🌀 Kerf Bending Plywood Calculator — Curve & Circle Cuts

Measure the circumference of the inside face — the face the grooves go on — then divide it by the pitch of each cut, which is kerf width plus the web you leave between kerfs. For a 36-inch-diameter circular tabletop from 3/4-inch plywood, the inside face circumference is π × (36 − 1.5) ≈ 108.4 inches; with a 1/8-inch blade and a 1/8-inch web the pitch is 1/4 inch, so you cut about 434 grooves. The calculator above runs this for any radius, diameter, kerf, and web you enter.

Kerf bending is cutting a series of evenly spaced parallel slots (kerfs) partway through a panel so the remaining thin layer of wood — the web — can flex, letting the panel wrap around a curve. The deeper and closer the kerfs, the tighter the bend. It is the standard shop method for curved cabinet faces, rounded window returns, column wraps, and circular floor curves, using nothing more than a saw with a crosscut sled or a radial-arm setup.

Deep enough that the panel bends without force, shallow enough to keep a strong web. The rule: never leave less than 1/8 inch of web thickness. On 3/4-inch stock that means grooves up to 5/8 inch deep; on 1/2-inch stock, grooves up to 3/8 inch. Going deeper than that — or leaving a thinner web — causes the web to snap on the bend, usually at the first kerf and usually audibly.

Spacing equals kerf width plus web width. With a 1/8-inch saw blade and a 1/8-inch web, that is a 1/4-inch pitch — about 150 grooves across the inside face of a 24-inch-radius half-circle in 3/4-inch ply. Tighter radii need tighter spacing: the quick-reference table above lists starting spacings from 3/16 inch on 1/4-inch stock up to 3/4 inch on 1-inch stock.

Because the bend concentrates at each kerf. A kerf-bent surface is a series of flat facets between slots, so thin ply with wide spacing shows pronounced scalloping. Three fixes, in order of effort: tighter kerf spacing, a thinner panel with more kerfs, or filling the kerfs with glue and sanding the curve fair before veneering. Filling with epoxy or wood filler plus a heavy sanding pass makes most scallops invisible under paint.

No — not in the bending direction. Kerfing severs every fiber that would carry bending load, so a kerf-bent panel has essentially zero structural strength across its own span. Kerf-bent panels lose all bending-direction strength, so they must be glued and fastened to a rigid substrate, framed, or laminated against a solid core. A kerf-bent circular floor, for example, must sit over conventional joist framing — the kerfed skin is decorative.

Odd-numbered ply construction with a single core species bends most predictably — Baltic birch is the shop favorite because its thin plies crack less than construction-grade fir ply. Avoid OSB (it crumbles at the webs) and avoid boards with interior voids, which open into visible holes at every kerf. For radii under 6 inches, skip kerfing entirely and use 1/4-inch bendy plywood, which needs no grooves at all.

Always the inside — the concave face. The kerfs must open up as the panel curves, which only happens on the face that gets shorter. Cut them on the outside and the slots close instead, the panel refuses to bend, and the webs crush. If your test bend tightens the kerfs, you have the panel facing the wrong way: flip it over.

Mask the outside face with packing tape first, then flood the kerfs from the inside with thin glue and let it cure. The tape stops squeeze-out from gluing your cauls to the show face. For the stiffest result, fill every kerf with glue and a sawdust slurry before bending — filled kerfs cure into solid hinges, and the panel holds its curve permanently even after the form comes off.