Kerf Bending Calculator
Tell it the curve you want and the stock you have — get the kerf count, spacing, and cut depth, derived from real geometry rather than guesswork.
Verified against independent hand calculation · sources cited below
8 kerfs, spaced 1.33″ apart
- Cut depth
- 0.63″
- Bend-zone length
- 10.6″
- Per-kerf rotation
- 11.3°
Kerfs go on the inside of the bend, evenly spaced across the bend zone. Cut a test piece from the same stock first, bend dry, then glue the kerfs (or fill with glue-soaked veneer) to lock the curve.
The geometry, shown honestly
When a kerfed board bends, the outer web acts as a hinge line and every kerf closes like a tiny pie slice. A kerf of width w in stock of thickness t with web d can close through w ÷ (t − d) radians before the faces touch. Divide the total bend angle by that per-kerf limit and you have the minimum kerf count; spread them evenly across the bend zone (arc length at the outside face) and the curve comes out fair instead of faceted.
Getting a clean result
- Cut every kerf at identical spacing — a story stick or a spacing jig on the miter gauge beats measuring each line.
- Bend around a form, not freehand; the form defines the radius your math assumed.
- Orient the kerfs inside whenever looks allow — the continuous face shows, the kerfs hide.
- Add one or two extra kerfs beyond minimum if the calculator warns the spacing is tight; slightly under-bent kerfs close sweeter than jammed ones.
Frequently asked questions
How does kerf bending actually work?
You cut evenly spaced kerfs across the inside face of the bend, leaving a thin continuous web on the outside. Each kerf can close by its own width, and every closed kerf contributes a small rotation — enough kerfs, evenly spaced, and the board wraps a smooth curve. The math: each kerf allows roughly kerf width ÷ (thickness − web) radians of bend.
How deep should the kerfs be?
Deep enough to leave a web of about 1/16″–1/8″. A thicker web fights the bend and can crack the outside face; thinner risks sawing through on a wavy board. Set your saw’s depth stop using a test scrap of the exact stock, and sneak up on it.
Why did my board crack while bending?
Usually one of three things: the web was too thick (too much wood resisting), the kerfs were too far apart (each one forced past its closing limit), or the stock was too dry and brittle. A spritz of water or a damp towel on the outside face before bending helps noticeably — and always bend slowly, closing kerfs progressively.
How do I make the bend permanent?
A kerf bend has almost no strength until you lock it: work glue into the kerfs and clamp to a form until dry, laminate a veneer or thin skin over the kerfed face, or fill the kerfs with glue-coated spline strips for the strongest result. Unglued kerf bends will relax and creak.
Sources & further reading
- First-principles closing geometry: kerfs n = θ(t − web) ÷ kerf width; spacing = bend-zone arc ÷ n
- Standard shop practice for webs, test pieces, and glue-locking from established woodworking references