Convert power and rpm to torque

What This Torque Calculator Does

This calculator converts mechanical power and rotational speed into torque, expressed in newton-metres (Nm). You enter the power output in kilowatts (kW) and the rotational speed in revolutions per minute (rpm), and it returns the torque the shaft is producing or transmitting at that operating point.

It is useful for engineers, mechanics, machine builders, and anyone working with motors, engines, gearboxes, or pumps. If you know an electric motor's rated power and rated speed, or an engine's power at a given rpm, this tool tells you how much twisting force is available on the output shaft.

How It Works: The Torque Formula (Nm)

Torque, power, and rotational speed are linked by the rotational power equation. Power equals torque multiplied by angular velocity. Rearranging to solve for torque and converting units gives the practical metric formula:

Torque (Nm) = 9548.8 x Power (kW) / Speed (rpm)

The constant 9548.8 comes from the unit conversions: it equals 60,000 divided by 2 x pi. The 60,000 converts kilowatts to watts (x1000) and minutes to seconds (x60), while the 2 x pi converts revolutions to radians. Because torque is inversely proportional to rpm, the same power produces more torque at low speed and less torque at high speed.

Worked Example

Suppose an electric motor is rated at 7.5 kW and runs at 1450 rpm (a typical 4-pole induction motor speed). Plug the numbers into the formula:

Torque = 9548.8 x 7.5 / 1450 = 71,616 / 1450 = 49.4 Nm

So the motor delivers about 49.4 Nm at its rated operating point. As a second check, a 150 kW engine producing peak power at 5500 rpm gives 9548.8 x 150 / 5500 = 260.4 Nm at that speed.

Tips and Common Mistakes

Getting an accurate result depends on using consistent, correct inputs. Keep these points in mind:

• Match power and speed to the same operating point. An engine's peak torque and peak power usually occur at different rpm, so torque at peak power is not the engine's maximum torque.
• Use kW, not horsepower. If your figure is in metric hp (PS), multiply by 0.7355 to convert to kW first; for mechanical hp, multiply by 0.7457.
• Use shaft (output) power. Rated nameplate power on a motor is mechanical output, but if you only have electrical input power, multiply by efficiency before calculating.
• Avoid zero or near-zero rpm. Dividing by very low speed produces unrealistically large torque values and the formula is undefined at exactly 0 rpm.

Factors That Affect the Result

The formula gives the ideal torque at the shaft for the power and speed you enter. Real systems lose some of that torque to friction in bearings, gears, belts, and seals, so usable torque at the load is slightly lower.

Gearing changes torque too: a reduction gearbox lowers output speed and raises torque by roughly the gear ratio (minus efficiency losses), while step-up gearing does the opposite. For motors, torque also varies across the speed range, peaking during starting and at the rated point rather than staying constant, so always state the rpm your torque figure refers to.