Mortise and Tenon Calculator
Calculates tenon thickness as a fraction of the stock thickness (one-third by default), rounded to the nearest 1/16 inch. Use it when laying out frame-and-panel furniture or door construction.
Last updated: September 2026
Formula below · 2 sources (fpl.fs.usda.gov, Wikipedia) · Updated Sep 2026
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About this calculator
The classic rule for a mortise-and-tenon joint is the one-third rule: make the tenon about one-third as thick as the stock, so the two cheeks removed from the tenon and the two mortise walls left standing are all about one-third each. The formula is: tenonThickness = round(rail_thickness × ratio × 16) / 16, with ratio 1/3 (standard), 0.4 (strong) or 0.5 (through tenon in thick stock), rounded to the nearest 1/16 inch to match chisel and bit sizes. When the rail is thinner than the leg it joins, base the ratio on the thinner piece and make sure each mortise wall keeps at least about 1/4 inch of wood. The shoulder on each face is (rail_thickness − tenon_thickness) / 2. Tenon length is typically about 2/3 to 3/4 of the mortised piece's width for a blind tenon. Following these proportions keeps the mortise walls thick enough to resist splitting while leaving the tenon strong.
How to use
Imagine a rail 1.5 inches thick. Enter rail_thickness = 1.5 and keep the standard 1/3 joint ratio. The calculator computes 1.5 × 1/3 = 0.50 inch tenon thickness (rounded to the nearest 1/16"). Each shoulder is then (1.5 − 0.50) / 2 = 0.50 inches wide. The one-third rule keeps the tenon strong while leaving adequate shoulders — the standard proportion for mortise-and-tenon joinery. Thicker stock or specific joints can use the 0.4 or 0.5 ratio options.
Frequently asked questions
Why is tenon thickness usually one-third of the stock thickness?
The one-third rule balances the tenon against the mortise walls. A thicker tenon is stronger in bending and has more glue area, but it leaves thinner mortise walls that can split under racking loads; a thinner tenon leaves strong walls but can snap. At one-third, the tenon and each mortise wall are about the same thickness, which gives a well-balanced joint in most furniture. Heavier joints (0.4) or through tenons in thick stock (0.5) are used where the mortised piece is much thicker than the rail.
What is the correct tenon length for a mortise-and-tenon joint?
A common guideline is to make the tenon length equal to five times its thickness, or roughly 80% of the mortised member's thickness, whichever is less. Longer tenons provide more glue area and resist pulling apart, but they also risk breaking through the far face of the mortised piece if the stock is narrow. For through-tenons — where the tenon passes completely through the stile — length equals the full stile thickness, often with a decorative wedge. Always leave at least 3/8 inch of solid wood beyond the bottom of a blind mortise.
How do I cut a mortise and tenon joint without a mortising machine?
You can cut mortises by drilling a series of overlapping holes with a sharp Forstner bit to remove the bulk of the waste, then paring the walls flat and square with a sharp bench chisel. For the tenon, a table saw with a dado stack or a tenoning jig produces consistent shoulders quickly. Alternatively, a router table with an upcut spiral bit and a fence gives clean, repeatable results. Hand-tool purists use a rip-filed tenon saw for the cheeks and a crosscut saw for the shoulders, finishing with a shoulder plane. Whichever method you choose, sneak up on the final dimension and test-fit frequently.