Mortise and Tenon Calculator (Rule of Thirds Sizes)

The mortise and tenon is the classic frame joint for tables, chairs, doors and beds. Enter the size of the leg or stile and the rail, and this calculator gives the tenon thickness from the rule of thirds, the tenon width after the shoulders, the mortise depth and tenon length for a blind or through joint, and the wall left each side of the mortise. It warns when a tenon is wide enough that cross-grain movement becomes a risk.

Mortise and tenon calculator

How it works

Tenon thickness = 1/3 of the thickness of the piece being mortised, rounded to the nearest 1/16 in or 1 mm (a chisel or mortise bit size), and never so thick that the rail has no shoulder on its faces
Tenon width = rail width − 2 x the edge shoulder
Mortise depth = 2/3 of the width of the mortised piece for a blind mortise, or all the way through
Tenon length = mortise depth − a small gap for glue at the end
Width check: no more than 5 x the tenon thickness, and about 4 in, before twin tenons are worth considering

Worked example

A rail of 3/4 in x 1-1/2 in stock joining a stile of the same size, blind mortise, 1/8 in shoulders at each edge and a 1/16 in glue gap:

Units: Inches
Thickness of the piece being mortised (leg or stile): 0.75 in
Width of the piece being mortised (the mortise depth direction): 1.5 in
Thickness of the rail or apron (the tenoned piece): 0.75 in
Width of the rail or apron: 1.5 in
Shoulder at each edge of the tenon: 0.125 in
Joint: Blind (stopped) mortise
Gap at the end of the tenon (blind): 0.0625 in

The calculator gives:

Tenon thickness (mortise width): 1/4" (0.25 in)
Tenon width (mortise length): 1-1/4" (1.25 in)
Tenon length: 15/16" (0.938 in)
Mortise depth: 1" (1 in): 2/3 of 1-1/2" (1.5 in)
Wall left each side of the mortise: 1/4" (0.25 in)
Shoulders: 1/8" (0.125 in) at each edge of the rail

The tenon is 1/4 in thick, leaving 1/4 in walls each side of the mortise, and 1-1/4 in wide, exactly the 5 x thickness limit. The mortise is 1 in deep (2/3 of 1-1/2 in) and the tenon 15/16 in long. These are the same numbers as the worked example in the Woodworker’s Journal article in the sources.

Tenon thickness by stock thickness (rule of thirds)

Mortised stock	Tenon thickness	Wall each side	Max tenon width (5 x)
1/2"	3/16"	5/32"	15/16"
5/8"	3/16"	7/32"	15/16"
3/4"	1/4"	1/4"	1-1/4"
7/8"	5/16"	9/32"	1-9/16"
1"	5/16"	11/32"	1-9/16"
1-1/4"	7/16"	13/32"	2-3/16"
1-1/2"	1/2"	1/2"	2-1/2"
1-3/4"	9/16"	19/32"	2-13/16"

Tenon rounded to the nearest 1/16 in. Width limit capped at 4 in.

Notes and limits

Size the mortise to a chisel or bit you own, then fit the tenon to the mortise, not the other way round.
The rule of thirds is written for parts of the same thickness. When a thin apron meets a thick table leg, a thicker tenon set back from the outside face is usually stronger.
Keep a mortise well away from the end of a leg, or leave the leg long (a horn) and cut it off after glue-up, so the end does not split out while you chop.
Wide rails on tables and doors usually get a haunched tenon or twin tenons to control movement and keep the mortise walls strong.
A tenon should slide in with hand pressure. If it needs a mallet, it is too tight to glue up.
Choosing between a mortise and tenon and other corner joints? See our guide to wood corner joints.

Frequently asked questions

How thick should a tenon be?

About one third of the thickness of the piece being mortised. In 3/4 in stock that is a 1/4 in tenon with 1/4 in walls each side.

How deep should a mortise be?

For a blind mortise, about two thirds of the width of the piece being mortised: 1 in deep in a 1-1/2 in wide stile.

How wide can a tenon be?

A common rule is no more than 5 x its thickness, and about 4 in before splitting it into two tenons, because the rail and leg move differently across the grain.

How long should a tenon be?

Slightly shorter than the mortise is deep, about 1/16 in, so excess glue has somewhere to go and the shoulders pull tight.

What size tenon for a 1-3/4 in table leg?

With a 3/4 in apron, the tenon is limited by the apron, not the leg: about 1/2 in thick leaves a 1/8 in shoulder on each face. The calculator shows this when you enter both sizes.

Sources

Woodworker’s Journal, "Real-life Mortise and Tenon Dimensions": tenon 1/3 the thickness of the mortised stock, width no more than 5 x its thickness (4 in before twin tenons), blind mortise 2/3 the width of the mortised board

Last reviewed: 16 October 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.