Spindle Speed Calculator (RPM from SFM)
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026Spindle Speed Calculator
Tooling manufacturers publish recommended cutting speeds in SFM (surface feet per minute) - this calculator converts that recommendation into the actual spindle RPM your machine needs.
RPM = (Cutting Speed x 12) / (π x Tool Diameter)
Example
100 SFM recommended cutting speed, 0.5" diameter tool:
RPM = (100 x 12) / (π x 0.5) = 764 RPM
Why Tool Diameter Changes the Required RPM
Cutting speed represents how fast the cutting edge itself moves through the material, independent of tool diameter - but a larger diameter tool covers more surface distance per revolution than a smaller one at the same RPM, since its circumference is larger. This is exactly why a larger drill bit or end mill always requires a lower RPM than a smaller one to hit the same recommended cutting speed.
Practical Use
Always start with the tooling manufacturer's recommended SFM for your specific material and tool coating, then use this calculator to find the correct starting RPM for your machine setup - adjust from there based on actual cutting performance, chip formation, and tool wear observed during the cut.