Cutting Speed to RPM Calculator
Enter surface speed and diameter to get spindle RPM — the first number every speeds-and-feeds calculation needs, in both metric and SFM.
Last updated: 2026-09-28
How the calculation works
- Surface speed at the tool's edge is π·D per revolution — RPM follows directly from the target Vc.
- SFM input converts to metric internally; the answer is the same RPM either way.
- The surface-speed check row back-calculates so you can verify a machine-limited RPM choice.
Formula
n = Vc × 1000 / (π × D)
| Symbol | Meaning | Unit |
|---|---|---|
n | Spindle speed | RPM |
Vc | Cutting (surface) speed | m/min |
D | Diameter | mm |
Worked example
Interpreting the result
Vc is the material-tool contract: too fast burns coatings and edges, too slow rubs and work-hardens. Machine the nearest available speed BELOW the calculation — the few percent lost RPM costs far less than the tool life gained. Diameter is the lever on small tools (a 3 mm cutter needs 12,700 RPM for the same Vc), which is why machines with high-speed spindles pay for themselves on small-diameter work.
Assumptions
- Constant surface speed idealization; real machines pick the nearest fixed RPM.
- Starting values — tool manufacturers' cutting data override any table here.
Limitations
- Does not account for machine rigidity, overhang or chatter conditions — reduce Vc for light machines.
- Ceramics, CBN and exotic tooling run outside the typical carbide ranges.
Frequently asked questions
How do I convert SFM to RPM?
RPM = SFM × 3.82 ÷ diameter (inches) — or enter SFM here; the conversion to metric happens internally.
What RPM for a 6 mm endmill in aluminum?
At Vc 300 m/min: 15,900 RPM — beyond most machine spindles, so run the machine's max RPM and feed by chip load. Small tools are surface-speed starved on standard spindles.