Spline Joint Calculator — Reinforced Mitre Joints

A spline joint uses a thin strip of wood (or plywood) inserted into matching grooves in two mating pieces. It adds strength and alignment. Splines are commonly used to reinforce mitre joints, which are inherently weak because they join end grain to end grain.

Plywood makes excellent splines because it is dimensionally stable and strong in all directions. Use birch plywood for best results. Solid wood splines should have the grain running across the joint (perpendicular to the joint line) for maximum strength.

The spline thickness should be about one-third of the board thickness. This is similar to a tenon. The groove width must match the spline exactly for a snug fit. Common plywood thicknesses (3, 4, 5, 6, 9 or 12mm) make convenient spline stock, so this calculator snaps the result to the nearest of those sizes.