Spindle Speed Calculator
Running a tool at the wrong RPM wears it out fast or leaves a rough finish. This calculator converts cutting speed (SFM) and tool or workpiece diameter into the correct spindle RPM using the standard machining formula. Works for milling, turning, drilling, and boring operations.
The formula: RPM = (Cutting Speed x 12) / (pi x Diameter). Cutting speed is in surface feet per minute (SFM), and diameter is in inches. The "12" converts feet to inches so the units work out.
Every material has an ideal cutting speed range. Aluminum runs fast at 300-800 SFM with carbide tooling. Mild steel sits around 60-100 SFM. Stainless steel and titanium need slower speeds, typically 40-80 SFM, because they work-harden. Tool manufacturers publish recommended SFM values for each material and insert grade.
If you are working in metric, the equivalent formula is RPM = (Cutting Speed x 1000) / (pi x Diameter) where cutting speed is in meters per minute and diameter is in millimeters. This calculator handles the conversion automatically when you switch units.