What is a spline joint?

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.

What material should I use for splines?

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.

How thick should a spline be?

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, 6mm) make convenient spline stock.

WoodCalcs

Spline Joint Calculator — Reinforced Mitre Joints

Calculate spline dimensions and groove depth for reinforced mitre and edge joints.

Board Thickness

Joint Length

Joint Type

How We Calculate This

This calculator determines spline and groove dimensions for reinforced joints.

Spline sizing

Thickness: &frac13; of board thickness, snapped to common plywood sizes
Width (mitre): 60% of the mitre face width
Width (edge): &frac23; of board thickness
Groove depth: Half the spline width + 1mm clearance

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.