Electric Motor Torque Calculator
Enter a motor power and shaft speed to get torque instantly, or switch to AC mode to solve synchronous speed, slip and breakdown torque, or use DC mode to calculate torque from voltage, current and efficiency. All three modes support metric (N·m, W/kW) and imperial (lb-ft, HP) units and show the full working.
How electric motor torque is calculated
Torque is the rotational equivalent of force - it is the turning moment at the motor shaft. The fundamental relationship between shaft power (P), speed (N in RPM) and torque (T in N·m) is P = T x 2pi x N / 60, which rearranges to T = (60 x P) / (2pi x N). This means torque and speed trade off against each other at constant power: double the speed and you halve the torque for the same power output. In imperial units the constant 63,025 replaces the metric constant when working in horsepower, RPM and pound-inches, or 5,252 when using pound-feet.
AC motor torque: synchronous speed, slip and the torque spectrum
An AC induction motor produces torque because the rotor always turns slightly slower than the rotating magnetic field - this speed difference is called slip. Synchronous speed (the field speed) is set by the supply frequency and pole count: Ns = 120f / p. A 4-pole motor on a 50 Hz supply runs at 1500 RPM synchronously but typically delivers rated torque at around 1440 to 1480 RPM because of slip. Three torque values matter in practice: full-load torque (FLT) is the rated continuous output; starting torque is the torque produced from standstill, typically 1.5 to 2.5 times FLT for a NEMA Design B motor; and breakdown torque is the maximum possible before the motor stalls, typically 2.0 to 3.5 times FLT. Staying below breakdown torque during transient overloads protects the motor.
DC motor torque and the voltage-current-efficiency relationship
A DC motor converts electrical power (P_in = V x I) into shaft power (P_out = P_in x efficiency). The shaft torque follows from P_out and speed in the same way as for any motor: T = (60 x P_out) / (2pi x N). Because a DC motor output power falls as speed rises (at constant voltage), torque falls roughly linearly with speed, reaching a maximum (stall torque) at zero speed and dropping to zero at no-load speed. Efficiency of 70 to 90 % is typical; BLDC and servo drives often exceed 92 %. The power lost to heat is P_in minus P_out, which sizes the thermal management requirements.
Torque units and conversion between metric and imperial
The SI unit of torque is the Newton-metre (N·m). One N·m equals 0.7376 pound-feet (lb-ft) or 8.8507 pound-inches (lb-in). US motor nameplates and the NEC often quote torque in lb-ft while European nameplates use N·m. The quick conversion is: N·m = lb-ft / 0.7376, or lb-ft = N·m x 0.7376. When combining with horsepower (HP), remember that 1 HP = 745.7 W. The commonly cited formula T (lb-ft) = HP x 5252 / RPM comes directly from substituting these constants into the main torque equation.
Common AC induction motor synchronous speeds
| Poles | Sync speed at 50 Hz (RPM) | Sync speed at 60 Hz (RPM) | Typical application |
|---|---|---|---|
| 2 | 3000 | 3600 | High-speed pumps, compressors |
| 4 | 1500 | 1800 | General-purpose industrial motors |
| 6 | 1000 | 1200 | Fans, blowers, centrifugal pumps |
| 8 | 750 | 900 | Conveyors, mixers |
| 10 | 600 | 720 | Low-speed drives |
| 12 | 500 | 600 | Rolling mills, hoists |
Synchronous speeds (RPM) for standard pole counts at 50 Hz and 60 Hz supply frequencies.
Frequently asked questions
What is the formula for electric motor torque?
The core formula is T = (60 x P) / (2 x pi x N), where T is torque in Newton-metres, P is shaft power in Watts and N is speed in RPM. In imperial units with horsepower and lb-ft this becomes T = HP x 5252 / RPM. The formula rearranges in three ways: solve for torque from power and speed, solve for power from torque and speed, or solve for speed from power and torque.
What is slip in an AC induction motor?
Slip is the percentage difference between the synchronous speed of the rotating magnetic field and the actual rotor speed. Without slip there is no relative motion between rotor and field, so no torque is induced. Slip increases as load increases: a lightly loaded motor might slip only 1 %, while a fully loaded motor slips 3 to 5 %. Excessive slip (above about 8 %) usually indicates an overloaded or starting-torque-limited motor.
What is breakdown torque and why does it matter?
Breakdown torque (also called pull-out torque or maximum torque) is the highest torque an AC induction motor can produce. If the load torque exceeds this value the motor cannot accelerate and stalls. For NEMA Design B motors the breakdown torque is typically 2.0 to 3.5 times the full-load torque. Knowing the breakdown torque lets you size the motor with enough headroom for transient overloads like conveyor starts or pump surges.
How does the number of poles affect torque?
More poles reduce the synchronous speed (Ns = 120f / p) and therefore increase torque for the same rated power, because P = T x angular velocity. A 4-pole 50 Hz motor runs at 1500 RPM synchronously and delivers twice the torque of a 2-pole motor (3000 RPM) at the same power. High-pole-count motors are used in applications needing high torque at low speed, such as hoists, cranes and rolling mills.
Why does DC motor torque decrease as speed increases?
At constant voltage, as a DC motor speeds up the back-EMF (a voltage generated by the spinning rotor) opposes the supply voltage, reducing the current and therefore the torque. The torque-speed characteristic of an ideal DC motor is a straight line from stall torque at zero speed to zero torque at no-load speed. Real motors deviate slightly from this line because of friction and core losses, but the linear model is a good approximation for sizing and application work.
How do I convert torque from N·m to lb-ft?
Multiply Newton-metres by 0.7376 to get pound-feet, or by 8.8507 to get pound-inches. To go the other way, divide pound-feet by 0.7376 to get N·m. For example, 50 N·m equals 50 x 0.7376 = 36.88 lb-ft. This calculator performs the conversion automatically when you select the torque unit.