Calculate motor torque from kW and RPM

By FieldCalc operator · October 13, 2026

Use shaft power and speed to calculate N·m, understand the 9550 constant, and distinguish rated torque from electrical input and gearbox output.

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What the calculation describes

Mechanical shaft power and rotational speed determine torque at that operating point. Torque is measured in N·m; power is the rate of doing work and can be expressed in kW. Electrical consumption is not the same as mechanical shaft output.

Understand the 9550 constant

T(N·m) ≈ 9550 × P(kW) ÷ n(RPM). The starting relationship is P(W) = torque × angular speed, with angular speed 2πn ÷ 60. Converting kW to W yields 60,000 ÷ 2π ≈ 9549.3, commonly rounded to 9550. This division formula cannot be used at zero RPM.

A 2.2 kW rated example

If the nameplate specifies 2.2 kW rated mechanical output at 1,450 RPM, torque is 9550 × 2.2 ÷ 1450 ≈ 14.49 N·m. Enter 2.2 and 1450 in the calculator. This describes the rated operating point, not torque continuously produced while the motor runs unloaded.

Check units before entering values

750 W equals 0.75 kW. At 1,500 RPM, torque is approximately 4.78 N·m. Entering 750 into a kW field makes the result 1,000 times too large. For horsepower specifications, distinguish mechanical hp from metric PS before converting.

Electrical input needs an efficiency estimate

For 2.5 kW measured electrical input, an assumed efficiency of 0.88 gives estimated shaft output of 2.2 kW. Efficiency depends on load and operating conditions; this assumption is not universal. Do not apply efficiency again if the nameplate value already states rated mechanical shaft output.

Estimate gearbox output separately

For 14.49 N·m input torque, reduction ratio 10 and assumed transmission efficiency 0.90, estimated output is 130.41 N·m at 145 RPM. This is not a permissible torque rating. Verify gearbox limits and duty conditions. The calculator does not automatically evaluate gearbox losses or allowable torque.

Lower speed does not guarantee higher available torque

The inverse relationship holds only at equal power. Real motor power changes across the operating range. Keeping rated kW fixed while entering a much lower RPM does not prove the motor can deliver the resulting torque. Check torque-speed curves, cooling and inverter limits.

Motor sizing needs more information

The formula does not predict starting or peak torque. Selection requires load torque, inertia, acceleration time, duty cycle and thermal conditions. Use the result as a preliminary check and complete selection using manufacturer specifications and actual machine conditions.

A guide to preliminary calculations. Check manufacturer specifications and operating conditions before applying results.