Some machines, such as car engines or drills are designed with rotational speed in mind. When a machine rotates, its speed is often measured in RPMs, or how many revolutions it completes per minute. You can convert this measurement to SFMs, or surface feet per minute, if you know the diameter, or the distance from one side of the rotational path to the other. The wider the diameter, the higher the surface feet per minute.

Measure the diameter of the rotating item in feet.

Multiply the diameter by 0.262. For example, if the diameter equals 0.05 feet, multiply 0.05 by 0.262 to get 0.0131.

Multiply the product of Step 2 by the RPMs. Finishing the example, if the item is rotating at 200 RPM, multiply 200 by 0.0131 to get 2.62 feet per minute.