The mortise and tenon is the strongest joint in woodworking — but only when the tenon is sized right. Too thick and the mortise walls split; too thin and the joint is weak. Enter your stock sizes below and this calculator gives you the classic proportions for a through, blind or haunched tenon.
Your joint
How the math works
The classic rule is simple: the tenon is one-third the thickness of the rail, leaving one-third of the stock on each side as the mortise walls. The tenon width leaves a ¼″ shoulder at the top and bottom of the rail, and its length depends on the tenon type:
Tenon thickness = stock thickness ÷ 3
Tenon width = rail width − ½″ (two ¼″ shoulders)
Tenon length: through = full mortise-piece thickness; blind or haunched = ⅔ of the mortise-piece thickness
Mortise depth = tenon length + 1⁄16″ (through tenons go all the way through)
Worked example: ¾″ stock, a 3″ rail and a 1½″ mortise piece with a through tenon give a tenon 0.25″ thick × 2.50″ wide × 1.50″ long, with a mortise depth of 1.50″ (the tenon passes all the way through). The same joint as a blind tenon would be 1.00″ long with a 1.06″ deep mortise.
Tips for a tight joint
- Cut the mortise first, then fit the tenon to it. It is far easier to pare a tenon down than to widen a mortise that is already cut.
- Leave the tenon a hair proud on a through joint. Cut it about 1⁄16″ long, then trim and sand it flush after the glue dries — it looks cleaner than a tenon that sits shy.
- Glue only the mortise walls, not the end grain. Glue on the tenon cheeks and mortise sides is what holds; end grain barely bonds. See our guide to the best wood glue for beginners for which glue suits the joint.
- Test-fit dry, every time. A tenon that needs a mallet to seat is too tight and will starve the glue joint; it should push in with firm hand pressure.
- Put a haunch on wide rails. On rails over about 4″ wide, a haunched tenon stops the rail twisting and keeps the groove (for a panel) from showing a gap.
Frequently asked questions
What is the one-third rule for mortise and tenon joints?
The tenon should be one-third the thickness of the stock it is cut from, leaving one-third on each side as the mortise walls. This balances the strength of the tenon against the strength of the mortise walls — a thicker tenon weakens the walls, a thinner one weakens the tenon itself.
How long should a tenon be?
A through tenon runs the full thickness of the mortise piece. A blind (stopped) tenon should be about two-thirds of the mortise-piece thickness — long enough for a strong glue surface, short enough to leave solid wood at the bottom of the mortise. Never make a blind tenon longer than the mortise is deep.
Through tenon vs blind tenon — which should I use?
Through tenons are stronger and show the end grain as a visible detail, which suits rustic and Craftsman-style work. Blind tenons hide the joint completely, so they are the choice when you want clean faces — on a table apron or a cabinet door, for example. Strength-wise they are close when sized to the same proportions.
What is a haunched tenon?
A haunched tenon is a blind tenon with a short, shallow step — the haunch — left at the top of the tenon. The haunch (about one-third of the tenon length) fills the groove in a panelled door or frame so you cannot see into the groove, and it resists twisting on wide rails. This calculator gives you the haunch length when you choose the haunched type.