RPM Calculator
Free RPM calculator: convert cutting speed (Vc) and diameter into spindle speed, metric or SFM.
Enter the tool or workpiece diameter plus the cutting speed the material needs, and this RPM calculator returns the spindle speed: n = Vc × 1000 / (π × D) in metric units, or rpm ≈ 3.82 × SFM / D in inches. It works the same for milling cutters, lathe workpieces, router bits and drills.
FAQ
How do you calculate RPM from cutting speed?
Divide the cutting speed by the tool circumference. Metric: rpm = (Vc × 1000) / (π × D), Vc in m/min and D in mm. Example: a 10 mm cutter in a material wanting Vc = 100 m/min runs at (100 × 1000)/(π × 10) ≈ 3,183 rpm.
What is the RPM formula in inches (SFM)?
rpm = (SFM × 12) / (π × D) with D in inches, usually shortened to rpm ≈ 3.82 × SFM / D. A 1-inch cutter at 100 SFM runs at 382 rpm; a 1/2-inch cutter needs twice that, 764 rpm.
How many RPM is 100 SFM?
It depends on diameter: 100 SFM is 382 rpm at 1 inch, 764 rpm at 1/2 inch and 1,528 rpm at 1/4 inch. Halve the diameter and the rpm doubles, because each revolution carries the edge half as far.
What RPM should a 6 mm router bit run at?
Hardwood (Vc 150–300 m/min) puts a 6 mm bit at 8,000–16,000 rpm; plywood and MDF (200–350 m/min) at 10,600–18,600 rpm — near the top speed of most hand routers. Overspeeding scorches glue lines and rounds over chip edges.
What is the difference between RPM and cutting speed?
RPM counts spindle revolutions; cutting speed (Vc) is how fast the edge moves through the material and grows with diameter at the same RPM. A 100 mm face mill at 500 rpm shears at 157 m/min while a 10 mm end mill at the same 500 rpm manages only 15.7 m/min — which is why catalogs quote Vc, not RPM.
Can the RPM be too high for the tool?
Yes. Beyond the recommended Vc, heat piles up at the edge: glue lines scorch, acrylic melts, carbide wears many times faster. If you must run fast, keep chip load per tooth up through the feed rate — the feeds & speeds calculator combines RPM, flutes and chip load into that number.