Lathe RPM and cutting speed calculator
Enter the diameter and cutting speed to get spindle RPM — or enter RPM to see the actual cutting speed. You also see whether the spindle hits its limit, with ready G96/G97 lines.
Quick answer
Spindle speed RPM = 12 × SFM / (π × D), D in inches. Example: Ø2″ at 600 SFM → 12 × 600 / (3.1416 × 2) ≈ 1146 RPM. Metric: n = 1000 × Vc / (π × D), Ø50 mm at 180 m/min ≈ 1146 RPM.
Spindle speed
— RPM
Feed: — mm/min
LinuxCNC lines
Starting cutting speeds (OD turning)
⚠ These numbers are approximate, not rules. Cutting data is chosen from the insert maker's recommendations and the rigidity of your particular machine, so yours may differ noticeably. Start conservatively and adjust by the result.
Click a number to load it into the calculator.
| Material | Carbide, m/min | HSS, m/min |
|---|---|---|
| Mild / medium carbon steel | ||
| Stainless steel | ||
| Cast iron | ||
| Aluminum | ||
| Brass, bronze | ||
| Plastics (nylon, acetal) |
Who needs it and why
Insert makers give the cutting speed in SFM or m/min, but the machine needs spindle RPM — and that depends on the diameter. Catalogs give feed per revolution, while a program often needs feed per minute. The calculator converts one into the other in a second.
- Setting up cutting data — from an insert catalog: S from the cutting speed and diameter, F per minute from feed per rev.
- Using G96 — to pick a sensible RPM cap D, so the spindle doesn't run away on a small diameter or when parting off.
- Checking — what speed the tool actually cuts at with your RPM — not burning the insert on a big diameter or rubbing on a small one.
- Machines without G96 — manual lathes or CNC with gear ranges: which speed step suits the diameter.
The formula
RPM = 12 × SFM / (π × D), with D in inches. Back: SFM = π × D × RPM / 12. Metric: n = 1000 × Vc / (π × D), Vc in m/min, D in mm.
G96 and G97 in LinuxCNC
G96 holds a constant surface speed: S is in m/min (G21) or ft/min (G20), and D is the RPM cap — without it the spindle runs away as the tool nears the center. G97 is plain constant RPM. Details with LinuxCNC and Fanuc examples are in the article G96 vs G97: constant surface speed on a CNC lathe.
To get a full turning program with these speeds — roughing, finishing and a 2D toolpath view — use the free online generator. All 12 lathe cycles are in the Turn G-code app.