RPM Calculator
Calculate rotational speed from linear surface speed and diameter, or AC induction motor synchronous speed from supply frequency and pole count.
Enter your values
Choose mode, then fill in the fields below.
How this calculator works
For a rotating part where a known point on its circumference travels at a given surface (linear) speed, rotational speed is:
V is surface speed, D is diameter — both must use consistent length units (the calculator uses m/min and mm, converting internally). For a three-phase AC induction motor, the theoretical synchronous speed of the rotating magnetic field is:
f is supply frequency in Hz, P is the number of motor poles. Actual induction motor speed under load runs slightly below this synchronous value due to slip — typically 1–5% lower.
Common synchronous speeds (50 Hz supply)
| Poles | Synchronous RPM |
|---|---|
| 2 | 3000 |
| 4 | 1500 |
| 6 | 1000 |
| 8 | 750 |
Frequently asked questions
Why is my motor's actual RPM lower than the calculated synchronous speed?
Induction motors always run slightly slower than synchronous speed — the difference (slip) is what induces current in the rotor and produces torque. Nameplate RPM already accounts for typical full-load slip.
Can I use the surface speed mode for belt or pulley speed?
Yes — this is the same relationship used to find pulley or wheel RPM from a known belt or surface speed and the pulley diameter.