What this calculator does
An electric motor is normally rated by its power output and its running speed in RPM, and torque, the twisting force at the shaft, follows from those two figures rather than the other way around. This calculator answers the how to calculate torque of motor question directly: given the power rating and RPM printed on a motor's nameplate, it works out the torque it delivers at that speed.
The torque formula for motor sizing is torque equals power divided by angular speed, with angular speed expressed in radians per second from the RPM figure. This is the reverse of starting with a known force and lever arm to find torque, which is covered by the mechanical torque calculator on this site; this page starts from the motor's power and speed rating instead, which is how motor torque is most often quoted in practice.
The formula
Angular speed in radians per second equals RPM × 2π ÷ 60. Torque then equals power in watts divided by that angular speed, which simplifies to torque = power × 60 ÷ (2π × RPM). The result is also shown converted to pound-feet.
| Term | Meaning |
|---|---|
| Torque | The twisting force delivered at the motor shaft, in newton-metres. |
| Power | The motor's rated power output, in watts. |
| RPM | Revolutions per minute, the shaft's rotational speed at the rated power. |
The inputs explained
| Field | What to enter |
|---|---|
| Power (W) | The motor's power rating, in watts (1 kW = 1,000 W). |
| Shaft speed (RPM) | The shaft speed at that power rating, in revolutions per minute. |
When to use it
Sizing a motor from a nameplate rating
Most motor nameplates list power and RPM rather than torque directly. This works out the torque so it can be compared against what a driven load actually needs.
Comparing motors running at different speeds
Two motors with the same power rating but different RPM deliver different torque, since torque falls as speed rises for a fixed power output. This makes that trade-off explicit.
Checking a gearbox or pulley reduction
Knowing the motor's output torque before a reduction stage is the starting point for working out torque after gearing.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does torque change with shaft speed at a fixed motor power?
A 750 W motor at a range of shaft speeds.
| Shaft speed | Torque | Torque in lb-ft |
|---|---|---|
| 500 RPM | 14.324 N·m | 10.565 |
| 750 RPM | 9.549 N·m | 7.043 |
| 1000 RPM | 7.162 N·m | 5.282 |
| 1450 RPM | 4.939 N·m | 3.643 |
| 2900 RPM | 2.470 N·m | 1.822 |
| 3600 RPM | 1.989 N·m | 1.467 |
Questions
What is the torque formula for a motor?
Torque equals power divided by angular speed: τ = P / ω, where ω (in radians per second) equals RPM × 2π ÷ 60. This rearranges to τ = P × 60 ÷ (2π × RPM).
Why does torque fall as RPM rises for the same power?
Power is the rate of doing work, equal to torque multiplied by angular speed. If power is fixed, torque and angular speed must move in opposite directions: spin faster and there is less twisting force available, spin slower and there is more.
Does this apply to petrol engines too?
The same power-torque-RPM relationship holds for any rotating machine, including internal combustion engines, though engine torque curves are usually far from constant across the RPM range, unlike many electric motors near their rated point.
How is this different from the torque and rotational power calculator?
That calculator starts from a known force and lever arm to find torque, then derives the power it delivers at a given RPM. This one runs the calculation the other way: starting from a motor's rated power and RPM to find the torque it produces.
To calculate torque from a force and lever arm instead, see the torque and rotational power calculator.