Mortise and Tenon Calculator
Calculates tenon thickness as two-thirds of the rail thickness, rounded to the nearest 1/16 inch — the classic rule for strong mortise-and-tenon joints. Use it when laying out frame-and-panel furniture or door construction.
About this calculator
The mortise-and-tenon joint relies on a tenon that is approximately one-third to two-thirds of the rail thickness to balance glue surface area against residual wall strength in the mortised piece. The widely accepted rule sets tenon thickness at 66% of rail thickness: tenon_thickness = round((rail_thickness × 0.66) × 16) / 16. Multiplying by 16 and then dividing converts the decimal result to the nearest 1/16 inch, matching standard chisel and router-bit sizes. The shoulder width on each face is then (rail_thickness − tenon_thickness) / 2, keeping the joint symmetrical. Tenon length is typically 80% of the stile (or leg) thickness. Following this proportional system ensures the mortise walls are thick enough to resist racking without reducing the tenon to a fragile sliver.
How to use
Imagine a rail that is 1.5 inches thick. Enter rail_thickness = 1.5. The calculator computes: 1.5 × 0.66 = 0.99 → 0.99 × 16 = 15.84 → round to 16 → 16 / 16 = 1.0 inch tenon thickness. Each shoulder is (1.5 − 1.0) / 2 = 0.25 inches wide. If your stile is 1.75 inches thick, set tenon length to roughly 1.75 × 0.8 = 1.4 inches. You now have a complete, proportional layout ready to mark directly onto your stock with a mortising gauge.
Frequently asked questions
Why is tenon thickness set to two-thirds of rail thickness?
The two-thirds rule balances competing structural demands. A thicker tenon offers more shear strength and glue surface, but it leaves thinner mortise walls that can split under racking loads. At roughly 66%, each mortise wall retains about 17% of the stile thickness, which is widely accepted as a minimum for most hardwoods. Softer species or highly loaded joints may benefit from a slightly thinner tenon — closer to 50% — to keep walls robust. The rule is a starting point, not an absolute law.
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.