The calculation is a simple rule of thumb used by woodturners everywhere.

1. Measure Max Diameter: Find the largest diameter of your rough, unbalanced blank in inches.
2. Apply the Formula: The calculator divides a constant (typically between 6000 and 9000) by the diameter to find the recommended maximum starting RPM. We use a constant of 8000 for a safe, middle-of-the-road estimate.
   RPM = 8000 / Diameter (in)

• RPM (Revolutions Per Minute): The speed at which the lathe's headstock and the workpiece are spinning.
• Blank: The rough, often unbalanced piece of wood that you mount on the lathe to begin turning.
• Catch: A dangerous event where the turning tool digs too deeply into the wood, which can shatter the workpiece or wrench the tool from your hands. Proper speed and tool presentation help prevent catches.

Example 1: An 8-inch rough bowl blank.
8000 / 8 = 1000 RPM. That is your safe top speed while the square, unbalanced blank is out of round. Once it is trued to a spinning disc you can step up toward 1500-1800 RPM for a cleaner cut.

Example 2: A 12-inch wet firewood slab.
A fat 12-inch off-cut gives 8000 / 12 = 667 RPM. Big diameters drop fast, which is exactly the point — the heavier the wobble, the slower you must start.

Example 3: A 4-inch pepper mill blank.
Small stock is far more forgiving: 8000 / 4 = 2000 RPM, leaving plenty of headroom to raise speed once it is round.

• Measuring the Wrong Diameter: Use the widest point of the blank, not the section under the tool. A bowl with a flared rim is governed by that rim.
• Skipping the Slow Start: Always bring the lathe up to speed gradually and stand to the side for the first revolutions while the imbalance is still severe.
• Treating the Number as a Ceiling for Round Stock: The rule is for rough blanks. Reassess once the piece is balanced — but only after the wobble is gone.
• Ignoring a Loose Mount: A speed you would trust on a sound mount can tear a blank off loose screws or a poorly seated faceplate. Check the fixturing before every run.

"Start low, always. I bring a rough blank up to about half the calculated speed, watch and listen for one rotation, then creep toward the number. The lathe will tell you when it has had enough — a vibration you can feel in the bed is your real limit, not the chart." - Experienced Turner

"Your faceguard and hand position matter more than 100 RPM either way. The speed rule reduces the chance of a disaster; the guard and standing off-line mean a disaster that does happen doesn't take your face or hands with it."

• Green bowl blanks: Freshly cut, out-of-round rounds that need the slowest start.
• Reclaimed firewood: Irregular, heavy slabs with hidden checks that demand a conservative first speed.
• Multi-facet spindles: Square stock mounted between centers that only smooths out after several light passes.