Spindle Speed Calculator
Calculate the spindle speed (RPM) for a mill, lathe, or drill based on tool diameter and the material’s recommended cutting speed.
What this calculator does
The Spindle Speed Calculator converts a material’s recommended cutting (surface) speed into the correct spindle rotation speed in revolutions per minute (RPM) for milling, drilling, or turning operations.
The formula
The standard machining formula is RPM = (SFM × 12) / (π × D), where SFM is the surface feet per minute recommended for the tool and material combination, and D is the tool (or workpiece, for turning) diameter in inches. This comes directly from the circumference the cutting edge travels each revolution.
Recommended SFM values vary widely by material and tool type (for example, high-speed steel drilling mild steel is often around 80-100 SFM, while carbide tooling can run much faster) — check your tool manufacturer’s chart for the specific SFM to enter.
- Running well above the recommended RPM can overheat the tool and shorten its life.
- This is a general planning reference; always follow your machine’s safe operating limits and the tool manufacturer’s guidance.