About the Boost Horsepower Calculator
Adding boost — via a turbocharger or supercharger — forces more air into an engine's cylinders than atmospheric pressure alone would provide. Because an engine's power output tracks roughly with how much air (and matching fuel) it burns per cycle, boosting the intake charge above atmospheric pressure increases the mass of air available and, with it, the horsepower the engine can produce. This calculator applies the standard rule-of-thumb formula used throughout the automotive tuning world to estimate that gain.
The formula
The calculation compares absolute pressure before and after boost is added:
- New HP = Stock HP × (Atmospheric Pressure + Boost PSI) ÷ Atmospheric Pressure
- Atmospheric pressure at sea level is standard at 14.7 psi. A boost gauge reads pressure above atmospheric, so 10 psi of boost means the intake manifold sits at 14.7 + 10 = 24.7 psi absolute.
- The ratio of absolute pressures (24.7 / 14.7 ≈ 1.68 for 10 psi) is used as a multiplier on the naturally aspirated horsepower to estimate the boosted figure.
How to get accurate results
- Enter your engine's naturally aspirated (unboosted) horsepower rating, not an already-boosted figure.
- Enter boost pressure in psi as read from a boost gauge, which is pressure above atmospheric, not absolute pressure.
- Adjust atmospheric pressure only if you are at meaningful altitude; 14.7 psi (sea level) is correct for most locations.
Practical context
This is a theoretical estimate based on the pressure ratio alone. It assumes the engine can supply enough additional fuel and appropriate ignition timing to safely use the extra air — it does not account for intercooler efficiency, injector or fuel pump capacity, turbo lag, or knock limits, all of which determine how much boost an engine can actually run in practice. Treat the result as a planning figure and validate real-world gains with a dyno.