Motor torque calculator determines full-load and peak shaft torque for AC motors from power rating, operating speed, and service factor used for coupling and gearbox sizing work.
Calculate Full-Load Motor Torque from Power and Speed
This tool calculates the continuous shaft torque an AC motor delivers at rated power and speed, along with imperial equivalents and service-factor design limits. Mechanical and electrical engineers use it to select couplings, gearboxes, and shafts, and to confirm a motor can meet a load’s torque requirement before installation.
Using the Motor Torque Calculator
Enter motor power rating (P) in kW, operating speed (N) in RPM, and service factor (SF) if listed on the nameplate. The calculator returns nominal full-load torque (T) in Nm, imperial equivalents in lb-ft, lb-in, and kg-m, safe design torque at the service factor, and dynamic torque thresholds for starting and breakdown conditions.
Motor Torque Formula and Calculation Steps
Torque is derived from the fundamental relationship between power and angular velocity:
$$P = T \times \omega$$
Rearranged and converted into standard motor units, this becomes the working formula used across mechanical and electrical engineering references, including Marks’ Standard Handbook for Mechanical Engineers and NEMA MG-1 motor performance calculations:
$$T (Nm) = \frac{9550 \times P (kW)}{N (RPM)}$$
Where 9550 is the rounded constant from converting kilowatts to watts and RPM to radians per second $(60 \times 1000) / (2\pi) \approx 9549.3$. Safe design torque is then $T \times SF$, and SF power limit is $P \times SF$. Common mistake: applying the 9550 constant to horsepower instead of kilowatts — the correct imperial constant is 7127, and mixing the two understates or overstates torque significantly.
Torque-Speed Curve for an Induction Motor
Why Breakdown Torque Isn’t at Zero Speed
Standard NEMA Design B motors don’t produce their maximum torque at standstill. The torque-speed curve dips slightly just after start (pull-up torque), then rises to a peak — breakdown torque — at roughly 15–25% slip, well before reaching synchronous speed, then falls to zero at $N_s$.
This means the calculated full-load torque from this tool is only one point on that curve, not the motor’s maximum capability; the breakdown torque figure (typically 200–300% of full-load torque) is what actually determines stall resistance under a sudden load spike, not the starting torque value.
Service factor also has a condition attached that’s easy to miss: NEMA MG-1 service factor ratings apply only at rated voltage, rated frequency, and an ambient temperature of 40°C at or below 3,300 ft (1,000 m) altitude. Operating above that altitude or ambient temperature reduces the safe continuous torque a motor can actually sustain, even though the calculated SF torque value stays the same.
Motor Torque Calculator FAQs
Does higher torque always mean a stronger motor?
Not by itself. Torque must be read alongside speed, since the same power rating produces different torque values at different speeds. A slower motor of equal power always delivers more torque, per $T = 9550P/N$.
Can I use this formula for a motor with a gearbox?
This formula gives motor shaft torque only. Output torque after a gearbox is the motor torque multiplied by the gear ratio (minus efficiency losses), not the value calculated here directly.
What’s the difference between starting torque and breakdown torque?
Starting (locked-rotor) torque is produced at zero speed. Breakdown (pull-out) torque is the highest torque the motor can produce anywhere on its speed curve, occurring at partial slip, and is usually higher than starting torque.
Why does my calculated torque differ from the nameplate value?
Nameplate torque is measured at rated load, voltage, and frequency. If you enter a different operating speed than the nameplate rated speed, the calculated torque will differ, since torque is inversely proportional to speed at constant power.
Should I size a motor using nominal torque or safe design torque?
Use safe design torque (nominal torque × service factor) as the sizing ceiling for continuous operation, not the peak. Running continuously at or beyond safe design torque risks thermal damage even if it’s below breakdown torque.