Mortise & Tenon Calculator
Calculate mortise and tenon joint dimensions in inches from workpiece thickness and standard ratios.
What Is a Mortise and Tenon Joint?
A mortise and tenon jointconsists of a rectangular projection (tenon) cut from one workpiece that fits into a matching cavity (mortise) in the other — one of woodworking’s strongest mechanical joints, used in furniture legs, frame-and-panel doors, and timber framing.
The Mortise and Tenon Calculator determines the correct tenon thickness, width, and length based on your workpiece dimensions. It applies the industry-standard one-third-thickness rule to ensure the tenon is proportioned for maximum glue surface area without weakening either the tenon or the mortised member.
Mortise and Tenon Formula
The traditional woodworking rule for sizing mortise and tenon joints is the one-third rule: the tenon thickness equals one-third of the workpiece thickness. This proportion provides optimal strength while preserving enough material on each side of the mortise.
In mathematical notation:
Example Calculation
For a 1.5" × 3.5" rail (a standard 2×4 actual dimension):
- Tenon width = 1.5 ÷ 3 = 0.500"
- Tenon length = 3.5 × 0.67 = 2.345"
- Mortise width = 1.5 ÷ 3 = 0.500"
- Mortise depth = 1.5 × 0.667 = 1.000"
Mortise & Tenon Reference Table
| Workpiece Thickness | Tenon Width | Mortise Width | Mortise Depth | Typical Use |
|---|---|---|---|---|
| 0.75 | 0.250 | 0.250 | 0.500 | Small chair rungs, small panels |
| 1.00 | 0.333 | 0.333 | 0.667 | Light furniture rails |
| 1.25 | 0.417 | 0.417 | 0.833 | Medium cabinet frames |
| 1.50 | 0.500 | 0.500 | 1.000 | Standard 2× framing, chairs |
| 1.75 | 0.583 | 0.583 | 1.167 | Heavy table aprons |
| 2.00 | 0.667 | 0.667 | 1.333 | Solid table legs |
| 2.50 | 0.833 | 0.833 | 1.667 | Timber frame posts |
| 3.00 | 1.000 | 1.000 | 2.000 | Heavy timber framing |
How to Cut a Mortise and Tenon Joint
Cutting the Tenon
- Mark the shoulder line (tenon length) around all four faces using a marking gauge.
- Mark the cheeks (tenon width/thickness) on the face and edge.
- Cut the shoulders with a tenon saw, dovetail saw, or table saw crosscut sled.
- Cut the cheeks by ripping with a tenon saw, bandsaw, or table saw with a tenoning jig.
- Pare to the line with a sharp chisel and test fit in the mortise.
Cutting the Mortise
- Mark the mortise location using a mortise gauge set to tenon width.
- Drill a series of overlapping holes inside the mortise outline to remove most of the waste.
- Chop the walls square and flat using bench chisels and a mallet.
- Alternatively, use a plunge router with an upcut spiral bit and a fence for clean results.