Mortise & Tenon Calculator
Tenon size and mortise depth from the classic 1/3 rule
Tenon Thickness
1/4″
Tenon Width
2 1/2″
Tenon / Mortise Depth
7/8″
How to use this calculator
Enter the thickness and width of the piece that will carry the tenon, plus the thickness of the piece that gets the mortise. This applies the traditional 1/3 rule — tenon thickness at roughly a third of stock thickness gives the best balance of strength on both the tenon and the mortise walls around it.
Rules applied
Tenon Thickness = Stock Thickness ÷ 3
Tenon Width = Stock Width − (2 × 1/4″ shoulder)
Blind depth = 60% of mortise piece thickness (leaves a solid wall behind it)
Through depth = full mortise piece thickness
Tenon Width = Stock Width − (2 × 1/4″ shoulder)
Blind depth = 60% of mortise piece thickness (leaves a solid wall behind it)
Through depth = full mortise piece thickness
These are starting numbers, not fixed law — thicker stock can carry a tenon slightly under a third for extra mortise wall strength, and shoulder width is a matter of taste as much as engineering. Cut a test joint in scrap before committing to your good stock, especially the first time you dial in a new mortiser or router setup.