Motor Power, Speed and Torque Calculator

How power, torque and speed relate

On a rotating shaft, power is torque multiplied by angular velocity: P = T × ω. Angular velocity follows from rotational speed as ω = 2 × π × n / 60. Knowing any two of the three quantities gives the third.

The form most often used in the workshop is T = 9550 × P / n, with power in kW, speed in rpm and torque in N·m. The number 9550 is 60000 / (2 × π) rounded.

Example

A 7.5 kW motor running at 1450 rpm has a rated torque of 9550 × 7.5 / 1450 = 49.4 N·m. A 2900 rpm motor of the same power delivers half of that. At constant power, torque rises as speed falls.

Gearbox output

A gearbox reduces speed by its ratio and raises torque by the same ratio. Losses come off the torque: n₂ = n / i, T₂ = T × i × η. Fitting the motor above with a 20 to 1 gearbox at 90 percent efficiency gives 72.5 rpm and 889 N·m at the output.

Limits

This calculator gives the steady operating point. Starting torque, load inertia, service factor and the permissible output torque of the gearbox must be checked separately. For gearbox selection, the service factor in the manufacturer's catalogue governs.