressure (a 2.0 pressure ratio), which is why power nearly doubles on paper.
Why Real Gains Fall Short of the Math
The formula assumes the engine breathes just as efficiently under boost as without — which never quite holds. Charge heat is the main culprit: compressing air heats it, making it less dense, so an intercooler is critical to recover power. Backpressure from the turbine, ignition timing pulled to prevent knock, and the engine's own volumetric efficiency all eat into the theoretical figure. A realistic street result is often 10–20% below the calculator's number unless the setup is well-intercooled and properly tuned.
Turbo vs Supercharger
Both raise manifold pressure, so this formula applies to either. A turbocharger is driven by exhaust energy — efficient and capable of big numbers, but with some lag before it spools. A supercharger is belt-driven off the crank — instant response, but it consumes some engine power to spin. For the same boost pressure the air-density gain is similar; the difference is in how and when that boost arrives.
Estimated Power by Boost Level
Projected crank HP for a 300 HP naturally aspirated baseline (theoretical, before heat/tuning losses):
| Boost (PSI) | Pressure ratio | Est. HP (300 NA) |
|---|---|---|
| 5 | 1.34 | 402 |
| 7 | 1.48 | 443 |
| 10 | 1.68 | 504 |
| 14.7 | 2.00 | 600 |
| 20 | 2.36 | 708 |
Treat these as ceilings. Knock-limited timing and intake heat mean a safe, reliable tune usually targets somewhat less, especially as boost climbs past 14–15 PSI on pump fuel.
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Frequently Asked Questions
Roughly 6-7% of the engine's naturally aspirated horsepower.
Turbos are generally more efficient because they run on exhaust gas, while superchargers rob some engine power via a belt to compress the air.
As a rough rule, each additional PSI of boost adds roughly 7% over the naturally aspirated power, though this varies with engine efficiency, intercooling, and tuning.
A turbocharger is driven by exhaust gas and is more efficient but can lag; a supercharger is belt-driven for instant response but consumes engine power to run.
Compressing air heats it, and hot air is less dense. An intercooler cools the charge back down, packing more oxygen into each cylinder for more power and less knock risk.