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Tool Belt Calc

Measure

SFM to RPM Calculator

Spindle rpm from cutting speed and diameter.

Your measurementsEvery assumption editable01

Surface feet per minute, from the tooling manufacturer for your material.

Spindle speedCalculated live02

Result

2292rpm

300 SFM on 0.5 in = 2292 rpm

Cutting speed91.4 m/min
300 SFM
Diameter
0.5 in
Spindle speedSFM × 12 ÷ (π × diameter)
2291.8 rpm

Cutting speed is a property of the material and the tool, not of the machine. It stays constant while the rpm needed to achieve it changes with diameter — which is why a small end mill spins so much faster than a large one for the same job.

The familiar SFM × 3.82 ÷ D shortcut is this same formula with 12/π folded into a constant. It is accurate to within a fraction of a per cent.

Published speeds assume adequate rigidity and coolant. A light machine, a long tool overhang or a dry cut all want the figure reduced, sometimes substantially.

Cutting speed belongs to the material and the tool; rpm is what the machine has to do to achieve it at a given diameter. That is why a small end mill spins so much faster than a large one for identical work.

Why use this tool?What it does differently03

Why use this tool?

Any diameter

Turning or milling, from a small end mill to a large workpiece.

Metric alongside

Surface metres per minute for tooling data published in metric.

Explains the constant

The familiar 3.82 shortcut is 12/π folded together — not a fudge.

Rigidity caveat

Published speeds assume a rigid setup and coolant. Light machines want less.

How this worksThe method04

How this sfm to rpm calculator works

Spindle speed is cutting speed times twelve, divided by π times the diameter. The twelve converts feet to inches; the π converts a circumference to a diameter.

The widely used shortcut of SFM × 3.82 ÷ D is this same formula with 12/π precomputed. It is accurate to within a fraction of a per cent and exists because it is easier to remember.

Cutting speed is a property of the material being cut and the tool cutting it, so it stays constant while the rpm needed to reach it changes with diameter. Halving the diameter doubles the rpm.

How to use itStep by step05

How to use it

  1. Step 1: Look up the cutting speed

    From the tooling manufacturer for your material and tool combination.

  2. Step 2: Enter the diameter

    Tool diameter for milling and drilling; work diameter for turning.

  3. Step 3: Set the spindle

    Or the nearest speed your machine offers.

  4. Step 4: Reduce for a light setup

    Long overhangs, thin walls or no coolant all want a lower figure.

Example usageWorked figures06

Example usage

A half-inch end mill
300 SFM on a 0.5 in cutter is 2,292 rpm.
A quarter-inch cutter
The same 300 SFM on 0.25 in needs 4,584 rpm. Halving the diameter doubles the speed.
Turning a larger part
300 SFM on a 3 in workpiece is only 382 rpm — the same cutting speed at the tool, a very different spindle figure.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert SFM to RPM?

Multiply SFM by 12 and divide by π times the diameter in inches. The common shortcut is SFM × 3.82 ÷ diameter, which is the same formula precomputed.

What is SFM?

Surface feet per minute — how fast the cutting edge travels past the material. It is a property of the material and tool rather than of the machine.

Why does diameter change the rpm?

Because a larger circumference covers more surface distance per revolution. To keep the same surface speed, a larger diameter has to turn more slowly.

Should I use tool or workpiece diameter?

Tool diameter for milling and drilling, where the tool rotates. Workpiece diameter for turning, where the work rotates.

Are published cutting speeds always safe?

They assume adequate rigidity, coolant and a healthy machine. A long tool overhang, a light machine or a dry cut all want the figure reduced.

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