Lathe Speed Calculator

Every point on a spinning workpiece traces a circle, and a wider workpiece covers more distance per rotation than a narrow one at the same RPM — which is exactly why lathe speed has to scale inversely with diameter to keep the cutting edge moving across the wood at a safe, consistent surface speed. This calculator solves that relationship directly: RPM = (SFPM × 12) / (π × Diameter), where SFPM is the target surface feet per minute for your material and the ×12 converts feet to inches to match a diameter entered in inches. Softwood is rated for 800 SFM since it's soft and forgiving at higher speeds, hardwood drops to 600 SFM because its density generates more friction and heat at the same surface speed, and green or wet wood is capped at 400 SFM since wet stock is more prone to catching, splitting along grain, or flinging water and debris at higher speeds. Rather than giving a single number, the tool reports a roughing speed at 75% of the calculated optimum (slower, for knocking down an out-of-round blank safely) and a finishing speed at 125% (faster, for a clean final surface), bracketing the calculated RPM by ±25%.

It also reports the workpiece's circumference (π × diameter) for reference. Keep in mind this formula assumes a single point of material at the widest diameter — as you turn down a blank, the safe speed actually increases, so these numbers represent your starting point on an unturned or roughed blank, not a fixed setting for the whole project. Always start on the slower end and listen/watch for vibration before pushing toward the finishing RPM.