Wood Bending Calculator — Steam, Kerf & Laminate

It depends on the species and thickness. As a rule of thumb, the minimum steam bend radius is 10-25 times the wood thickness. Ash and oak bend the tightest at about 8-10 times thickness. Pine and cherry are poor steam benders, needing 18-25 times thickness. Using a metal backing strap allows tighter bends by preventing the outer fibres from stretching.

The general rule is one hour of steaming per 25mm of thickness. So a 20mm board needs about 45-50 minutes. Do not over-steam — the wood becomes weak and mushy. Under-steaming causes cracking. The wood should be at or above fibre saturation point (around 28% MC) throughout its thickness.

Kerf bending uses a series of parallel saw cuts on the inside face of a curve. The cuts remove material so the wood can flex. The spacing between kerfs determines the bend radius — closer spacing gives tighter curves. After bending, the kerfs are filled with glue and/or veneer. It is best for hidden surfaces or laminated assemblies.

Laminate bending (gluing thin strips around a form) is the most reliable method for consistent, spring-back-free curves. It works with any species and can achieve very tight radii. The downsides are visible glue lines, more material prep and clamping time. It is the preferred method for chair parts, handrails and musical instruments.

Cracking occurs when the outer fibres of the bend stretch beyond their limit. The main causes are: bending too tight, under-steaming, cross grain in the stock, knots or defects, and bending too slowly (the wood cools). Using a metal backing strap on steam bending dramatically reduces cracking by limiting the outer fibre stretch.

Kerf spacing ≈ kerf width × bend radius ÷ cut depth, where cut depth is your stock thickness minus the material you leave uncut — one third of the thickness or 3mm, whichever is greater. For example, a 300mm bend radius in 20mm stock with a 3mm saw kerf leaves 6.7mm uncut, giving a cut depth of about 13.3mm and a kerf spacing of about 67.5mm. A 90° bend at that radius has an arc length of about 471mm, so you would need 7 kerfs. Closer spacing gives a tighter, smoother curve — the calculator works this out for any radius, thickness and kerf width in mm.