How thick should a tenon be?
A third of the stock, leaving equal shoulders — 1/4" on 3/4" material. Proportions for every thickness, and how deep a blind mortise should go.
A third of the stock thickness: 3/4" material gives a 1/4" tenon.
That leaves 1/4" of shoulder on each face. In practice you round to the chisel or router bit you own and let the shoulders take up the difference.
How that is worked out
- The rule of thirds: tenon = stock ÷ 3. 3/4" ÷ 3 = 1/4".
- It balances two failures against each other. A thicker tenon is stronger in itself but leaves thin walls either side of the mortise, which is where the joint actually breaks. A thinner one keeps the walls stout and snaps at the shoulder instead.
- A blind mortise stops at about 67% of the way through the piece it lands in, so the socket keeps a sound floor — 2 5/16" into a 3 1/2" leg.
Tenon proportions, and blind depth into a 3 1/2" leg
| Stock thickness | Tenon | Shoulder, each face | Nearest chisel | Max blind depth |
|---|---|---|---|---|
| 1/2" | 3/16" | 3/16" | 11/64" | 2 5/16" |
| 5/8" | 3/16" | 3/16" | 13/64" | 2 5/16" |
| 3/4" | 1/4" | 1/4" | 1/4" | 2 5/16" |
| 7/8" | 5/16" | 5/16" | 19/64" | 2 5/16" |
| 1" | 5/16" | 5/16" | 21/64" | 2 5/16" |
| 1 1/4" | 7/16" | 7/16" | 27/64" | 2 5/16" |
| 1 1/2" | 1/2" | 1/2" | 1/2" | 2 5/16" |
| 1 3/4" | 9/16" | 9/16" | 37/64" | 2 5/16" |
| 2" | 11/16" | 11/16" | 43/64" | 2 5/16" |
Worth knowing
- Round the tenon to the tool, not the other way round. A 3/4" rail wants a 1/4" tenon and a 1/4" mortise chisel exists, which is not a coincidence — the rule and the tool sizes grew up together.
- Glue surface is what the joint lives on, and length gives far more of it than thickness does. A long tenon in a deep mortise beats a fat one in a shallow one every time.
- A through tenon goes the whole way and is wedged from the outside, which is both stronger and a visible detail. A blind one stops short and is invisible.
- End grain to end grain glues badly, so the shoulders do the work of keeping the joint square while the cheeks do the work of holding it together. Cut the shoulders cleanly and the rest forgives a lot.
Work out your own numbers
- 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.
Related questions
- What size tenon for 3/4 inch stock?
- 1/4" thick, with 1/4" shoulders each side. That is the rule of thirds landing exactly on a common mortise chisel, which is why 3/4" stock is such a comfortable size to work in. Length depends on what it goes into — about two thirds of the way through for a blind mortise.
- How deep should a mortise be?
- About two thirds of the way through the piece for a blind mortise, so there is still sound material under the socket. A through mortise goes the whole way and gets wedged. Either way, depth buys you glue surface, which matters more to the joint's strength than thickness does.
- Should the tenon be exactly one third?
- No — it should be the nearest size you can actually cut cleanly. A third is a starting point that balances the tenon's strength against the mortise walls', and both are forgiving either side of it. Rounding 0.29" up to a 5/16" chisel you own beats chasing an exact figure with a tool you do not.