🪵 Lathe Speed Calculator

Enter your workpiece diameter and a target surface speed (surface feet per minute) to get a recommended lathe RPM setting. Lower RPM for larger, rougher blanks; higher RPM for smaller, finished spindle work.

Blank diameter (in)

Target surface speed (SFM)

How surface speed becomes RPM

The rim travels πD inches per revolution, so the script converts feet to inches and divides:

RPM = (SFPM × 12) / (π × D)

D    = blank diameter, inches
SFPM = target surface feet per minute

With the defaults - a 10 in blank at 300 SFPM - that is 3600 / 31.416, or 115 RPM. Hold 300 SFPM and drop to a 4 in spindle and the same sum gives 286 RPM. Diameter sits on the bottom of the fraction, so the bigger the blank, the lower the speed that holds its rim at one surface speed.

Reading the two inputs

Measure across the widest point the blank will actually swing - the diagonal of a square blank, not the finished bowl. At 300 SFPM:

Diameter	RPM at 300 SFPM
2 in	573
4 in	286
8 in	143
12 in	95
16 in	72

The tool neither picks a surface speed for you nor caps its answer. Leave the diameter box empty and the script substitutes 0.1 in, returning over 11,000 RPM.

This is an arithmetic conversion, not a safety rating. Your lathe's speed range and the ratings on your chuck and faceplate govern; where any is lower than the number here, the lower figure wins. A blank that is out of balance, cracked, bark-edged or wet needs slower still - bring the speed up only once it runs without vibration.

Frequently asked questions

What RPM should I use for a 10 inch bowl blank?

At the default 300 SFPM, a 10 in blank works out at 115 RPM. That figure is only as good as the surface speed you chose, and assumes a sound, balanced blank on a lathe rated for it - check your machine's manual first.

Why does the recommended speed fall as the blank gets bigger?

Because the rim covers more distance per turn. Doubling the diameter doubles the circumference, so RPM must halve to keep the same surface feet per minute, and the forces thrown by any imbalance rise as well.

What surface speed should I enter?

The page starts at 300 SFPM and leaves the choice to you. Published figures vary with the wood, the cut and the machine, so take it from your lathe's documentation rather than treating one value as standard.