Mortise and tenon sizing
Enter the stock thickness and the piece being mortised. This sizes the tenon by the rule of thirds and the length a blind or through mortise allows.
in
= 0.75"
in
What limits a blind tenon's depth
Joint
Cut the tenon
Thickness
1/4"
Rule of thirds
Length
2"
Cutting it
- Tenon thicknessA third of 3/4" stock
- 1/4"
- ShoulderEach face, cut square to the tenon
- 1/4"
- Tenon lengthTwo thirds of 3", leaving sound wood at the bottom
- 2"
- Maximum blind depth
- 2"
About mortise and tenon joints
- Why a third of the stock thickness?
- It is the balance point between the tenon and the two cheeks either side of it. Much thinner and the tenon itself is the weak point under racking; much thicker and the mortised piece is left too thin on each face to resist splitting out. 3/4" stock landing on a 1/4" tenon with a 1/4" shoulder either side is the version most joinery books use as the default.
- Why does a blind tenon stop at two thirds of the mortised piece's width?
- So there is solid wood left at the bottom of the socket. Go deeper and you risk breaking through the far face, or leaving so little material there that it splits the first time the joint racks under load.
- When should I use a through tenon instead of blind?
- When you want to wedge or pin it from the far face — a through tenon with a fox wedge or a pair of angled wedges locks itself tighter the more the joint is loaded, which a blind mortise can't do. It also shows on the outside face, which is a look some joinery wants and some doesn't.
- Does this account for a haunch?
- No — this sizes a plain tenon. A haunched tenon (common at the top of a frame-and-panel door stile, where the tenon meets a groove) keeps the same thickness and shoulder math but adds a short return near the top to fill the groove; add that by hand once you know the plain tenon's dimensions.