Motor Torque Calculator — kW, hp & rpm

Coupling a motor to a conveyor, a pump or a gearbox means knowing how hard it twists, and nameplates rarely print that. They print power and speed. Torque follows from those two, and this Motor Torque Calculator does the conversion in N·m, lb·ft, lb·in and kgf·m at once.

Any one of torque, power or speed can be the unknown. Add a gear ratio and the output shaft’s torque and speed appear as well, reduced by the gearbox efficiency you enter.

Worked Example: A 7.5 kW Motor at 1,450 rpm

The tool opens on 7.5 kW and 1450 rpm, a common four-pole motor on a 50 Hz supply:

• Torque: 49.39 N·m, which is 36.43 lb·ft, 437.16 lb·in or 5.04 kgf·m
• Angular speed: ω = 2π × 1,450 ÷ 60 = 151.84 rad/s
• Power check: 7.5 kW is 10.06 hp at 745.7 W per horsepower

A later sample puts a 2.2 kW motor behind a 20:1 gearbox at 90% efficiency: 14.79 N·m at the motor becomes 266.30 N·m at 71 rpm. The pole buttons fill in synchronous speed, 120 × f ÷ poles, so a 6-pole motor on 60 Hz gives 1,200 rpm with one click.

How the Math Links Power, Speed and Torque

Power is torque times angular speed, P = T × ω. Rearranged for the units on a nameplate, T (N·m) = 9,549.3 × kW ÷ rpm, and the US form is T (lb·ft) = 5,252.1 × hp ÷ rpm. Both constants are just 60 ÷ 2π with unit conversions folded in. A gearbox multiplies torque by its ratio and divides speed by it; the efficiency figure takes its losses out of torque and power, not speed.

Limits: Rated Torque Is Not Starting Torque

The answer is the steady torque at the speed you enter. Starting and breakdown torque are often two to three times higher on an induction motor and come from the manufacturer’s curve, not from this formula. Enter the loaded nameplate rpm rather than the synchronous speed, or the torque comes out a few percent low.

Speed is capped at 200,000 rpm. Anything outside the range is refused with “Speed must be greater than 0 and no more than 200,000 rpm.” Results show two decimal places.