Joinery math

Mortise and tenon proportions calculator

A good mortise and tenon is two-thirds math and one-third feel. The math is the rule of thirds: the tenon is one-third of stock thickness, with a one-third wall on each side. The feel is knowing when to push the shoulder thickness for visual balance or tenon length for long-grain glue area.

The formula

tenon_thickness = stock_thickness / 3

The traditional rule of thirds keeps the mortise walls structurally equal to the tenon. For 3/4" stock, that’s a 1/4" tenon with 1/4" walls. Tenon length is usually 2/3 to 3/4 of the mating piece’s width for blind tenons, full-through for exposed work.

Example: 3/4" stock, 2-1/2" rail

Stock is 3/4" thick → tenon thickness 1/4", mortise walls 1/4" each. Rail is 2-1/2" wide, so a blind tenon runs 1-5/8" deep (about 2/3 of the leg width at typical 2-1/4" mortise depth). Shoulder is 1/4" all around for classic proportions.

Why it matters

Get the proportions wrong and the joint either splits a mortise wall when you glue it up or fails under racking loads in ten years. The rule of thirds is the reason antique furniture is still standing.

Step-by-step

  1. 1

    Measure stock thickness

    Use actual finished thickness, not nominal.

  2. 2

    Apply rule of thirds

    Tenon = stock / 3. Walls = stock / 3 each side.

  3. 3

    Pick tenon length

    2/3 to 3/4 of the mating member’s width for blind tenons.

  4. 4

    Set shoulder thickness

    Usually 1/4 of stock thickness each side, adjust for visual weight.

  5. 5

    Test the joint dry

    Assemble without glue first — a snug fit, not hammer-tight.

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Frequently asked questions

Why the rule of thirds?

It balances tenon strength with mortise wall strength. A fatter tenon weakens the walls; a thinner one fails in long-grain shear. Thirds is the structural sweet spot.

How long should the tenon be?

Blind tenons: 2/3 to 3/4 of the mating piece’s width. Through tenons: the full width, wedged on the far side for extra mechanical lock.

Can I use a different ratio?

Yes — some shop traditions use 1:2:1 or other ratios for specific stock. Dead On lets you override the default but warns you when walls get structurally thin.

Does Dead On draw the joint?

Yes. The Pro version renders the mortise and tenon to scale with all dimensions, including offsets for haunched joints and drawbored pins.

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