Enter stock sizes and joint preferences
Sample inputs and outputs
| Stock thickness (mm) | Width (mm) | Shoulder (mm) | Fit | Tenon t (mm) | Tenon w (mm) | Tenon L (mm) | Mortise depth (mm) |
|---|---|---|---|---|---|---|---|
| 38 | 90 | 6 | Standard | 12.67 | 78 | 26.60 | 27.60 |
| 25 | 70 | 5 | Snug | 8.33 | 60 | 17.50 | 18.50 |
| 50 | 120 | 8 | Loose | 16.67 | 104 | 35.00 | 36.00 |
Examples use automatic sizing with a 1 mm glue gap.
Core sizing relationships
- Recommended tenon thickness:
t ≈ T / 3, clamped to practical limits. - Tenon width:
w = W − 2S(two shoulders). - Recommended tenon length:
L ≈ 0.70T(typical joinery proportion). - Mortise depth:
D = L + g(adds glue gap). - Mortise width/length with allowance:
mw = t + a,ml = w + a. - Long-grain glue area (approx):
A = 2(Lw) + 2(Lt). - Mortise removal volume:
V = mw × ml × D.
Note: end-grain area is shown for reference only; long-grain bonding dominates strength.
Practical workflow
- Select units and enter stock thickness and member width.
- Set a shoulder margin that preserves edge strength.
- Choose fit type based on your cutting accuracy.
- Leave overrides blank to use automatic recommendations.
- Use the outputs to mark mortise and tenon layouts.
- Download CSV or PDF to share measurements on-site.
Mortise and tenon sizing for dependable construction joinery
1) Why this joint stays reliable on site
Mortise and tenon joints resist racking because the tenon is trapped on several faces, spreading stress into long grain. For doors, gates, benches, and frames, repeatable dimensions improve alignment and reduce twist during clamping, especially when faces must stay flush.
2) Start with stock thickness for strength balance
The calculator anchors sizing to stock thickness (T). A common rule uses tenon thickness near one-third of T to balance strength and material left as cheeks. Example: 38 mm stock suggests about 12.7 mm, keeping the mortise centered and stable.
3) Shoulder margins protect edges and hide minor error
Tenon width is computed as w = W − 2S, where W is member width and S is shoulder margin. Shoulders improve visual gaps, create a clean reference for squareness, and reduce splitting near edges. Many interior frames use 5–8 mm shoulders.
4) Tenon length and glue gap help full seating
Tenon length commonly ranges 0.60–0.75 × T; the default here is 0.70 × T. Mortise depth follows D = L + g, adding a glue gap so the tenon will not bottom out. A 1 mm gap is practical.
5) Fit allowance turns tool accuracy into clearance
To match real cutting tolerances, mortise size is expanded with allowance (a): mw = t + a and ml = w + a. Tight uses 0.00 mm, snug 0.10 mm, standard 0.20 mm, and loose 0.40 mm for easier assembly.
6) Use glue area and volume outputs for planning
The calculator estimates long-grain glue area on cheeks and edges, where bonding is most effective. It also reports mortise removal volume, helpful for time planning when chopping by hand or routing in passes. Larger volumes usually require staged cuts to keep walls straight.
7) Haunched tenons can stiffen edge members
When a joint sits near an open edge, a small haunch can limit twist and conceal the mortise at the top. The optional haunch suggestions are conservative starting points based on member width and tenon thickness, often useful for doors and exterior frames.
8) Field checks that prevent costly rework
Dry-fit first, confirm shoulders seat fully, then measure diagonals to verify squareness. If the joint is too tight, pare evenly instead of forcing it. If too loose, reduce allowance or increase bearing. Export results to keep dimensions consistent across repeats.
Common questions
1) Should I always use one-third thickness for the tenon?
It’s a reliable baseline for many frame members. For very thin stock, reduce thickness to keep cheeks strong. For heavy exterior work, you may go slightly thicker if edge distances remain safe and splitting risk stays low.
2) What fit type should I choose for site-built frames?
Standard works well when you expect minor swelling or paint. Snug suits accurate machine-cut joints. Tight is best for test pieces and controlled environments. Loose can help when you need fast assembly and generous glue space.
3) Why add a glue gap at the mortise bottom?
A glue gap prevents hydraulic lock and ensures the tenon seats fully on the shoulders. It also provides space for squeeze-out and dust. Without a gap, a bottomed-out tenon can leave visible shoulder gaps.
4) How do I pick shoulder margins?
Choose margins that preserve edge strength and leave enough meat around the mortise. Small cabinetry often uses 5–8 mm. Wider exterior members may need 10–12 mm or more. Increase margins for knotty material.
5) Can I use inches and still get accurate allowances?
Yes. The calculator converts your inputs to millimeters internally, applies the allowance values, then converts back to inches for display and exports. This keeps clearance behavior consistent across unit systems.
6) What does the glue area output tell me?
It estimates long-grain bonding surface, which strongly influences joint strength. Use it to compare design options, like longer tenons or wider rails. It is an estimate, not a structural certification for engineered loads.
7) When should I use a haunched tenon?
Use a haunch when you need extra anti-twist support near the edge, or when you want to conceal the mortise at the top of a stile. It’s common in doors, gates, and exposed frames.
Measure carefully, cut accurately, and dry-fit before glue-up always.