Mortise and Tenon Proportions Calculator
Get proportioned mortise-and-tenon dimensions — tenon thickness, shoulder size, width and recommended depth — from your stock thickness and rail width.
How it works
Tenon thickness ≈ stock ÷ 3 (snap to a chisel size) · Shoulder = (stock − tenon) ÷ 2 · Length ≈ 5 × tenon thickness, capped at ~3/4 of the leg
A strong mortise-and-tenon centres the tenon in the rail with a tenon thickness of roughly one-third the stock — thick enough to resist shear, with enough shoulder on each side to hide gaps and keep the joint square. The classic guidance is to snap that third to your nearest mortise chisel (e.g. 1/4″ for 3/4″ stock) so the mortise is cut in clean, repeatable passes. Length matters as much as thickness: aim for a tenon about five times its own thickness for good long-grain glue area, but never so deep that it weakens the leg — keep it inside about three-quarters of the leg’s thickness. On rails wider than about 2″, leave a small shoulder at each edge (a haunch) so seasonal movement and racking don’t open the joint.
Worked example
Rail 3/4″ thick and 3″ wide into a 1-1/2″ leg: tenon ≈ 3/4 ÷ 3 = 1/4″, shoulders (3/4 − 1/4) ÷ 2 = 1/4″ per face. Ideal length 5 × 1/4 = 1-1/4″, but 3/4 of the 1-1/2″ leg is 1-1/8″, so cap the tenon at about 1-1/8″. Width 3 − 2×3/16 = 2-5/8″ with a small shoulder at each edge.
Frequently asked questions
Why one-third the thickness?
It balances the two failure modes: a thicker tenon leaves thin, weak walls on the mortise; a thinner tenon shears too easily. One-third gives a tenon and two mortise walls of roughly equal strength, and it conveniently matches common chisel sizes.
What is a haunch and do I need one?
A haunch is a short, shallow extra shoulder at the edge of a wide tenon. It fills the end of a groove and resists twisting without weakening the joint, which is why this calculator trims the tenon width on rails over about 2″ wide.
Should the tenon be a tight fit?
Aim for a snug slip fit — the tenon should push home with hand pressure and hold together before glue. Too tight and you squeegee all the glue out (or split the mortise); too loose and the joint starves. A few thousandths of clearance is ideal.
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This calculator is for educational purposes. Double-check important results before acting on them.