Engine Horsepower Calculator

Estimate engine horsepower using torque & RPM, quarter‑mile ET, trap speed, displacement & boost, or simple HP↔kW conversion. Ideal for tuners, racers, and engine builders.

Horsepower from Torque & RPM

Use this when you know engine torque at a given RPM (dyno sheet or manufacturer data).

% (0–40%)

Use ~10–15% for FWD/RWD manuals, 15–20% for automatics, 20–25% for AWD.

Engine horsepower formulas used

This engine horsepower calculator combines several popular methods used by racers, tuners, and engine builders. Here are the core formulas.

1. Horsepower from torque and RPM

Imperial units (lb·ft):

\( \text{HP} = \dfrac{T_{\text{lb·ft}} \times \text{RPM}}{5252} \)

Metric units (N·m):

\( \text{HP} = \dfrac{T_{\text{N·m}} \times \text{RPM}}{7127} \)

The constants 5252 and 7127 come from unit conversions between torque, rotational speed, and power.

2. Horsepower from 1/4‑mile ET

A commonly used empirical drag‑race formula is:

For weight in pounds and ET in seconds:

\( \text{HP} \approx \dfrac{W_{\text{lb}}}{\left(\dfrac{\text{ET}}{5.825}\right)^3} \)

If you enter weight in kilograms, the calculator converts it to pounds internally.

3. Horsepower from trap speed

Trap‑speed based horsepower is less sensitive to traction:

For weight in pounds and trap speed in mph:

\( \text{HP} \approx W_{\text{lb}} \times \left(\dfrac{V_{\text{mph}}}{234}\right)^3 \)

When you enter km/h, the calculator converts to mph before applying the formula.

4. Rough horsepower from displacement & boost

This tab uses a simplified airflow‑based estimate:

  1. Convert displacement to liters if needed.
  2. Compute pressure ratio: \( \text{PR} = 1 + \dfrac{P_{\text{boost}}}{P_{\text{atm}}} \), with \( P_{\text{atm}} \approx 14.7\ \text{psi} \).
  3. Estimate airflow and brake specific fuel consumption (BSFC).
  4. Approximate power using a typical 9–11 hp per lb/min of air rule of thumb.

This is only a planning tool. Real engines are limited by fuel, timing, intercooling, knock resistance, and mechanical strength.

5. Wheel horsepower vs crank horsepower

If you know drivetrain loss as a percentage, you can convert between wheel and crank horsepower:

\( \text{BHP} = \dfrac{\text{WHP}}{1 - \dfrac{L}{100}} \)

where \( L \) is drivetrain loss in percent.

How to use this engine horsepower calculator

Method A – Using torque & RPM

  1. Select the Torque & RPM tab.
  2. Enter torque from your dyno sheet or spec (choose lb·ft or N·m).
  3. Enter the corresponding engine speed in RPM.
  4. Optionally enter drivetrain loss to estimate crank horsepower from wheel horsepower.
  5. Click Calculate horsepower to see WHP and BHP in hp and kW.

Method B – Using 1/4‑mile ET

  1. Select the 1/4‑Mile ET tab.
  2. Enter your best ET and race weight (car + driver).
  3. Choose lb or kg for weight.
  4. Click Calculate horsepower to get estimated flywheel horsepower.

Method C – Using trap speed

  1. Select the Trap Speed tab.
  2. Enter quarter‑mile trap speed and race weight.
  3. Click Calculate horsepower to see estimated crank horsepower.

Method D – Using displacement & boost

  1. Select the Displacement & Boost tab.
  2. Enter engine size, boost level, volumetric efficiency, and target RPM.
  3. Click Estimate horsepower for a rough crank HP estimate.

Method E – Converting HP ↔ kW

  1. Select the HP ↔ kW tab.
  2. Enter either horsepower or kilowatts.
  3. Click the appropriate conversion button.

Typical drivetrain loss percentages

  • FWD manual: 10–15%
  • RWD manual: 12–18%
  • Automatic (RWD/FWD): 15–22%
  • AWD performance car: 20–25%+

These are only guidelines. The best way to know actual loss is to compare engine‑dyno and chassis‑dyno data for the same setup.

Engine horsepower calculator FAQ

How do I calculate engine horsepower from torque?

Use the classic formula:

\( \text{HP} = \dfrac{T_{\text{lb·ft}} \times \text{RPM}}{5252} \)

Enter torque and RPM in the Torque & RPM tab and the calculator will do this automatically.

What is the difference between wheel HP and crank HP?

Crank HP (bhp) is measured at the engine’s crankshaft. Wheel HP (whp) is measured at the driven wheels. Friction and inertia in the transmission, differential, and tires cause losses, so whp is always lower than bhp.

Are ET and trap‑speed horsepower estimates accurate?

They are good for ballpark comparisons but not a replacement for a proper dyno. Track conditions, gearing, launch technique, and aerodynamics all influence ET and trap speed.

Can I use this calculator for motorcycles, boats, or diesel engines?

Yes. The physics is the same, but empirical drag‑race formulas (ET and trap speed) were tuned for typical cars. For non‑automotive applications, torque & RPM or dyno data will be more reliable.

Is brake horsepower (bhp) the same as mechanical horsepower?

In most automotive contexts, yes. Brake horsepower is power measured at the crankshaft on a brake dynamometer, and it is usually expressed in mechanical horsepower (1 hp ≈ 0.7457 kW).