Lay out a mortise and tenon from the size of the rail: tenon thickness by the one-third rule, how much to remove from each cheek, the tenon width after shoulders, and how deep to cut the mortise.
The traditional starting point is a tenon one-third the thickness of the rail, centered, so each cheek removes one-third as well. It leaves equal amounts of wood in the tenon and on each side of the mortise, which is a sensible balance between the strength of the tenon and the walls of the mortise.
In practice you then round the tenon to a size you can cut: a chisel width, a hollow-chisel mortiser bit or a router bit. The calculator rounds to the nearest 1/32 in (or 0.5 mm) so you can see how close the nearest tool size is.
t = rail thickness ÷ 3 (rounded) · cheek = (rail thickness − t) ÷ 2
tenon width = rail width − 2 × shoulder
stub mortise depth = tenon length + 1/16 in (1.5 mm) · through mortise depth = stile thickness
The extra 1/16 in on a stub mortise leaves room for excess glue so the tenon can seat fully without hydraulic pressure stopping it short.
| Rail thickness | T ÷ 3 | Rounded tenon (in) | Cheek each side | Nearest common chisel |
|---|---|---|---|---|
| 0.625 in | 0.2083 | 0.21875 (7/32) | 0.203125 in | 3/16 or 1/4 |
| 0.75 in | 0.2500 | 0.25 (1/4) | 0.25 in | 1/4 |
| 0.875 in | 0.2917 | 0.28125 (9/32) | 0.296875 in | 1/4 or 5/16 |
| 1 in | 0.3333 | 0.34375 (11/32) | 0.328125 in | 5/16 or 3/8 |
| 1.25 in | 0.4167 | 0.40625 (13/32) | 0.421875 in | 3/8 |
| 1.5 in | 0.5000 | 0.5 (1/2) | 0.5 in | 1/2 |
| 1.75 in | 0.5833 | 0.59375 (19/32) | 0.578125 in | 1/2 or 5/8 |
When you round to a chisel, round the tenon down rather than up in thin rails, so the mortise walls do not get too thin. Recalculate the cheeks for the size you choose.
A ¾ in frame-and-panel door. Rail ¾ in, 2 in wide, ⅛ in shoulders, 1 in stub tenon: tenon ¼ in thick, cheeks ¼ in, tenon width 1.75 in, mortise 1.0625 in deep. A ¼ in chisel or mortiser bit is exactly right.
A 1 in table apron. One-third is 0.333 in, rounded to 11/32 in (0.34375). Most woodworkers would choose a ⅜ in chisel here and accept slightly thinner mortise walls, or a 5/16 in chisel for more wall — both are reasonable for a 1 in apron.
A through tenon in a 1¼ in leg. Choose “Through”: the mortise depth becomes the full stile thickness, 1.25 in. Cut from both faces to the middle to avoid blowout.
Metric. 20 mm rail, 70 mm wide, 5 mm shoulders, 25 mm stub tenon: tenon 6.5 mm, cheeks 6.75 mm, width 60 mm, mortise 26.5 mm deep. A 6 mm chisel is the nearest common size.
The one-third rule is a starting point, not a strength calculation. Very thin rails, highly figured wood, chairs and other racking-loaded joints often need different proportions. Haunched, wedged and tusk tenons are outside what this page models.
A common guide is about 3 to 5 times the tenon thickness, or about two-thirds of the stile width for a stub tenon. Longer is not always better if it weakens the leg.
Edge shoulders hide any ragged mortise ends and stop the joint from showing a gap if the rail shrinks in width.
The tenon is sized from the rail. Center the mortise on the stile or offset it to keep the faces where you want them.
Yes. Switch units and the rounding changes to 0.5 mm steps.