Forced induction packs more air – and therefore more fuel – into each cylinder. As a first approximation, power rises in proportion to the absolute manifold pressure, so 14.7 psi of boost at sea level roughly doubles the power. In practice heat, pumping losses and compressor efficiency take a share, so many tuners subtract 10–20%.
How to use the Boost Horsepower Calculator
- Naturally aspirated power
- Boost pressure
- Atmospheric pressure
- Real-world efficiency allowance
- Every result field is editable: type into Boosted power and Boost pressure plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the engine’s naturally aspirated power, the boost pressure and, if you like, an efficiency allowance. Type the power you want into the result to find the boost needed.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Naturally aspirated power | In hp |
| Boost pressure | Units: psi, bar, kPa |
| Atmospheric pressure | Units: psi, bar, kPa |
| Real-world efficiency allowance | In % |
What each result means
- Boosted power = [Naturally aspirated power]×(1 + ([Boost pressure]÷[Atmospheric pressure])×[Real-world efficiency allowance]÷100)
- Power gain = [Boosted power] – [Naturally aspirated power]
- Pressure ratio = ([Boost pressure] + [Atmospheric pressure])÷[Atmospheric pressure]
Example values
How the main result changes when Boost pressure changes and everything else stays at the example values above:
| Boost pressure (psi) | Boosted power (hp) |
|---|---|
| 3 | 241 |
| 4.5 | 261 |
| 6 | 282 |
| 7.5 | 302 |
| 9 | 322 |
| 12 | 363 |
Formula
Boosted power = naturally aspirated power × (boost + atmospheric pressure) ÷ atmospheric pressureWorked example
Inputs — Naturally aspirated power: 200 hp; Boost pressure: 6 psi; Atmospheric pressure: 14.6959 psi; Real-world efficiency allowance: 100 %.
Results — Boosted power = 282 hp; Power gain = 82 hp; Pressure ratio = 1.41.
Frequently asked questions
How much horsepower does 10 psi of boost add?
In theory about 68% (10 ÷ 14.7); real gains are often 50–60% after heat and losses.
Does an intercooler help?
Yes – cooler, denser air gets closer to the theoretical gain and reduces knock.
Is boost at altitude different?
Atmospheric pressure is lower at altitude, so the same gauge boost gives a smaller pressure ratio increase relative to sea-level power.
What formula does the Boost Horsepower Calculator use?
Boosted power = naturally aspirated power × (boost + atmospheric pressure) ÷ atmospheric pressure.