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

To calculate the surface speed, multiply pi by the diameter, then multiply again by the RPM.

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.


What is the significance of measuring surface speed in machining?
Measuring surface speed is crucial in machining because it helps in determining the optimal feed rates and cutting speeds for machining operations. This ensures efficient material removal, prolongs tool life, and prevents damage to the workpiece.

How does the diameter of the stock affect the surface speed?
The diameter of the stock directly influences the surface speed. A larger diameter means the material’s surface moves faster at the same spindle RPM compared to a smaller diameter. This is because the circumference of the stock, which determines the distance covered per rotation, increases with diameter.

Can the surface speed formula be used for materials of any shape?
The surface speed formula provided is specifically designed for cylindrical components. For non-cylindrical shapes, different formulas or considerations may be necessary to accurately calculate surface speed, as the shape affects how the material moves past the cutting tool.

surface speed calculator