Calculator Inputs
Formula Used
Tenon thickness = stock thickness × selected thickness ratio.
Face shoulder = (stock thickness − tenon thickness) ÷ 2.
Tenon length = smaller of receiving thickness × length ratio, or receiving thickness − back wall.
Mortise width = tenon thickness + fit clearance.
Glue cheek area = 2 × tenon length × tenon height × number of tenons.
Safe design capacity = glue area × allowable shear × grain factor × joint modifier ÷ safety factor.
Safety ratio = safe design capacity ÷ applied load.
How To Use This Calculator
Choose metric or imperial units first. Enter all dimensions in the selected system. Add the thickness of the rail or part that will carry the tenon. Enter the width of that part and the thickness of the receiving member. Then set the expected load, safety factor, tenon count, and sizing ratios.
Use the shear strength field for your conservative design value. Use lower grain factors for weak, short, or uncertain stock. Press the calculate button. Review the result card, chart, and notes before laying out the final joint.
Example Data Table
| Project | Stock Thickness | Rail Width | Receiver Thickness | Suggested Ratio | Typical Use |
|---|---|---|---|---|---|
| Chair rail | 25 mm | 55 mm | 38 mm | 0.33 | Light frame load |
| Table apron | 32 mm | 90 mm | 50 mm | 0.33 | Moderate racking load |
| Door stile | 45 mm | 120 mm | 60 mm | 0.35 | Wide frame layout |
| Workbench stretcher | 50 mm | 140 mm | 75 mm | 0.40 | Heavy service load |
Mortise And Tenon Sizing Guide
A mortise and tenon joint works by joining two members with matching shapes. The tenon enters the mortise. The shoulders close the seam. The cheeks give glue area. The surrounding stock keeps the joint aligned under load.
Why Proportion Matters
A common starting point is one third of the stock thickness. This leaves useful shoulders on both faces. It also keeps enough material around the mortise. Thin tenons may flex. Thick tenons can weaken the receiving member. The best size balances both problems.
Length And Depth
Tenon length depends on the receiving member. A deeper tenon gives more cheek area. Yet it also removes more wood. The calculator checks the back wall allowance. That helps prevent breakthrough. It also leaves space for glue and seasonal movement.
Shoulders And Height
Shoulders improve appearance and resist twisting. They hide small layout errors. They also stop the rail from moving deeper into the mortise. Wide rails may need twin tenons. This lowers stress and leaves stronger webs between mortises.
Strength Check
The strength estimate uses glue cheek area and allowable shear. It then applies grain condition, joint condition, and safety factor. This is a planning model, not a certified engineering design. Real wood varies. Fit, glue, moisture, and grain direction change performance.
Practical Layout Advice
From a physics view, the joint transfers force through shear, bearing, and bending. Longer cheeks increase shear area. Wider shoulders improve restraint. More tenons spread stress. No calculator can remove workmanship risk. Square walls, clean fibers, and good glue coverage remain important. Use clamps evenly. Avoid starving the joint with excess pressure during final glue-up assembly work.
Use the result as a layout guide. Match dimensions to your chisel, router bit, or hollow chisel. Dry fit before adding glue. The joint should slide with firm hand pressure. It should not crush fibers. It should not rattle. Make test cuts in scrap first.
Final Review
After calculating, compare the safe capacity with the expected load. Read every design note. Increase tenon count for wide or heavily loaded rails. Leave enough back wall. Keep shoulders clean. Good sizing makes cutting easier and improves final durability.
FAQs
1. What is the best tenon thickness?
A common starting thickness is about one third of the rail thickness. Adjust it for cutter size, load, species, and shoulder width.
2. How deep should the mortise be?
The mortise should be slightly deeper than the tenon length. Leave a sound back wall so the mortise does not break through.
3. Why does the calculator include load?
Load helps compare glue cheek area with expected service force. It gives a practical safety ratio for early design decisions.
4. Can I use twin tenons?
Yes. Twin tenons suit wide rails. They reduce large mortise removal and can keep stronger wood between openings.
5. What is fit clearance?
Fit clearance is the small extra space added for assembly. Keep it small, because loose joints reduce strength and alignment.
6. Should shoulders be equal?
Equal face shoulders are common. Unequal shoulders can work when layout, appearance, or offset stock requires a special fit.
7. Does wood species matter?
Yes. Dense, straight-grained wood usually handles stress better. Use conservative shear values when the species or grain is uncertain.
8. Is this a certified structural tool?
No. It is a planning calculator for joinery layout. Critical structural work needs testing, standards, or professional review.