SFM ↔ RPM Calculator

Same formula, either direction

RPM = (SFM × 12) ÷ (π × diameter) and SFM = (RPM × π × diameter) ÷ 12 are the same relationship solved for different variables — both come from converting the tool or workpiece circumference into feet and relating it to how many revolutions per minute produce the target surface speed.

When you'd use the reverse direction

Going from RPM back to SFM is useful when you're handed a program with a spindle speed already set and want to sanity-check whether it's actually appropriate for the material and tool diameter in front of you — a legacy program written for a different tool diameter can carry over an RPM that's no longer the right cutting speed once the tool changes.

Diameter means different things depending on the operation

For milling, use the cutter diameter. For turning, use the workpiece's outer diameter at the point of cut — and remember that diameter shrinks as you turn down a part, so the RPM needed to hold a constant SFM increases as the cut progresses, which is exactly what a lathe's constant-surface-speed (CSS) mode is doing automatically.