How thick should a tenon be?

The classic rule is one third of the rail thickness. That ratio leaves two mortise walls each roughly a third of the thickness, balancing the strength of the tenon against the strength of the material around the mortise. The calculator applies this automatically, and you can override it if your stock or design demands.

How long should a tenon be?

A common range is two and a half to three times the tenon thickness for furniture rails, which gives ample long-grain gluing surface without weakening the receiving piece. Deeper mortises up to about five times thickness add strength where the receiving member is wide enough to accept them.

Where does a mortise and tenon get its strength?

Almost all of the glue strength comes from the two tenon cheeks, which meet the mortise long grain to long grain where PVA and similar glues bond strongly. The shoulders are largely end and edge grain and contribute little adhesion, but they close the joint and resist racking.

Is the shear capacity figure exact?

No, it is a conservative estimate. It multiplies the cheek glue area by a typical allowable shear stress for the species group, assuming a clean, well-fitted, fully clamped joint. Real strength depends on fit, glue, clamping, moisture, and grain, so treat the number as guidance, not a rating.

Should I add a haunch or draw-bore pin?

For frame joints where the rail meets a grooved member, a haunch fills the groove and resists twist. A draw-bored peg adds mechanical lock and clamps the shoulders without glue. Both are good practice on doors and frames, though they sit outside the basic sizing this tool covers.

Mortise & Tenon Sizing Calculator

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.

How it works

The dominant rule is the rule of thirds: make the tenon roughly a third of the rail thickness so that the tenon and the two mortise walls share the load evenly.

tenon_thickness = rail_thickness / 3
mortise_wall    = (rail_thickness - tenon_thickness) / 2   (each side)
tenon_length    = 2.5 to 3 x tenon_thickness
glue_area       = 2 x tenon_width x tenon_length           (both cheeks)
shear_capacity  = glue_area x 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, they carry the glue strength, so the capacity estimate multiplies that cheek area by a conservative allowable shear stress for the chosen species group.

Tips, example, and notes

For a 20 mm thick, 60 mm wide oak rail, the tool suggests a tenon close to 6.7 mm thick with mortise walls about 6.7 mm each, a length around 17 to 20 mm, and a generous cheek glue area that yields a healthy shear estimate for dense hardwood.

Keep the fit snug but not forced; a tenon that needs hammering squeezes glue out of the joint and can split the mortise. Cut the mortise first and pare the tenon to it. The shear figure assumes a clean, fully clamped glue-up, so for chairs and other high-stress pieces consider adding a draw-bored peg or a haunch rather than relying on glue area alone.