Calculator inputs
Use the fields below to size a balanced joint for frames, doors, carcasses, benches, and similar construction work.
Example data table
| Case | Rail Thickness | Rail Width | Mortised Thickness | Mortise Depth | Shoulder | Tenon Thickness | Tenon Width | Tenon Length |
|---|---|---|---|---|---|---|---|---|
| Door frame rail | 38 mm | 100 mm | 50 mm | 32 mm | 12 mm | 12.47 mm | 75.00 mm | 29.00 mm |
| Cabinet divider | 25 mm | 60 mm | 32 mm | 20 mm | 8 mm | 8.13 mm | 44.00 mm | 17.00 mm |
| Bench stretcher | 45 mm | 120 mm | 65 mm | 40 mm | 15 mm | 14.80 mm | 90.00 mm | 37.00 mm |
Formula used
1. Recommended tenon thickness = clamp((rail thickness ÷ 3) − fit allowance, minimum, half the rail thickness).
2. Tenon width = clamp(rail width − 2 × shoulder, 35% of rail width, 75% of rail width).
3. Cheek thickness each side = (rail thickness − tenon thickness) ÷ 2.
4. Blind tenon length = minimum of usable mortise depth, 75% of mortised member thickness, and six times tenon thickness.
5. Through tenon length = mortised member thickness.
6. Estimated glue area = 2 × (tenon width × tenon length) + 2 × (tenon thickness × tenon length).
7. Joinery score blends thickness ratio, width ratio, and length ratio into one quick screening value for layout quality.
How to use this calculator
- Choose millimeters or inches and select a blind or through joint.
- Enter the rail thickness and rail width for the member receiving the tenon layout.
- Enter the mortised member thickness and the actual mortise depth you can cut safely.
- Add a shoulder size if you already have a design target. Leave a small practical fit allowance.
- Press Calculate Joint and review the result panel above the form.
- Use the chart, ratio checks, and exports to compare alternatives before cutting timber.
FAQs
1. What is a common tenon thickness rule?
A common starting rule is about one-third of the rail thickness. This keeps cheeks thick enough while leaving a tenon that still resists shear and twisting in typical frame construction.
2. How deep should a blind mortise be?
A blind mortise is often cut slightly deeper than the tenon length. The extra clearance prevents hydraulic glue pressure and gives a little room for dust and imperfect cleanup.
3. Why do shoulders matter?
Shoulders improve alignment, reduce visible gaps, and help the joint resist racking. Balanced shoulders also make the layout look cleaner on rails, stretchers, and frame members.
4. Can I use this for through tenons?
Yes. Switch the joint type to through tenon. The calculator then uses the mortised member thickness as tenon length and removes bottom clearance from the sizing logic.
5. What does fit allowance do?
Fit allowance slightly reduces the theoretical tenon thickness. It helps account for saw marks, chisel cleanup, seasonal movement, and the practical need for assembly without crushing fibers.
6. Is glue area the same as strength?
No. Glue area helps compare options, but strength also depends on wood species, grain direction, moisture, shoulder support, machining accuracy, and the loads carried by the finished assembly.
7. Should I always follow the calculator exactly?
Use the results as a strong starting point. Then adjust for timber defects, show faces, haunches, wedges, appearance requirements, hardware conflicts, and how much material remains around the mortise.
8. What if my rail is very narrow?
Very narrow rails may need smaller shoulders, twin tenons, a haunched tenon, or a different joint. Always confirm that enough wood remains around the mortise walls.