The mortise and tenon is the backbone of furniture and timber-frame joinery because it combines a large long-grain glue surface with a mechanical interlock. Sizing it well is mostly about one ratio and a couple of length rules, which this calculator applies to your actual stock dimensions to give you tenon thickness, mortise wall dimensions, and an estimated shear capacity.
Why the rule of thirds works
The tenon must be strong enough to resist bending and shear forces, but the two mortise walls flanking it must also be strong enough not to split. Making the tenon one-third of the rail thickness distributes material evenly between these three elements. If the tenon were half the thickness, the mortise walls would be too thin and prone to splitting; if it were a quarter, the tenon itself becomes weak. The rule of thirds is not arbitrary — it produces the most balanced failure mode, where the joint is equally resistant to all failure paths.
How it works
tenon_thickness = rail_thickness / 3
mortise_wall = (rail_thickness − tenon_thickness) / 2 (each side)
tenon_length = 2.5 to 3 × tenon_thickness
glue_area = 2 × tenon_width × tenon_length (both cheeks)
shear_capacity = glue_area × allowable_shear(species)The bonded width is the tenon width minus a small shoulder on each edge. Because the cheeks are long-grain to long-grain contact, they carry virtually all of the glue strength. The capacity estimate multiplies the cheek area by a conservative allowable shear stress for the chosen species group.
Species groups and what they mean for joint strength
| Species group | Examples | Relative shear allowable |
|---|---|---|
| Softwood | Pine, spruce, cedar | Lower |
| Medium hardwood | Cherry, walnut, ash | Moderate |
| Dense hardwood | Oak, maple, beech | Higher |
| Very dense | Teak, mahogany, Iroko | Highest |
Species with higher density generally take glue better and have higher long-grain shear strength. However, very dense tropical species can have oily grain that requires careful surface preparation (degreasing) before gluing, or the actual joint strength may be lower than the species classification suggests.
Worked example: 20 mm × 60 mm oak rail
For a 20 mm thick, 60 mm wide oak rail in a chair back:
- Tenon thickness:
20 / 3 ≈ 6.7 mm - Mortise wall each side:
(20 − 6.7) / 2 ≈ 6.7 mm - Tenon length:
6.7 × 2.5 to 3 = 17–20 mm - Glue area (both cheeks):
2 × 56 mm × 18 mm ≈ 2,016 mm²
At a conservative allowable shear for dense hardwood, this cheek area produces a solid shear estimate for typical furniture loads. For a chair where the joint takes dynamic loading (rocking, tilting), adding a draw-bored peg through the tenon significantly improves resistance to the cyclic forces that gradually work glued joints loose.
Cutting sequence and fit
Cut the mortise first, then pare the tenon to fit it. This is the traditional sequence because it is easier to pare a tenon smaller than to enlarge a mortise. The fit should be snug but hand-pushable: if you need a mallet to assemble dry, the tenon is too tight and will squeeze glue out, weakening the bond. If the tenon rattles, the joint is loose and will rack.
The shear estimate assumes a clean, well-fitted, fully clamped glue-up. Surface roughness (but not torn grain) actually improves adhesion slightly; a few passes with a card scraper on the cheeks is ideal. For chairs, high-stress frames, or pieces without glue (dry-assembled timber frames), a draw-bore pin or wedged tenon is good practice.