Torque Calculator
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RPM Calculator
Find machining spindle speed from cutting speed and diameter.
Spindle speed
—
RPM.
| Cutting speed | — |
|---|---|
| Diameter | — |
| — | |
| — |
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How the RPM Calculator Works
For machining, spindle speed comes from the material’s cutting speed and the tool or workpiece diameter. Multiply the cutting speed (in surface feet per minute) by 12 to get inches, then divide by the circumference (π times diameter). At 100 SFM on a 1-inch diameter, that’s about 382 RPM.
Cutting speed depends on the material and tool — aluminum runs fast, hardened steel slow. Smaller diameters need higher RPM to keep the same surface speed, which is why drills spin faster for small bits. A common shop shortcut is RPM ≈ 3.82 × SFM ÷ diameter.
RPM = cutting speed (SFM) × 12 ÷ (π × diameter)
Frequently Asked Questions
How do you calculate machining RPM?
Multiply the cutting speed in surface feet per minute by 12, then divide by pi times the diameter in inches.
What is SFM?
Surface feet per minute — how fast the cutting edge moves across the material. It’s set by the material and tool, not the machine.
Why do small drills spin faster?
To keep the same surface speed, a smaller diameter needs more RPM. Cutting speed depends on the edge speed, which is RPM times circumference.