Motor Torque Calculator
Calculate motor torque, shaft power or rotational speed with automatic kW, horsepower, N·m and lb-ft conversions.
Motor inputs
How to use the Motor Torque Calculator
The Motor Torque Calculator solves the relationship between shaft torque, mechanical power and rotational speed.
- Choose whether to calculate torque, power or speed.
- Enter the two known values.
- Select the units shown beside power, speed and torque.
- Click Calculate to see all converted results.
- Download a printable PDF view or spreadsheet-compatible Excel file if needed.
Motor torque formulas
Torque: T = P ÷ ω
Power: P = T × ω
Speed: RPM = P × 60 ÷ (2π × T)
Metric shortcut: T(N·m) = 9,549.30 × P(kW) ÷ RPM
US shortcut: T(lb-ft) = 5,252.11 × P(hp) ÷ RPM
These equations use mechanical shaft power. Electrical input power is normally higher because a real motor has losses. If you only know electrical input, estimate shaft output with the motor efficiency before calculating torque.
Worked example
A motor delivers 15 kW at 1,500 RPM. Angular velocity is 2π × 1,500 ÷ 60 = 157.08 rad/s. Torque is 15,000 ÷ 157.08 = 95.49 N·m, equivalent to approximately 70.43 lb-ft. The same shaft power is approximately 20.12 mechanical horsepower.
Common motor torque conversions
| Quantity | Conversion |
|---|---|
| Power | 1 mechanical hp = 745.699872 W |
| Torque | 1 N·m = 0.737562 lb-ft |
| Torque | 1 lb-ft = 12 lb-in |
| Speed | 1 RPS = 60 RPM |
Understanding motor torque
Torque is the turning effect available at the shaft. Power describes how quickly work can be done, while speed describes how quickly the shaft rotates. At the same power, reducing speed increases torque. At the same speed, increasing shaft power increases torque.
Rated torque is not the same as starting, breakdown or transient torque. Gearboxes can multiply output torque while reducing speed, subject to transmission losses. Always compare the calculated operating point with the manufacturer’s speed-torque curve and duty rating.
Important engineering limits
The Motor Torque Calculator assumes steady mechanical shaft values. It does not model acceleration, inertia, service factor, thermal duty, harmonics, voltage variation, gearbox efficiency or dynamic load changes. Final motor, gearbox, coupling and drive selection should follow manufacturer data and qualified engineering review.
For general motor-system efficiency resources, consult the U.S. Department of Energy motor systems guidance.
Common mistakes to avoid
- Entering electrical input power as though it were mechanical shaft output.
- Mixing lb-ft and lb-in.
- Using synchronous speed when the actual loaded shaft speed is lower.
- Assuming rated torque equals starting or breakdown torque.
- Ignoring gearbox and transmission losses.
Frequently asked questions
Why does torque increase when RPM decreases?
For constant power, torque is inversely proportional to angular speed. Lower shaft speed therefore requires more torque to transmit the same power.
Should I use electrical input power or shaft power?
Use mechanical shaft output. If only electrical input is known, apply a reliable efficiency value to estimate output power first.
Is horsepower different from kilowatts?
They are units of power. This calculator uses mechanical horsepower, where one horsepower equals 745.699872 watts.
Can this calculator size a motor?
It provides a steady-state torque relationship only. Motor sizing also requires starting torque, load profile, duty cycle, inertia, overload capacity, environment and safety factors.
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Disclaimer: The Motor Torque Calculator provides educational planning estimates and does not replace motor data, machinery standards or professional engineering.