Kerf Bending Calculator
How far apart to cut the kerfs to bend a board or sheet to a radius, how deep to cut them, and how many cuts the curve takes.
Result
2 3/8 in
Ideal geometry; wood varies, so cut a test piece on scrap before the real board.
- Kerfs to cut
- 8
- Depth of each cut
- 5/8 in
- Length of the bend on the outside face
- 18.8 in (1.6 ft)
- Turn at each closed kerf
- 11.46 °
- Note
- An estimate from ideal geometry: cut a test piece first. Not for structural or load-bearing parts.
https://calcgallery.com/lumber-carpentry/kerf-bending-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Kerf Bending Calculator
- Enter the board's thickness, how much wood to leave uncut, and your blade's kerf.
- Enter the radius of the curve and whether it is measured to the outside or the inside face.
- Enter the angle of the bend, 90° for a quarter circle.
- Mark the kerfs at the spacing shown, cut them to the depth shown, and test on scrap first.
How it is calculated
cut depth = board thickness − wood left uncutspacing = outside radius × kerf width ÷ cut depthbend length = outside radius × bend angle (in radians)kerfs = ⌈bend length ÷ spacing⌉When the board bends, each kerf closes until its two sides touch. A kerf of width w and depth d closing about the thin web turns the board by w ÷ d radians, so the number of kerfs in a bend is the angle divided by that, and they are spread over the length of the curve. This is the usual idealised geometry behind kerf spacing.
Limits
Real wood does not close every kerf perfectly: it springs back, the web can stretch or crack, and knots and grain change how a board bends. Treat the spacing as a starting point, cut a test piece, and adjust. Thinner kerfs, spaced closer, give a smoother curve than a few wide ones. Not for structural or load-bearing parts.
With your numbers
- Cut depth = 0.75 in − 0.125 in = 0.625 in
- Spacing = outside radius × kerf ÷ cut depth = 12 in × 0.125 in ÷ 0.625 in = 2.4 in
- Kerfs = ⌈18.85 in ÷ 2.4 in⌉ = 8
Quick reference
Ideal geometry; wood varies, so cut a test piece on scrap before the real board.
| Job | Kerf spacing, centre to centre |
|---|---|
| 3/4 in board, 12 in radius, 90° | 2 3/8 in |
| 3/4 in board, 6 in radius, 90° | 1 3/16 in |
| 3/4 in board, 24 in radius, 180° | 4 13/16 in |
| 1/2 in plywood, 3/32 in kerf, 8 in radius | 1 11/16 in |
| 12 in inside radius, 90° | 2 9/16 in |
| 18 mm board, 300 mm radius, 90° | 60 mm |
| 18 mm board, 500 mm radius, 180° | 100 mm |
| 12 mm ply, 2.5 mm kerf, 200 mm radius | 50 mm |
Common questions about the Kerf Bending Calculator
- How far apart should kerf cuts be?
- Far enough apart that each cut closes up exactly when the board reaches the curve. That spacing is the radius times the kerf width divided by the depth of the cut. Halving the radius halves the spacing, and a thicker blade or a shallower cut spreads the kerfs further apart.
- How deep should kerf cuts be?
- Deep enough to leave a thin skin of wood that bends without cracking. The right thickness depends on the species, the grain and the moisture in the wood, so cut a few test kerfs in scrap from the same board before committing.
- Which side do the kerfs go on?
- On the inside of the curve, so the cuts close up as the board bends. The uncut face becomes the smooth outside. That is why the page measures the radius to the outside face; if you only know the inside radius, choose that option and it adds the board thickness.
- Is a kerfed board strong?
- No. Kerfing removes most of the wood across the bend, leaving a thin web. It suits trim, face frames, curved panels and cabinet sides that are supported or glued up. Do not use kerf bending for structural or load-bearing parts.