Spindle RPM Calculator

CORE CNC

Solve for spindle speed (RPM) from a target surface speed (SFM) and tool diameter, the reverse of the SFM calculation.

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Spindle Speed (RPM)

Formula: RPM = (SFM times 3.82) divided by D, with D in inches.

How it works

This rearranges the surface-speed formula to solve for RPM directly, so the SFM from a tooling chart and the tool diameter give the spindle speed to dial in.

FAQ

What if the RPM comes out higher than the spindle max? Use the highest RPM the machine supports; running below the ideal SFM is safer than exceeding it.

Related Guides

CNC Calculators · SFM Calculator · Feed Rate Calculator

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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.