Horsepower Calculator

Calculate horsepower, torque, or RPM

Select the value you want to find, then enter the other two values measured at the same operating point. The calculator solves the rotational power relationship in either direction and reports mechanical horsepower, kilowatts, metric horsepower, pound-feet, newton-meters, and revolutions per minute.

horsepower = torque (lb-ft) x RPM / 5252.113

torque (lb-ft) = horsepower x 5252.113 / RPM

RPM = horsepower x 5252.113 / torque (lb-ft)

Horsepower calculation example

An engine producing 300 lb-ft at 4,000 RPM makes about 228.48 mechanical horsepower. That is approximately 170.38 kW or 231.65 metric horsepower (PS). The same entered torque equals about 406.75 N-m. These values describe one point on the engine's torque curve, not necessarily its peak output.

Why the formula uses 5252

One mechanical horsepower is 33,000 foot-pounds of work per minute. A rotating shaft travels 2 pi radians per revolution, so dividing 33,000 by 2 pi produces approximately 5252.113. At about 5,252 RPM, horsepower and torque in lb-ft have the same numerical value. The relationship still applies above and below that speed; only the numerical comparison changes.

Mechanical hp, kW, and metric PS

This calculator treats hp as mechanical horsepower, the unit commonly used for US vehicle specifications. One mechanical horsepower is approximately 745.7 watts. Metric horsepower, often written PS, is slightly smaller at approximately 735.5 watts. The NIST Guide to the SI provides unit definitions and conversion references, while the watt is the SI unit of power.

Crank horsepower versus wheel horsepower

The formula does not estimate drivetrain loss. Use torque measured at the crankshaft to calculate crank horsepower, or wheel torque from a chassis dynamometer to calculate wheel horsepower. Do not mix crank torque with wheel RPM or compare results from different test conditions as if they were the same measurement.

Rated power and dynamometer results

Vehicle power claims depend on the test method, correction conditions, installed accessories, and reporting standard. SAE J1349 describes a repeatable method for installed net engine power and torque ratings. This browser calculator performs the mathematical conversion only; it does not certify an engine or correct a dyno run for atmospheric conditions.

Use values from the same RPM

Torque and horsepower vary across the operating range. To verify a dyno point, pair the torque reading with the RPM shown at that exact point. Peak torque and peak horsepower usually occur at different RPM, so combining two separate peak specifications produces a result that does not represent a real operating point.

Related performance calculators

After calculating power, use the power-to-weight ratio calculator to account for vehicle mass. The tire size calculator compares diameter and speedometer effects, while the towing capacity calculator checks vehicle and trailer limits.

Frequently asked questions

Why do horsepower and torque cross at 5,252 RPM?

When RPM equals the 5252.113 conversion constant, multiplying torque by RPM and dividing by that same number leaves the torque value unchanged.

Can I use newton-meters?

Yes. Select N-m and the calculator converts the entered torque to lb-ft before solving, then reports both units.

Is horsepower constant at every RPM?

No. Horsepower changes with both torque and shaft speed. A single calculation represents one operating point.