Calculators
Surface Speed Calculator (SFM ↔ RPM)
Convert either way between cutting/surface speed (SFM or m/min) and spindle RPM for a given diameter — the core relationship behind every speeds-and-feeds decision.
Last verified: 2026-07-09
Surface speed (how fast the cutting edge moves past the material) and spindle RPM are linked by the tool or part diameter. Pick which one you want to solve for, enter the other plus the diameter, and switch between inch (SFM) and metric (m/min).
Units
Solve for
Spindle speed
764RPM
RPM = (cutting speed × K) ÷ (π × Ø), where K = 12 (inch) or 1000 (metric). Surface speed is the inverse. Smaller diameters spin faster for the same surface speed.
Frequently asked
- What's the formula?
- RPM = (12 × SFM) ÷ (π × diameter in inches), or RPM = (1000 × m/min) ÷ (π × diameter in mm). Rearranged, surface speed = π × diameter × RPM ÷ (12 or 1000).
- Do I use the tool diameter or the part diameter?
- The rotating diameter that's cutting — the tool diameter for drilling/milling, or the part diameter for turning.
Sources
- Standard surface-speed / RPM relationship (V = π·D·N)