Lathe Cutting Speed (RPM) Calculator
Calculate the correct lathe RPM for any material and cutting tool diameter.
Avoid burning tools or damaging workpieces with the wrong spindle speed.
Why cutting speed matters:
Running a lathe at the wrong RPM damages tools and workpieces. Too fast and the tool tip overheats, loses hardness and fails, or the workpiece chatters and comes out rough. Too slow and the tool rubs instead of cutting, which gives a poor finish and wastes time.
The fundamental formula:
RPM = (Cutting Speed × 12) ÷ (π × Diameter)
Or using the simplified machinist’s formula:
RPM = (CS × 3.82) ÷ Diameter (inches)
Where:
- CS = Cutting Speed in Surface Feet per Minute (SFM)
- Diameter = workpiece diameter in inches at the cut
- 3.82 = 12 ÷ π (a machinist’s shortcut)
For metric (m/min and mm):
RPM = (CS × 1,000) ÷ (π × Diameter in mm)
Recommended cutting speeds (SFM) by material:
| Material | HSS Tool | Carbide Tool |
|---|---|---|
| Aluminum (6061) | 200–300 SFM | 600–1,000 SFM |
| Mild steel (1018) | 80–100 SFM | 300–500 SFM |
| Stainless steel (304) | 50–80 SFM | 200–350 SFM |
| Cast iron | 60–80 SFM | 200–300 SFM |
| Brass / bronze | 150–200 SFM | 400–600 SFM |
| Copper | 100–150 SFM | 400–600 SFM |
| Titanium | 25–50 SFM | 100–200 SFM |
| Nylon / plastics | 200–400 SFM | 600–1,000 SFM |
Worked example:
Turning 1-inch diameter mild steel with an HSS tool at 90 SFM:
- RPM = (90 × 3.82) ÷ 1.0 = 344 RPM
- Select the nearest lower lathe speed (e.g., 315 or 340 RPM depending on your lathe’s speed steps)
One thing the formula will happily hide from you: on a big diameter in a slow material the answer drops below anything the machine can do. A 6-inch titanium billet at 25 SFM works out to 16 RPM, and the slowest step on most hobby lathes is somewhere around 50. When that happens the honest answer is that the machine is running the cut faster than the material wants, so take a lighter cut, flood the coolant, and accept shorter tool life.
Tool life and feeds:
RPM gives you spindle speed; combine it with feed rate (inches per revolution) for surface finish:
- Rough cuts: 0.010–0.020" per revolution
- Finishing cuts: 0.002–0.005" per revolution
Coolant:
Steel and stainless: use cutting fluid (sulfurized oil or water-soluble coolant). Aluminum: use kerosene or WD-40 for better finish. Brass and cast iron: typically run dry.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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