Sizing a mortise and tenon: thickness, width, length and fit

Starting proportions for a mortise-and-tenon joint in table and chair work: tenon thickness, shoulders, haunches, length where two mortises meet, and how tight the fit should be.

The mortise and tenon is the standard joint for connecting a rail to a leg or stile. A tongue on the end of the rail (the tenon) fits a rectangular pocket in the leg (the mortise). It is strong because the broad faces of the tenon, its cheeks, glue long grain to long grain, and because the shoulders bear against the leg and resist racking.

There is no single correct size, but there are well-worn starting points. This article gives them for the most common case: a 3/4- to 1-inch apron or rail joined to a square leg.

The parts

Cheeks: the two wide faces of the tenon. Most of the glue strength is here.
Shoulders: the faces of the rail around the tenon that meet the leg. They make the joint look tight and resist twisting.
Edges: the narrow faces of the tenon, top and bottom.
Mortise walls: the wood left on either side of the mortise in the leg.
Haunch: a short stub of tenon near the top of the rail that fills the groove or the gap between the tenon and the end of the leg.

Thickness: about a third of the rail

The traditional rule is a tenon about one third the thickness of the rail, rounded to a chisel or cutter size you own:

Rail thickness	Tenon thickness
3/4”	1/4”
7/8”	5/16”
1”	5/16” or 3/8”
1-1/4”	3/8” or 7/16”
1-3/4”	1/2” to 5/8”

Thicker is not automatically stronger. A thicker tenon leaves thinner mortise walls, and a mortise wall that is too thin can split when the joint is assembled or loaded. As a rough guide, keep at least a quarter inch of wood on each side of a mortise in a leg, more in softwood.

Offset toward the inside

Aprons are often set back from the face of the leg to leave a small step, or reveal, of 1/8 to 1/4 inch. A centred tenon on a set-back apron puts the mortise closer to the outside of the leg than it needs to be. Many makers offset the tenon toward the inside face of the apron so the mortise sits deeper in the leg, with a thicker outside wall.

Width: shoulders at top and bottom

The tenon is usually narrower than the rail, with shoulders on its top and bottom edges as well as its faces:

A top shoulder keeps the mortise away from the end of the leg. Without it, the short grain above the mortise can break out. A haunch fills the space so the top of the joint still looks solid.
A bottom shoulder of about 1/4 inch hides any small error at the bottom of the mortise.

On wide rails, a single very wide tenon means a wide cross-grain glue joint in the leg, which fights wood movement. Many makers split tenons on rails wider than about 4 to 6 inches into two smaller tenons with a gap between them.

Length: as long as the leg allows

Longer tenons have more glue area, up to a point. For aprons into legs, 1 to 1-1/4 inches is typical. The limit is usually the leg: at a table corner, two aprons enter the same leg from adjacent faces, and their mortises meet inside it.

Cut the mortises so they intersect, and mitre the ends of the two tenons at 45 degrees so they do not collide.
Make each mortise about 1/16 inch deeper than its tenon is long. The space holds excess glue and any debris so the shoulders can close.

Fit

A good fit slides together with firm hand pressure, or a few light taps, and does not fall apart under its own weight. If it needs a mallet to close, it is too tight: it scrapes the glue off as it goes in and can split the mortise. If it rattles, the glue is being asked to fill a gap it cannot bridge.

Adjust the tenon, not the mortise. A few passes with a shoulder plane, a rasp or a sanding block on the cheeks thins it evenly. Keep the cheeks flat; a tenon that is tight at the shoulder and loose at the end is only glued on part of its length.

Variations

Through tenons pass all the way through the leg and show on the outside. They can be wedged for mechanical strength.
Drawbored tenons use a peg through slightly offset holes to pull the shoulders tight without clamps.
Loose tenons use a separate piece of wood in mortises cut in both parts, which is often easier to machine.

Starting points for a table

For a side table with 1-1/2- to 1-3/4-inch square legs and 3/4-inch aprons:

Tenon 1/4 inch thick, offset so the apron has a 1/8- to 1/4-inch reveal.
Top shoulder about 1/2 inch with a haunch, bottom shoulder about 1/4 inch.
Tenon about 1 inch long, mitred at the end; mortise about 1-1/16 inches deep.
Test the fit in scrap of the same thickness before cutting the real parts.

In GrainDraft

Mortise and tenon is one of GrainDraft’s eight joint families. You choose the rail and the leg, confirm the mating faces, set the shoulders and engagement, and preview both cuts before the joint is created. The joint preview shows the assembled, exploded and section views, so you can check wall thickness and where two mortises meet inside a leg. Your first joint walks through one from start to finish.

Numbers here are starting points from common shop practice and published references. Check your own stock, tools and hardware instructions, and cut a test piece before committing project wood. Spotted an error? The contact address is on the About page.