Enter the stock diameter and rotations per minute (RPM) into the calculator to determine the surface speed. The calculator will display the result in feet/min, feet/second, and in/min, and m/s.

Surface Speed Formula

The following formula is used to calculate the surface speed of a rotating cylindrical component. The surface speed is often measured in surface feet per minute denoting it SFM.

SFM = pi * D * RPM

  • Where SFM is the surface speed (distance/minute)
  • D is the stock diameter
  • RPM is the rotations per minute of the spindle

In this scenario, the surface speed will be in the same distance or length units as the stock diameter. For example, if the stock diameter is measured in feet the surface speed will be feet/minute.

Surface Speed Definition

A surface speed, often referred to as SFM, is a term used in machining to describe the tangential velocity of a rotating piece of stock in a spindle.

This unit is used in converting the spindle rotation into the velocity of this stock which is used for determining certain feed rates etc. for machining the material.

Example Problem

How to calculate surface speed?

First, determine the stock diameter. For this example, we are machining a 2-inch diameter rod.

Next, determine the spindle RPM. In this case, the spindle is moving at 4000 RPM.

Finally, calculate the surface speed. Using the formula above, the surface speed is found to be:

SP = PI * D * RPM

= 3.14159*2*4000

= 25132.72 in/min.

To convert this into feet per minute, we divide the result by 12.

= 25132.72 / 12 = 2094.39 ft/min.

To convert this into feet per second, divide the result above by 60.

= 2094.39/60 = 34.906 ft/s

To convert this into meters per second divide the result by 3.281.

= 10.638 m/s.

surface speed calculator