SFM to RPM Calculator

Formula and worked example

Formula

RPM = (SFM × 12) ÷ (π × diameter in)
Metric: RPM = (Vc × 1,000) ÷ (π × diameter mm)

Example

375 SFM with a 0.500 in cutter requires 2,865 RPM.

Frequently asked questions

How do I convert SFM to RPM?

Multiply SFM by 12, then divide by pi times diameter in inches. A smaller cutter needs more RPM to reach the same surface speed. Always compare the result with the machine’s spindle limit and the tool manufacturer’s recommended cutting-speed range.

How do I convert RPM back to SFM?

Multiply RPM by diameter in inches and pi, then divide by 12. This reverse calculation is useful when the spindle has a fixed speed or maximum RPM and you need to know the surface speed the tool or workpiece will actually see.

What is Vc, and how is it different from SFM?

Vc is the metric cutting speed, normally expressed in meters per minute. SFM expresses the same surface-speed concept in feet per minute. For metric calculations, RPM equals Vc times 1,000 divided by pi times diameter in millimeters.