SFM to RPM Calculator
Convert cutting speed (SFM or SMM) to spindle RPM for a given tool diameter, or read the cutting speed from an RPM. Inch and metric, in your browser.
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Show the math
Spindle speed comes from the cutting speed and the tool (or work) diameter:
RPM = (SFM × 12) ÷ (π × D″)
RPM = (SMM × 1000) ÷ (π × D mm)
Cutting speed (SFM/SMM) is the speed at the tool's edge. Pick it from your material and tooling, then this gives the spindle RPM, or enter RPM to read the resulting cutting speed.
What this does
This calculator converts cutting speed (SFM or SMM) to spindle RPM for a given tool or part diameter, and can also read the cutting speed back from a known RPM. It works in inch and metric units.
How to use it
- Enter the cutting speed from your tooling chart.
- Enter the tool or part diameter.
- Read the spindle RPM to set.
- Or enter RPM to find the cutting speed.
How it works
Cutting speed is the speed at the tool edge; RPM is how fast the spindle turns. In inches RPM = (SFM × 12) ÷ (π × diameter); in metric RPM = (SMM × 1000) ÷ (π × diameter in mm).
RPM = (SFM × 12) ÷ (π × D)
Understanding your result
The RPM figure is the spindle speed that holds your chosen cutting speed at that diameter, so it changes whenever the tool or part size does, with smaller diameters needing higher RPM for the same surface speed. Treat it as a starting point and fine-tune to the material and the tooling makers guidance.
Example
A 0.5-inch tool at 100 SFM needs about 764 RPM. The same tool at 300 SFM needs about 2,292 RPM.
Sources & methodology
Last updated .
Frequently asked questions
How is RPM found from cutting speed?
In the inch system RPM = (SFM × 12) ÷ (π × diameter in inches). In metric, RPM = (SMM × 1000) ÷ (π × diameter in mm). Cutting speed is the speed at the tool edge; RPM is how fast the spindle turns.
What cutting speed should I use?
It depends on the material and tool. Pick the recommended SFM/SMM from your tooling chart, enter the diameter, and this gives the spindle speed to set.
Is my data stored?
No. Everything is calculated locally in your browser.
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