Lathe RPM calculator

Turn a cutting speed into spindle speed, or find the surface speed you’re actually getting. On the lathe the diameter is the work; on the mill or drill it is the cutter.

How it works

Bigger work needs a slower spindle. Pick a cutting speed for your material (the speeds & feeds page suggests one), enter the diameter, and the calculator gives the rev/min. Switch to Detailed for the formula.

Cutting (surface) speed is how fast the work passes the tool edge. For a given material and tool it should stay about the same, so a smaller diameter needs a higher spindle speed. The formula is n = 1000 × Vc / (π × D) with Vc in m/min and D in mm. The imperial version is n = 12 × SFM / (π × D), with D in inches.

Worked example: a 25 mm bar of EN1A with an HSS tool at 30 m/min: n = 1000 × 30 / (π × 25) = 382 rev/min. On a back-geared Myford, pick the nearest speed below that.

Not sure what cutting speed to use? The speeds & feeds calculator suggests one for each material and tool.

Safety: never exceed your chuck’s rated speed. Large or out-of-balance work needs much lower speeds than the formula gives. Keep guards in place and remove the chuck key.

Spotted a mistake? Report an error.