Mortise and Tenon Joint Dimension Calculator
Size strong mortise and tenon joints following the classic 1/3 stock rule. Get exact tenon cheek, haunch, and depth specs based on stock thickness, stile width, and standard mortise chisel dimensions.
What Are the Correct Proportions for a Mortise and Tenon?
A mortise and tenon joint reaches maximum mechanical efficiency when tenon thickness equals exactly one-third of the rail thickness, and the tenon is centered so that the two side cheek shoulders each equal one-third. Tenon length should measure between two-thirds and three-quarters of the receiving stile or leg width, with an extra 1/16″ mortise depth allowance to accommodate excess glue squeeze-out.
Mortise & Tenon Proportion Calculator
Calculate structural tenon thickness, mortise wall clearance, and haunch dimensions using traditional 1/3 rules.
How to Cut Strong, Perfectly Sized Mortise and Tenons
Follow these four practical shop steps to machine crisp, gap-free mortise and tenon joints without fitting headaches:
Choose Your Mortise Chisel First
Always cut the mortise first. Mortise chisels and hollow chisel mortiser bits come in fixed imperial sizes (1/4″, 5/16″, 3/8″, 1/2″). Select the tooling size closest to one-third of your rail stock thickness before cutting tenon cheeks.
Chop or Route the Mortise Slot
Center the mortise on the stile or leg edge, leaving equal cheek walls on both sides. Drill or plunge route to depth, ensuring you add a 1/16″ extra clearance allowance at the bottom to prevent hydraulic glue lock.
Cut Tenon Shoulders and Cheeks
Using a table saw dado stack, tenoning jig, or handsaw, cut the outer cheek shoulders to the exact calculated shoulder depth. Test fit the tenon directly against your chopped mortise for a firm sliding fit.
Add a Haunch for Table Aprons or Grooved Frames
If building a table apron located flush with the top of a leg or a frame with panel grooves, notch a haunch into the outer edge of the tenon. The haunch fills the exposed groove slot and locks the rail against twisting.
Why the 1/3 Proportion Delivers Maximum Strength
A mortise and tenon joint represents an engineering balance between two competing failure points: the shear strength of the tenon blade versus the bursting resistance of the mortise cheek walls. Violating the classic 1/3 proportion compromises one of these components.
Tenon Too Thick (> 1/2 Stock)
Making a tenon overly thick creates a massive tongue, but leaves the mortise sidewalls dangerously thin. When the joint is clamped or subjected to racking leverage, the thin mortise walls split along the grain.
Tenon Too Thin (< 1/4 Stock)
Cutting a tenon too thin leaves thick, strong mortise walls, but weakens the tenon itself. Under vertical shear loads, a thin tenon snaps off cleanly at the shoulder line.
Side-Grain Glue Contact
End-grain absorbs glue like drinking straws and provides zero bond strength. A mortise and tenon derives 95% of its holding power from side-grain to side-grain contact across the broad flat cheeks.