Mortise and tenon proportion chart
Tenon thickness and shoulder for any stock, by the rule of thirds, with the deepest a blind mortise should go into a given piece.
Not on the chart? Work out your own: Mortise and tenon sizing →
| Stock thickness | Rule of thirds | Chisel or bit to use | Shoulder it leaves, each face | Max blind depth into a 3 1/2" leg |
|---|---|---|---|---|
| 1/2" | 0.167" | 3/16" | 5/32" | 2 5/16" |
| 5/8" | 0.208" | 3/16" | 7/32" | 2 5/16" |
| 3/4" | 0.250" | 1/4" | 1/4" | 2 5/16" |
| 7/8" | 0.292" | 5/16" | 9/32" | 2 5/16" |
| 1" | 0.333" | 5/16" | 11/32" | 2 5/16" |
| 1 1/4" | 0.417" | 3/8" | 7/16" | 2 5/16" |
| 1 1/2" | 0.500" | 1/2" | 1/2" | 2 5/16" |
| 1 3/4" | 0.583" | 5/8" | 9/16" | 2 5/16" |
| 2" | 0.667" | 5/8" | 11/16" | 2 5/16" |
Reading it
- The rule of thirds: the tenon is 33% of the stock thickness, leaving equal shoulders. It balances the tenon's own strength against the wall left either side of the mortise.
- A third of the stock almost never lands on a tool. Mortise chisels, hollow chisels and spiral bits come in eighths and sixteenths, so the third is a target and the chisel column is what you actually cut — the shoulders then take up whatever is left, which is why they are not identical to the tenon on most rows.
- A blind mortise is held to about 67% of the way through, so the socket keeps a sound floor. A through tenon goes the whole way and is wedged.
- Glue surface is the point of the joint: a long tenon in a deep mortise beats a thick one in a shallow one.
For the case this table does not cover
- Mortise and tenon sizing
Tenon thickness and shoulder by the rule of thirds, and the length a blind or through mortise allows.
- Panel glue-up calculator
How many boards it takes to glue up a target panel width, and how much wider to glue up so the rip lands clean.