Mortise and Tenon Sizes?

[Jim Fox]

What's the general rule? Thirds? I found this by searching in google, what do you think?

http://www.woodcraft.com/articleprint.aspx?ArticleID=252

So with 3/4" stock, that's a 3/8" mortise, leaving 3/16" on each side. Would you do 3/16" all the way around then?

[Brian Kent]

You just named a good range - from 1/3 to 1/2 of the thickness of the stock. have 2 mortise chisels - 1/4" and 3/8" - and just go for whichever feels right on 3/4" stock. I determine the length of the mortise not by a formula to width, but base it on the size of the piece with the tenon, leaving about the same sized shoulder all the way around.

[Frank Drew]

On 3/4" stock I'd probably use a 1/4" tenon; the glue line strength along the tenon is the same whether the tenon is 1/4" or 3/8", and you'd risk more weakness with the 3/16" cheek than you would in using a 1/4" tenon. IMO.

[Jesse Cloud]

Agree with the above, 1/4 or 3/8. I usually leave at least 1/2 inch on the top, to minimize stress on the mortised piece - easy to crack a leg when you are doing a test fit. For convenience, I usually use 1/2 on the bottom, too.

You don't want your mortises to get crazy wide either. After about 4 inches you would need to worry about wood movement, and probably make two smaller tenons instead of one huge one.

[Chris Friesen]

So with 3/4" stock, that's a 3/8" mortise, leaving 3/16" on each side. Would you do 3/16" all the way around then?

For hand tools, 1/4" is easier to cut and is less likely to blow out the side of the mortise. With a hollow-chisel mortiser, all the pressure is straight down (so little chance of cracking the sides) and the 1/4" chisel clogs easily so it's easier to use the 3/8".

If your mortise is near the end of a piece, you might want to leave a longer shoulder on that end or else use a haunch (possibly hidden) to prevent twisting of the tenon piece. If it's in the middle of another piece you may as well keep a common size of shoulder all the way around...makes it easier to lay out, and easier to cut by machine.


You don't want your mortises to get crazy wide either. After about 4 inches you would need to worry about wood movement, and probably make two smaller tenons instead of one huge one.

With two tenons, one of the mortises should have some extra width to accomodate movement in the tenon piece. To keep the joint snug, you could put a peg through an elongated hole in that tenon.

Alternately, use a single tenon with unglued haunches on both sides to keep the tenon piece from twisting.

The stretchers on my workbench-in-progress are 8" wide. I used a 4" wide tenon, with 3/4" long haunches on either side to keep the stretcher from twisting. Each tenon is drawbored with two 3/8" pegs. Those joints are _solid_.