Choose a safe starting speed for a wood lathe from the diameter of the blank. The calculator applies the widely used diameter × RPM rule for roughing and finishing and can cap the result at your lathe’s maximum.
On a lathe the work spins, not the tool, and the outside of a large blank is moving far faster than the center. A 12 in bowl blank at 1,000 RPM has its rim passing the tool at more than 3,000 ft/min; a 2 in spindle at the same speed is barely a sixth of that. Out-of-balance blanks make the problem worse, because the force trying to throw the blank off grows with the square of the speed.
A rule that has served woodturners for a long time keeps diameter × RPM between about 6,000 and 9,000: the low end for roughing, the high end as a ceiling once the piece is round and balanced. The calculator does that division for you.
Rough RPM ≈ 6,000 ÷ D and finishing maximum ≈ 9,000 ÷ D, with D in inches. You can change both constants if you prefer a more cautious or more aggressive rule, and enter your lathe’s top speed to cap the result.
Rough ≈ 6000 ÷ D · Finish max ≈ 9000 ÷ D
| Diameter | Rough RPM | Finish max RPM |
|---|---|---|
| 2 in | 3,000 | 4,500 |
| 3 in | 2,000 | 3,000 |
| 4 in | 1,500 | 2,250 |
| 6 in | 1,000 | 1,500 |
| 8 in | 750 | 1,125 |
| 10 in | 600 | 900 |
| 12 in | 500 | 750 |
| 14 in | 429 | 643 |
| 16 in | 375 | 563 |
| 18 in | 333 | 500 |
| 20 in | 300 | 450 |
For small diameters the rule gives speeds above what most lathes can reach. That is a ceiling, not a target — pen blanks and small spindles turn cleanly well below it.
A 10 in bowl blank, freshly bandsawn. Rough at about 600 RPM. If the lathe shakes at that speed, drop lower until it runs smoothly, and true the outside first. Once round and balanced, you can work up toward 900 RPM for finishing cuts.
A 14 in platter. 6,000 ÷ 14 = 429 RPM roughing, 643 RPM maximum finishing. On many midi lathes this is near the bottom of the speed range, so be sure the lathe can go that slow before mounting a blank that size.
A 3 in table-leg spindle on a lathe that tops out at 2,400 RPM. The rule says rough at 2,000 and finish up to 3,000. Enter 2,400 in the cap field and the finishing figure is held to 2400 RPM.
Wet, heavy, or natural-edge blanks deserve less than the rule. So do segmented work with uncertain glue joints and anything with voids. Very small diameters can usually run faster than a lathe goes, but sanding at top speed heats the surface and can check the wood. The rule exists to keep the rim speed sensible, not to replace judgment.
The largest diameter that is spinning right now. Recalculate as the piece gets smaller, or when a square blank becomes round (a square’s diagonal is about 1.41 × its side).
The blank is out of balance or the lathe is light. Lower the speed, true the blank, and add weight to the stand if you can.
Convert the diameter to inches (mm ÷ 25.4). The constants are inch-based.
Many turners sand at or below the finishing speed to avoid heat checks. Faster is not always smoother.