Torque, power and RPM.

Worked example

An engine makes 300 lbf·ft of torque at 5,000 rpm. Quick check: 300 × 5,000 ÷ 5,252 = 285.6 hp. In SI units: 300 lbf·ft = 406.7 N·m, and power = torque × 2π × rpm ÷ 60 = 406.7 × 2π × 5,000 ÷ 60 = 212,971 W = 213.0 kW. Dividing by 745.6999 W per mechanical horsepower gives 285.6 hp; in metric horsepower (735.4988 W) it is 289.6 PS.

How it works

Power equals torque times angular speed. With speed in rpm the angular speed is 2π × rpm ÷ 60 radians per second, so P (W) = T (N·m) × 2π × rpm ÷ 60. Solving for torque or speed just rearranges the same line. Mechanical horsepower is 745.6999 W, metric horsepower (PS) is 735.4988 W and electrical horsepower is 746 W.

In imperial units the familiar shortcut is hp = lbf·ft × rpm ÷ 5,252. The constant 5,252 is the rounded value of 33,000 ft·lbf per minute divided by 2π, so the answer from the shortcut differs from the exact one by about 0.002%. It also means torque in lbf·ft and power in hp are equal at about 5,252 rpm.

The calculator uses exactly the torque you type. Engine-crank torque and wheel torque differ by drivetrain losses, which it does not estimate.

Sources and basis: Horsepower (Wikipedia); Torque (Wikipedia); NIST Guide to the SI, Appendix B: Conversion Factors. Checked 2026-10-08. Planning and learning aid only. It is not for structural, pressure-system or other safety-critical design, and it is not professional engineering advice. Have a qualified engineer check anything that carries people or could fail dangerously.

FAQ

How do you convert torque to horsepower?

Multiply torque in lbf·ft by rpm and divide by 5,252 to get mechanical horsepower. In SI, power in watts is torque in N·m times 2π times rpm divided by 60, and one mechanical horsepower is 745.6999 W.

Why is the constant 5,252?

It is the rounded value of 33,000 ft·lbf per minute divided by 2π. Because of it, torque in lbf·ft and power in hp are numerically equal at about 5,252 rpm.

What is the difference between hp, PS and kW?

A mechanical horsepower is 745.6999 W, a metric horsepower (PS) is 735.4988 W and an electrical horsepower is 746 W. One kW is about 1.341 mechanical hp.

How do I find torque from kW and rpm?

Torque in N·m is power in watts divided by 2π × rpm ÷ 60. For 100 kW at 6,000 rpm that is 100,000 ÷ 628.3 = 159.2 N·m. Leave the torque field blank and enter power and rpm.

Is this engine power or wheel power?

It is whatever the torque you enter represents. Crankshaft torque gives crankshaft power; wheel torque gives wheel power. Drivetrain losses are not estimated.