Enter the surface speed (cutting speed) and diameter into the calculator to determine the rotational spindle speed.

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## Spindle Speed Formula

The following formula is used to calculate the rotational speed of a spindle.

SS = CS/(PI*D)

- Where SS is the spindle speed in rotations per time
- SS is the surface speed also known as cutting speed
- D is the diameter

In this case, the formula is will work for any units as long as they create a rotation per unit time, however, this is often simplified to the following formula:

SS = 3.82*SFM/D

- Where SFM is the cutting speed in surface feet per minute
- SS is in RPM

For the first equation, to make the units equal RPM, make sure the CS is in distance per minute and the diameter is in the same units as distance in cutting speed. For example, if the CS (surface speed) is 50ft/min, then the diameter should be in feet.

## Example Problem

**How to calculate spindle speed? **

First, determine the surface speed, also known as cutting speed. For this example problem, the surface speed is determined to be 5000 feet per minute (ft/min)

Next, determine the diameter of the spindle. The diameter of the spindle is measured to be 18 inches. Since the units should match the surface speed length unit, divide by 12. This yields 1.5 ft.

Finally, calculate the spindle rotational speed using the formula above:

SS = CS/(PI*D)

= 5000/(3.14159*1.5)

= 1061.33 RPM.