Mortise and tenon sizing
Enter the stock thickness and the piece being mortised. This sizes the tenon by the rule of thirds and the length a blind or through mortise allows, with a true-size template below to glue straight onto the rail.
= 0.75"
What limits a blind tenon's depth
Cut the tenon
Thickness
1/4"
Rule of thirds, and a stocked chisel size
Length
2"
Cutting it
- Tenon thicknessA third of 3/4" stock
- 0.250"
- Chisel or bit to useThe rule of thirds lands on a stocked size here
- 1/4"
- ShoulderEach face, whatever that chisel leaves — cut square to the tenon
- 1/4"
- Tenon lengthTwo thirds of 3", leaving sound wood at the bottom
- 2"
- Maximum blind depth
- 2"
Tenon template, at true size
Print at 100%, check the ruler, then glue this to the end of the rail and cut around it — the same wrap-and-cut approach the dovetail templates use.
Put a tape on the printed ruler. If it does not read exactly 6 inches, the print was scaled — set the print dialog to 100% or “Actual size” and print it again. Do not cut wood from a template that failed this.
Glue this to the end of the rail. Hatched areas are waste; where the strip ends is the shoulder line — the rail is full thickness again beyond it.
Printing this properly
In the print dialog, set the scale to 100% or “Actual size” and turn off “Fit to page”. Measure the printed ruler with a tape before you trust the shoulder lines — a joint cut a sixteenth off leaves a gap you can see from across the room.
Worked example: a blind tenon in 3/4" stock
A third of 3/4" stock is 0.250", which is a stocked 1/4" chisel. Each shoulder is what is left: 1/4" a face.
Into a 3" wide piece, a blind tenon is 2" long — two thirds of the width, leaving sound wood at the bottom of the mortise.
- Chisel or bit
- 1/4"
- Shoulder, each face
- 1/4"
- Tenon length
- 2"
- Maximum blind depth
- 2"
These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.
About mortise and tenon joints
- Why a third of the stock thickness?
- It is the balance point between the tenon and the two cheeks either side of it. Much thinner and the tenon itself is the weak point under racking; much thicker and the mortised piece is left too thin on each face to resist splitting out. 3/4" stock landing on a 1/4" tenon with a 1/4" shoulder either side is the version most joinery books use as the default.
- Why does a blind tenon stop at two thirds of the mortised piece's width?
- So there is solid wood left at the bottom of the socket. Go deeper and you risk breaking through the far face, or leaving so little material there that it splits the first time the joint racks under load.
- When should I use a through tenon instead of blind?
- When you want to wedge or pin it from the far face — a through tenon with a fox wedge or a pair of angled wedges locks itself tighter the more the joint is loaded, which a blind mortise can't do. It also shows on the outside face, which is a look some joinery wants and some doesn't.
- Does this account for a haunch?
- No — this sizes a plain tenon. A haunched tenon (common at the top of a frame-and-panel door stile, where the tenon meets a groove) keeps the same thickness and shoulder math but adds a short return near the top to fill the groove; add that by hand once you know the plain tenon's dimensions.
You might also need
- Dowels and biscuitsDowel diameter and length by the rule of thirds, which biscuit the stock takes, and how many along the joint.
- Dovetail and box joint templatesSet out the joint, print it at true 1:1 scale, and glue the paper straight onto the board.
- Cabinet boxesA carcass cut list from the finished outside dimensions, face-frame or frameless.