I always go by a simple rule, it may not be right, but it seems to work in most cases.

Thickness of my tenon is 1/3 of the width of the piece it fits into (3/4" stock would get you a 1/4" tenon)
Width of the tenon is 3 times the thickness (if Thickness is 1/4", I would make the width about 3/4"). Using those measurements, you would have a tenon about 1/4" x 3/4".

Length is totally determined by the material it intersects, obviously full length if a through tenon, otherwise I insert them about half the distance of the material it mates to.

Hope that helps a bit…

 

I don't think there really is a set rule of thumb, just a more ok, 1/8th inch on the sides of the tenon may fail kinda thing. depending on if it's load baring of course.

 

I've heard the thickness rule (1/3rd) but have to say I've never heard (or thought) about length. since I do a lot of mission style work, my tenons are not that long (because they run into each other in the corners). from a strength point of view I can't imagine that a longer tenon is going to do much (in fact I'm guessing that the strongest part of the tenon is nearest the shoulders/cheeks).

 

How long and how wide a tenon is can be variable. If I am using 3-5" wide rail stock, I will often use 1/2" shoulders. IE, a 4" rail would have a 3" wide tenon. This makes the math easier, and a decent sized shoulder helps prevent racking once the project is assembled. That is really one of the best things about mortise and tenon joints - they are self-squaring, which makes assembly easier.

How long varies with through tenons vs. standard tenons. Through tenons typically project past the leg 1/8"-1/2" depending on the look you want. Standard tenons should be at least 3/4" long for strength, and can extend well into the leg. In projects with 1-3/4" leg stock, my tenons are usually 1" long.

 

Most of my tenons are done cope and stick. The tenon size is set by the cope run on the shaper for the panel. it is typically close to the 1/3 rd rule (7/16 on 13/16 stock) but only 5/8 on 1 3/8 to 2/14 doors. Of course then there is the through tenon. I think this is the kind of thing that has many correct answers, and a lot of variable. When in doubt, break things. I learned a lot about glue ups by breaking my off cuts. For awhile make test pieces while you are tenonning, and then try and break them. The failures will teach you a great deal about what actually works for you, in what materials and configurations, and how the piece you just made is going to stand up, or fail.

 

The length of he tenon shoulder plays a major role in the
strength of a tenon. Applications vary. If you made
a chair with long, fat tenons in a rail only 1" deep, your
tenons would likely soon start wiggling at the shoulder.
Even if the glue doesn't fail the joint can get loose
in that sort of situation. It doesn't mean the piece
will fall apart though.

 

Loren…thanks…I never made a chair but I can see where longer would be better (I picture a prybar).

So as you say "Applications vary".

In a table setting with side rails meeting front/back rails inside the leg, depth will be dictated by where they bump into each other and I guess they are not bearing as much load but rather simply hold the legs together. Where there is no intersecting tenon (say a side rail on a chair with no front rail), go as deep as you can.

From an engineering point of view, would seem that more meat on the uncut shoulder would be better, in effect creating stress on the uncut portion of the rail against the leg…what Pinto says above.

 

These problems come up most when you want to
build something refined like a fine table or an elegant
chair. In the sort of heavier and more Craftsman
style furniture most people get started with in
furniture building, the thicker cross-sections of
the part makes joinery more forgiving.

As a practical matter, try to make the tenons as
robust as possible without sacrificing too much
strength in the mortised part. Stub tenons
when glued (as in cope and stick doors) are
pretty strong in cupboard door applications
but in entry doors where there's a lot of weight
cantilevered, the joint will often fail if the panels
aren't glued in (veneered ones usually can
and should be, imo).

