The mortise and tenon calculator generates the correct dimensions for mortise depth, width, and length, plus the tenon thickness and shoulder sizes. Based on the time-honored 1/3 rule for tenon thickness, ensuring strong joint walls on both pieces.
Mortise & Tenon Joint Calculator
How to Cut a Mortise and Tenon Joint
The mortise and tenon is the strongest wood joint for connecting rails to stiles, legs to aprons, and frames to panels. This mortise and tenon calculator eliminates the guesswork on dimensions.
Cut Mortise First
Always cut the mortise first, then fit the tenon to it. Mark the mortise centerline, set your chisel or router to the mortise width, and chop or rout to depth in multiple passes. Clean the walls with a sharp chisel. Leave the corners square for a hand-cut mortise, or round the mortise corners to match a router bit and round the tenon shoulders to match.
Fit the Tenon
Cut the tenon slightly fat (1/64\" oversized), then use a shoulder plane or tenon saw to sneak up on the final fit. Test with the mortise piece — the joint should seat 3/4 of the way with hand pressure, fully with a mallet tap. Glue up dry-fitted first to test clamp reach before applying glue.
FAQ
Is this mortise and tenon calculator free?
Yes, completely free. All calculations run locally in your browser.
How thick should a tenon be?
The tenon thickness should be one-third the thickness of the tenon piece (rail). For a 1-1/2" thick rail, the tenon is 1/2" thick. This rule ensures the mortise walls are thick enough not to blow out, and the tenon has enough material strength. A tenon that's too thick weakens the mortised piece.
How long should a mortise and tenon joint be?
Tenon length should equal 4–5× the tenon thickness, or fill 2/3 of the mortised piece's thickness. For a 3/4" thick tenon, aim for 3" to 3-3/4" length. Leaving 1/4" to 3/8" of wood beyond the mortise bottom protects against blowout during assembly.
What is a haunch in a mortise and tenon joint?
A haunch is a small shoulder cut at the top of the tenon that fills the groove in a door or panel frame. It prevents racking (twisting) without requiring a separate floating panel. Haunched tenons are standard for frame-and-panel doors — the haunch fills the panel groove so it doesn't show.
What is the fit tolerance for a mortise and tenon?
A properly fit mortise and tenon should slide together by hand with light pressure, but not fall together under its own weight. Gap between 0.001" and 0.003" (0.025–0.075mm) is ideal. Too tight risks splitting during assembly. Too loose results in a weak, sloppy joint after glue dries.