Electric Motor Torque Calculator

Enter your motor's power (kW or HP) and rotational speed (RPM) to calculate shaft torque in Newton-metres and lb-ft. The Electric Motor Torque Calculator uses the standard torque formula (T = 9550 × P / N) to give you the rated torque output alongside a clear power-torque breakdown — useful for motor selection, drivetrain design, and nameplate verification.

Enter the rated power from the motor nameplate or datasheet.

RPM

Use the rated (load) speed from the nameplate, not the synchronous speed.

%

Enter efficiency % to calculate shaft output torque accounting for losses. Leave at 100% to ignore.

Results

Shaft Torque

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Torque (lb·ft)

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Torque (lb·in)

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Power (kW)

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Power (HP)

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Angular Velocity

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Torque in Multiple Units

Frequently Asked Questions

How do you calculate electric motor torque?

Motor torque is calculated using the formula T = (60 / 2π) × (P × 1000 / N), which simplifies to T = 9550 × P / N, where T is torque in N·m, P is rated power in kW, and N is rated speed in RPM. For horsepower, the equivalent is T = 5252 × HP / RPM (result in lb·ft). This gives the rated shaft torque at full-load conditions.

What is the difference between rated speed and synchronous speed?

Synchronous speed is the theoretical speed of the motor's rotating magnetic field, determined by the supply frequency and number of poles. Rated speed is the actual shaft speed under full load, which is slightly lower due to 'slip'. Always use the rated (nameplate) speed when calculating torque, not the synchronous speed — using the wrong value will give an inaccurate result.

What is slip in an electric motor?

Slip is the difference between a motor's synchronous speed and its actual (loaded) speed, expressed as a percentage: s = (RPM_noload − RPM_load) / RPM_noload × 100%. Slip is inherent in induction motors and is what allows them to generate torque. Typical slip for standard induction motors ranges from 1% to 5% at full load.

How much torque does a 1 HP motor produce at 1800 RPM?

Using the formula T = 5252 × HP / RPM: T = 5252 × 1 / 1800 ≈ 2.92 lb·ft, or about 3.96 N·m. This is the standard rated torque for a 1 HP, 4-pole induction motor running at a typical full-load speed of 1800 RPM at 60 Hz.

What is the torque of a 1 kW, 4-pole motor?

A 4-pole motor at 50 Hz has a synchronous speed of 1500 RPM, with a typical rated speed of around 1450 RPM. Using T = 9550 × 1 / 1450 ≈ 6.59 N·m. At 60 Hz (synchronous 1800 RPM, rated ~1750 RPM), the torque would be T = 9550 × 1 / 1750 ≈ 5.46 N·m.

Why does torque decrease as motor speed increases for the same power?

Torque and speed have an inverse relationship at constant power: T = P / ω, where ω is angular velocity in rad/s. If power stays fixed and speed doubles, torque is halved. This is why high-speed motors produce lower torque than low-speed motors of the same power rating, and gearboxes are used to trade speed for torque.

What is breakdown torque in an AC electric motor?

Breakdown torque (also called pull-out torque) is the maximum torque an induction motor can produce before it stalls. If the load torque exceeds this value, the motor will lose synchronism and stop. Breakdown torque is typically 2 to 3 times the rated (full-load) torque and is specified on the motor's performance curve.

Does motor efficiency affect the shaft torque?

Yes. If efficiency is less than 100%, some of the input electrical power is lost as heat. The actual mechanical (shaft) power delivered is P_shaft = P_input × η. This calculator lets you enter efficiency so the torque result reflects true shaft output rather than input power. For nameplate calculations, the rated power is typically the output (shaft) power, so efficiency can be left at 100%.

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