Bowl Blank Calculator

That 1" allowance covers the truing cuts you make once the bowl is mounted on the lathe — the outside is rarely perfectly round or centered coming off the bandsaw, so you need extra stock to true up the rim and remove any tearout or bark inclusion before the outside profile is finished. Cutting a blank to the exact finished diameter leaves no margin for that step, so you'd end up with a smaller bowl than planned or a rim that never gets fully cleaned up.

The calculator compares the cylindrical blank's volume against an estimated finished bowl volume, where the bowl is modeled as the difference between two hemisphere-like shapes (the outer form minus the hollowed interior sized by your wall thickness). Because a cylinder is a much denser shape than a shallow hollowed bowl, most of that cylinder — often well over half — ends up as shavings on the floor, which is a normal and expected outcome of bowl turning, not a sign of a poor blank choice.

Treat it as a starting ceiling, not a target: the 6000/diameter rule is meant to keep rim speed in a safe range once the blank is round and balanced, but a freshly bandsawn or green blank is usually lopsided and should be started well below that number and increased only after the outside is trued up and running smoothly. Green wood in particular can throw bark or fling water at speed, so many turners start noticeably slower than the suggested RPM until the blank is balanced.

Wood shrinks primarily in the tangential direction as it dries — roughly 8-12% across the grain — which shows up as a reduction in diameter, not thickness, so the calculator scales the blank diameter rather than the wall allowance. This assumes you're rough-turning oversized and letting the piece move before a final finishing pass; if you're turning a green bowl to final dimensions in one pass, expect it to warp out-of-round as it dries regardless of this allowance.