Motor RPM Calculator
The speed of an AC motor is set by the supply it runs on and the way its windings are arranged, not by anything you can adjust on the shaft. This calculator finds the synchronous speed, the speed of the rotating magnetic field, using the standard formula: 120 multiplied by the supply frequency in hertz, divided by the number of poles. The result is in revolutions per minute. The constant 120 folds together the 60 seconds in a minute and the fact that poles come in pairs, so you can read speed straight off the frequency and pole count. Synchronous speed is the no-slip figure; a real induction motor runs a few percent slower under load, while a true synchronous motor holds this speed exactly. Enter your own frequency and pole count to check a nameplate, work out the speed on a 50 Hz versus a 60 Hz supply, or see how a variable-frequency drive will change the shaft speed. Because speed is directly proportional to frequency, drives control motors simply by changing the supply frequency. Every figure here is computed deterministically from the synchronous-speed formula, shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step.
Synchronous speed is 120 times the supply frequency divided by the pole count: RPM = 120 x f / poles. A motor on a 60 Hz supply with 4 poles turns at a synchronous speed of 1,800.00 RPM.
Synchronous speed formula
RPM = 120 x f / P
f = supply frequency (Hz)
P = number of poles
RPM = synchronous speed (revolutions per minute)
The rotating field completes one cycle per supply cycle per pole pair, so the speed is proportional to frequency and inversely proportional to the pole count.
Worked example
A 4-pole induction motor runs on a 60 Hz supply.
- 120 x frequency = 120 x 60 = 7,200
- Divide by poles: 7,200 / 4 = 1,800
- Synchronous speed = 1,800.00 RPM
The synchronous speed is 1,800.00 RPM. These are the calculator's default inputs, so the result above matches the widget exactly.
Synchronous speed at 60 Hz
More poles mean a slower motor for the same supply frequency.
| Poles | Speed at 50 Hz | Speed at 60 Hz |
|---|---|---|
| 2 | 3,000.00 | 3,600.00 |
| 4 | 1,500.00 | 1,800.00 |
| 6 | 1,000.00 | 1,200.00 |
| 8 | 750.00 | 900.00 |
Frequency and measurement standards: US National Institute of Standards and Technology (NIST).
Motor RPM calculator: frequently asked questions
What is synchronous speed?
Synchronous speed is the rotational speed of the rotating magnetic field in an AC motor. It equals 120 times the supply frequency in hertz, divided by the number of poles. At 60 Hz with 4 poles, the synchronous speed is 1,800 revolutions per minute.
Why is the constant 120?
The 120 combines two conversions: 60 seconds per minute and the fact that poles come in pairs, so a pole pair count is half the pole count. Writing speed as 120 times frequency divided by poles folds both factors into one tidy constant for RPM.
Does the motor actually run at synchronous speed?
An induction motor runs slightly slower than synchronous speed by an amount called slip, typically a few percent under load, because the rotor must lag the field to produce torque. A true synchronous motor locks to the synchronous speed exactly. This calculator gives the synchronous speed, the no-slip figure.
How does frequency change the speed?
Speed is directly proportional to frequency, which is why variable-frequency drives control motor speed by changing the supply frequency. Doubling the frequency doubles the synchronous speed; halving it halves the speed, for the same number of poles.
What pole counts are common?
Industrial AC motors commonly have 2, 4, 6 or 8 poles. At 60 Hz these give synchronous speeds of 3,600, 1,800, 1,200 and 900 RPM. More poles mean a slower motor for the same supply frequency.
Official sources
- Measurement units, frequency and electrical metrology: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.