Bowl Blank Calculator — Blank Dimensions for Turned Bowls

Allow at least 10mm extra on the diameter (5mm each side) for truing up the blank on the lathe. For depth, add 10-15mm for the mounting tenon or waste block. For green (wet) wood, add an extra 10% to allow for shrinkage as the bowl dries.

For dry-turned bowls, 6-10mm is typical for decorative pieces, 8-12mm for functional bowls. For green-turned bowls that will distort as they dry, turn to 10% of the diameter (e.g., 25mm walls for a 250mm bowl) and re-turn after drying.

Use the safe-speed band: the blank diameter in inches multiplied by RPM should stay between roughly 6,000 (low) and 9,000 (absolute maximum). A 200mm finished bowl gives a 220mm (~8.7 inch) blank, so the upper guide is about 900 RPM (8,000 ÷ 8.7) and the calculator shows this as the "suggested upper speed". For roughing an unbalanced blank, START nearer 600-700 RPM (6,000 ÷ 8.7) and raise the speed only as the blank trues up and runs smoothly.

The faceplate diameter should be roughly one-third of the bowl diameter. For a 200mm bowl, use a 60-75mm faceplate. The faceplate must be fully supported by solid wood - never let screws go into the part of the blank that will become the bowl interior.