RPM to hertz converter

How the RPM to hertz converter works

Enter a mechanical speed in either field. The converter divides revolutions per minute by 60 to show complete revolutions per second in hertz, or multiplies hertz by 60 for the reverse conversion.

One minute is the whole conversion factor

RPM counts complete turns during 60 seconds. Hertz counts cycles during one second. When one cycle means one complete turn, the relationship is:

Here n is rotational speed in rpm and f is mechanical rotational frequency in hertz. Wheel diameter, motor power and shaft material do not change this unit conversion.

Useful speeds to recognise

A spindle at 3000 rpm turns at 50 Hz. It completes fifty full revolutions every second. This is a mechanical frequency, even if the number happens to match a 50 Hz mains supply.

A fan at 1200 rpm turns at 20 Hz. The blades pass a fixed point more often than twenty times per second when the fan has several blades, so blade-pass frequency is a separate calculation.

A record at 33⅓ rpm turns at about 0.555556 Hz. One revolution takes about 1.8 seconds. The fractional answer is expected because the record does not complete a full turn each second.

A cycle is not always a revolution

The conversion is exact only after the cycle has been defined as one mechanical revolution. An encoder can emit hundreds of pulses per turn, a multi-blade fan produces several blade passages per turn, and an electric motor can rotate at a speed different from its supply frequency.

• Use pulses per revolution to obtain an encoder signal rate.
• Use blade count to estimate a blade-pass frequency.
• Use motor poles and machine data to relate supply frequency to shaft speed.

Use the calculator for a known shaft, wheel, disc or rotor speed. To find sensor pulse frequency, multiply the rotational frequency by pulses per revolution. To derive motor speed from electrical frequency, the motor type, pole count and sometimes slip are also required.

Decimals, zero and direction

Values that are not multiples of 60 naturally produce decimal hertz. Keep only as many digits as the original measurement supports: a tachometer reading rounded to whole rpm does not justify twelve meaningful decimal places.

Zero rpm equals zero hertz because nothing completes a revolution. The form uses non-negative speed; clockwise or counter-clockwise direction must be recorded separately when it matters.

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