CNC Spindle Speed Calculator
Spindle speed (RPM) is determined by the recommended cutting speed for the material and tool combination, and the diameter of the cutting tool. The formula converts surface speed (how fast the cutting edge moves across the workpiece) into the rotational speed needed to achieve that surface speed with a given tool diameter. This is the fundamental formula from Machinery's Handbook used by machinists worldwide.
Spindle speed formula
RPM = (SFM * 12) / (pi * D)
Where SFM is surface feet per minute and D is cutter diameter in inches. The factor 12 converts feet to inches. This formula appears in Machinery's Handbook (Industrial Press) as the standard RPM calculation for milling, drilling, and turning operations.
Common cutting speeds (SFM) by material
- Aluminum (carbide): 600 to 1,000 SFM
- Brass/bronze (carbide): 300 to 600 SFM
- Mild steel (carbide): 200 to 400 SFM
- Stainless steel 304 (carbide): 100 to 200 SFM
- Cast iron (carbide): 150 to 300 SFM
- Hard wood (carbide): 400 to 800 SFM
- Acrylic/HDPE (carbide): 500 to 900 SFM
CNC spindle speed calculator: frequently asked questions
What is surface feet per minute (SFM)?
Surface feet per minute (SFM), also called surface speed or cutting speed, is the speed at which the cutting edge moves across the workpiece surface. It is the recommended speed for a specific tool and material combination, published in tool manufacturer datasheets and Machinery's Handbook.
What SFM should I use for aluminum?
Carbide end mills in aluminum typically run 600 to 1,000 SFM. High-speed steel (HSS) tools run lower, around 200 to 400 SFM for aluminum. Always confirm with your specific tool manufacturer recommendation.
What SFM should I use for mild steel?
Carbide end mills in mild steel (1018, A36) typically run 200 to 400 SFM. Stainless steel is lower at 100 to 200 SFM. HSS tooling requires lower speeds still.
What is the formula in metric (m/min instead of SFM)?
For metric cutting speed (Vc in m/min): RPM = (Vc * 1,000) / (pi * D), where D is diameter in mm. The calculator above uses imperial SFM; multiply SFM by 0.3048 to convert to m/min if needed.
Why does the calculated RPM sometimes exceed my spindle maximum?
Small-diameter tools at high SFM require very high RPM. If the calculated RPM exceeds your spindle's maximum, use the maximum spindle RPM and accept a lower cutting speed. Avoid reducing cutter diameter to fit the RPM limit when possible, as smaller tools are more fragile.
Official sources
- Industrial Press: Machinery's Handbook (RPM formula, cutting speed tables).
- NIST Manufacturing Systems: Manufacturing Systems Integration Division.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.