Joinery
Mortise & Tenon Dimension Calculator
Size mortise width, tenon thickness, and shoulder length to your stock — with a rule-of-thumb that avoids weak joints.
Calculator
Optional: leave 0 for 1/3 rule.
Yes / No
Yes / No
What makes this different
Most online mortise/tenon calculators only output the tenon thickness as 1/3 of the stock. This one computes mortise width vs. tenon thickness (allowing 0.005 in of slip fit), suggests a haunched version for blind joints, and flags joints where the tenon would be wider than 1/3 of the rail.
Who it's for & when to use it
Building tables, chairs, doors, face frames — anywhere a strong mechanical joint is required.
How to use
- Enter the dimensions of your material or the parameters of your project.
- Adjust the optional knobs (waste factor, kerf, planer allowance) to match your shop practice.
- Click Calculate and read the output. Use the per-item breakdown for ordering, the totals for budgeting.
- Compare the result to your lumber yard's quote — the effective $/BF column is the apples-to-apples number.
Formula: Tenon thickness ≈ 1/3 of rail thickness, capped so the mortise leaves at least 1/8 in of wall on each side. Shoulder ≈ 1/3 of tenon thickness.
Notes & tips from the shop
- A through-tenon is stronger than a blind one, but only if you can show it off — otherwise pocket the look with a haunch.
- Mortise walls should be at least 1/8 in thick; less than that risks blowout when chopping.
- Tenon length ≈ 3× tenon thickness, or until you hit the back of the mortise.
Frequently asked questions
Can I skip the haunch?
Yes, but the haunch resists racking and keeps the shoulder flush during glue-up. Worth the extra five minutes.
What if my mortise is too loose?
Adjust the chisel or bit by 0.005 in. Test fit before glue — should slide with hand pressure only.
Is loose-tenon (Domino / floating) weaker than through-tenon?
Loose tenons glued in long-grain mortises on both sides are roughly 70–85 % the strength of an integral through-tenon of the same glue area.
Limitations
Strength is a function of glue area, wood species, and grain orientation — not modeled here.
All calculations happen in your browser — nothing you type is sent to a server.