Motor Size Calculator
Enter the load torque, its speed, the duty type, motor efficiency and the supply. The calculator works out the shaft power, adds the service factor and picks the next standard motor.
- Motor size
- Shaft power
- Current
Take torque from the machine's spec or measure it.
Power (kW) = torque (N·m) × rpm ÷ 9,550. A load needing 50 N·m at 1,450 rpm takes 7.6 kW. With a 1.25 service factor that is 9.5 kW, so an 11 kW, 4 pole motor.
Motor power chart
Power from torque and speed. Search for a speed.
| Torque | At 960 rpm | At 1,450 rpm | At 2,900 rpm |
|---|---|---|---|
| 5 N·m | 0.5 | 0.76 | 1.52 |
| 10 N·m | 1.01 | 1.52 | 3.04 |
| 20 N·m | 2.01 | 3.04 | 6.07 |
| 50 N·m | 5.03 | 7.59 | 15.18 |
| 100 N·m | 10.05 | 15.18 | 30.37 |
| 200 N·m | 20.1 | 30.37 | 60.73 |
| 500 N·m | 50.26 | 75.92 | 151.83 |
kW.
Torque and motor
At 1,450 rpm. Slide the torque.
The motor is drawn to size.
How to size a motor
Load to motor. Step through it.
Torque
Of the load.
Check the starting torque for heavy loads.
2, 4 or 6 pole
Motor speeds. Compare them.
Fast.
General.
Slow.
4 pole motors are the most common.
Before you buy
Motor checks. Tick each item.
Tick each item as you go.
Match the frame size and mounting to the machine.
What you get
Motor sizing. Each tile explains one result.
kW and hp.
Shaft.
Amps.
Speed.
Factor.
An oversized motor runs at poor efficiency and power factor.
Frequently Asked Questions
No question matches that search.
How do I calculate motor size?
Multiply torque by speed and divide by nine thousand five hundred and fifty for kilowatts.
How do I convert kW to horsepower?
Divide kilowatts by point seven four six.
What is a service factor?
A margin for shock loads or frequent starts.
Why does speed matter for motor size?
Power is torque times speed, so slow loads need more torque.
What are standard motor sizes?
Such as one point one, one point five, two point two, three, four and five point five kilowatts.
Should I oversize a motor?
Only a little, big oversizing wastes energy.
The right motor
Size the motor to the load and it runs cool, efficient and reliable.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.