Turning surface speed into spindle RPM
Every cutting tool has a sweet spot measured in surface speed — how fast the cutting edge should travel past the material. But your machine is set in RPM. This calculator bridges the two: feed in the recommended cutting speed for your material and the tool diameter, and it returns the spindle speed to dial in.
The RPM formula
Imperial: RPM = (SFM x 12) / (pi x D), with diameter in inches and surface speed in surface feet per minute. Metric: RPM = (Vc x 1000) / (pi x D), with diameter in millimetres and cutting speed in metres per minute. Small tools spin fast, big tools spin slow — the diameter sits in the denominator.
Typical cutting speeds
These are ballpark carbide speeds for milling. High-speed-steel (HSS) tooling runs at roughly one third of these, so cut them down if you are not running carbide.
| Material | Carbide (SFM) | HSS (SFM) |
|---|---|---|
| Aluminum | 600 – 1000 | 250 – 350 |
| Brass / bronze | 400 – 600 | 150 – 250 |
| Mild steel | 350 – 450 | 90 – 120 |
| Stainless steel | 150 – 250 | 50 – 80 |
| Cast iron | 250 – 350 | 60 – 90 |
| Titanium | 100 – 175 | 30 – 50 |
Worked example
A half-inch carbide end mill in mild steel at 400 SFM: RPM = (400 x 12) / (pi x 0.5) = 3056 RPM. Drop to HSS at about 120 SFM and the same tool wants roughly 917 RPM.
FAQ
Do I use the tool diameter or the part diameter?
For milling and drilling, use the tool diameter — the cutter is what is spinning. For turning on a lathe, use the workpiece diameter, since the part spins and the tool is stationary.
Why did my speed come out so high?
Small diameters drive RPM up quickly. A tiny end mill at full carbide speed can ask for tens of thousands of RPM — more than many machines can deliver, in which case you run at the spindle maximum and accept a lower surface speed.