 

Loren I would have to disagree with your comments about entry doors. It is even more important for the panels to be loose for expansion in an entry door, than a cupboard door, because of greater temp and humidity variations between in and out of a house, as well as exterior variations. Your previous comments about the length of the tenon is what gives the strength to the joint is on point. (I am not using length to mean the depth) Hence the bottom rail is typically larger than the others in a door. A stub tenon provides a great deal of strength. It is not just the increased surface area for glue, but the conversion of the force over the length of the tenon from tension to compression. (The "prybar effect", or torque) See drawing below This requires good shoulders as well, not just to resist the torque of opening and closing, but to create the torque in the joint, as the joint is pulling at the top shoulder and pushing at the bottom, as well as along the entire tongue. The measure of the strength can be expressed as the length of the mortise from the inside top of the mortise, to the bottom outside of the shoulder. The longer this line the stronger the joint, according to the properties of the species. In theory a long shallow tenon should have the same strength as a short deep one. However the depth of the tenon changes the force on the parts joined, in this case the style and rail of the door. A short deep tenon could fail by splitting the rail, because the torque beyond the joint increases as the depth of the tenon increases. A shallow long tenon can fail by splitting the style, because of the same. When all of these are considered The 1/3 rule does manage the strength of the wood vs. the forces of the joint well for most species and joints. For other applications it is important to think of these forces. I.e. a joint facing mostly compression, a table apron, or bottom rail in a chest construction, will benefit from a longer tongue.
To keep this relevant to the original question, I am off on a tangent about door construction. One of the things left out of the question in this thread is what type of joint is being made with the mortise and tenon.
The joint is always a compromise between the forces, the strength of the wood, and other materials making the joint (glue, wedges, pegs, etc.) so I am gonna to close the door on my thoughts.

 

An aside: In my doors I try and have equal shoulder widths and tenon thickness. 3/4,3/4,3/4 is a good example. So the largest doors are always 2 1/4" thick. This dimension also works well with a 3/4 tongue on the panel, or a 3/4" thick flat panel, depending on the style of door. On hard woods (oak, mahogany, elm) a 5/8" deep mortise and tenon provides plenty of extra surface area for strength. On soft woods I go 3/4" (knotty pine, or less stable grains like mesquite) The only other time to go deeper is if the width of the style and is reduced.

 

SO you don't glue in plywood or MDF panels?

Ok. I glue them. Makes a strong door. Never had a problem.

Gluing in solid wood panels is stupid and of course I never do that.

 

No i don't. When you said entry I took that to mean exterior doors. while mdf or plywood panels shouldn't expand enough to cause a problem. I have seen it happen before with other manufacturer's doors. Especially on south facing doors here in texas. (this can be brutal) We actually use space balls on all panels. I can see this working on interior passage doors, but the joints are strong enough that we allow the expansion. Actually out here another problem, you might be familiar with this one, is contraction. Specifically when things built in our high ambient humidity get installed, and the HVAC is clicked on (new construction typically) Even MDF will shrink after acclimating to %90 humidity and 110 degrees, and then suddenly dealing with %30 humidity or less and 65 degrees. Even worse for an entry that is 110 degrees one side and 65 the other. IMO it is best to make the joint strong and leave those panels free range!! Free range panels are happy panels!!

i actually made a good amount of money off of a house that left zero room for expansion on plywood panels, and the doors all warped and bowed. good times

 

I'm in Texas. I know of few situations where you have 110 degrees exterior and 65 degrees interior temps. A delta of about 35 degrees is just about the best you should expect in typical homes. Though you could expect better efficieny if you design and build the home yourself.

As far as gluing in man-made materials, I don't have an issue with but I'd never do it personally. While expansion is minimal, I have measured this on plywood and mdf in my shop over time and there some reason for concern in certain situations.

As far as the OP's question, I make my tenons bigger for thinner stock. For 3/4" stock, half would be devoted to the tenon with the other half split over the shoulders. This way, I feel I have a beefier joint without sacrificing lateral stability. But yeah, the 1/3 rule is pretty standard.

 

• I know of few situations where you have 110 degrees exterior and 65 degrees interior temps.*
  Tarrytown Austin, tx 65 days in a row, summer 2011. I admit to eggageratin' it was only 100 degrees. But to be fair the south side of those houses was easy 120 on the surface if not in the shade. (incidentally, I also had 3 carpenters look up and quit on me the end of that stretch.)

Agree with all else been' said

 

Ok…

Don't forget that there are "through" tenons, too! Which you see all the time on A&C furniture. I've always used the rule of 1/3 as described above. when doing through tenons, I allow them to extend about 3/16 beyond the the plane of the piece it's joining. Here is a picture of a project I'm working on now….a mantle clock with through tenons.

Hope this helps.