How this instrument works
Forced induction — a turbocharger or supercharger — works by packing more air into the engine's cylinders than atmospheric pressure alone would deliver, and more air (with proportionally more fuel) means a bigger combustion event and more power per stroke. The boost-ratio estimate used here treats horsepower gain as roughly proportional to how much you've raised the total absolute pressure above the engine's normal, naturally-aspirated baseline.
Standard atmospheric pressure at sea level is about 14.7 psi — that's what a naturally-aspirated engine breathes against with zero boost. Adding, say, 10 psi of boost raises the total absolute pressure feeding the engine to 24.7 psi, a ratio of roughly 1.68x over the naturally-aspirated baseline. This calculator applies that same ratio directly to horsepower: boosted HP = naturally-aspirated HP x (boost PSI + atmospheric PSI) ÷ atmospheric PSI.
This is a widely-used back-of-envelope rule of thumb, not a precise engineering simulation — real boosted horsepower also depends on intercooler efficiency, fuel delivery capacity, ignition timing, engine internals' tolerance for higher cylinder pressure, and plenty else that a single pressure-ratio formula can't capture. Treat the output as a reasonable ballpark for comparing boost levels, not a guaranteed dyno number for a specific build.
- Enter Naturally-aspirated horsepower — your engine's rated or measured HP with no boost applied.
- Enter Boost pressure (PSI) — the boost level you're running or considering.
- Leave Baseline atmospheric pressure at 14.7 PSI unless you're at meaningfully high altitude, where it's lower.
- Read Estimated boosted horsepower — the boost-ratio estimate for that combination.
- Try a few boost levels back to back to see how sensitive estimated power is to each additional PSI of boost.
Worked example — 300 hp at 10 psi of boost
A 300 hp naturally-aspirated engine running 10 psi of boost, against standard 14.7 psi atmospheric pressure: boosted HP = 300 x (10 + 14.7) ÷ 14.7 = 300 x 1.6803 = 504.08 hp — roughly a 68% power increase for 10 psi of boost.
Push the same engine to exactly one full atmosphere of boost (14.7 psi, doubling total absolute pressure) and the estimate becomes a clean doubling: 300 x (14.7 + 14.7) ÷ 14.7 = 600 hp exactly — a useful sanity check showing the formula behaves the way basic pressure-ratio intuition predicts at that specific round number.
Questions
Is this formula accurate enough to predict my actual dyno numbers?
No — treat it as a rough planning estimate, not a substitute for an actual dyno pull. Real boosted power depends on fuel system capacity, intercooling efficiency, ignition timing adjustments, and how much cylinder pressure the engine's internals can safely tolerate, none of which this simple pressure-ratio formula accounts for.
Why does the formula use 14.7 psi as the baseline?
14.7 psi is the standard sea-level atmospheric pressure a naturally-aspirated engine already breathes against with zero added boost — it's the 'zero boost' reference point the ratio is measured from. At meaningfully high altitude, ambient atmospheric pressure is genuinely lower, which is why the baseline is left as an editable input rather than hard-coded.
Does more boost always mean proportionally more horsepower?
Only up to a point, and only in this simplified model — in reality, an engine's fuel system, ignition timing, and structural limits eventually cap how much extra boost can be safely and effectively converted into extra power, and pushing boost past what the supporting hardware can handle risks engine damage rather than more power. This calculator's linear ratio doesn't know where that real-world ceiling is for your specific engine.
What's the difference between a turbocharger and a supercharger for this calculation?
Nothing, for this specific formula — both are forced-induction systems that raise intake air pressure above atmospheric, and the boost-ratio estimate only cares about the resulting PSI figure, not which mechanism produced it. Turbos and superchargers differ in how they're driven and in their power delivery characteristics, but that distinction doesn't change this particular pressure-ratio calculation.
What does 0 psi of boost return, and why does that matter?
It returns your original naturally-aspirated horsepower figure unchanged — 300 x (0 + 14.7) ÷ 14.7 = 300 hp exactly — which is a useful sanity check confirming the formula correctly reduces to 'no change' when there's genuinely no boost being applied.