How thick should a tenon be?

The traditional rule is one-third of the stock thickness. For a 45mm thick rail, the tenon should be about 15mm thick. This leaves equal-sized shoulders on each face, providing good glue surface and resistance to racking.

How long should a tenon be?

For a through tenon, the length equals the full width of the mortise piece. For a stub (blind) tenon, the length should be about two-thirds of the mortise piece width. Never make a stub tenon so long that it weakens the mortise piece.

What is a haunched tenon?

A haunched tenon has a small step cut into one edge. The haunch fills a groove (typically for a panel) and prevents the joint from twisting. It is commonly used in frame-and-panel door construction.

Should the mortise be wider than the tenon?

The mortise should match the tenon thickness exactly for a snug fit. It should be 1–2mm deeper than the tenon length to allow space for excess glue. Some woodworkers make the tenon 0.1–0.2mm thicker and pare to fit.

WoodCalcs

Mortise and Tenon Calculator — Joint Dimensions

Calculate mortise and tenon dimensions based on your timber sizes. Supports through, stub, and haunched joints.

Stock Thickness

Thickness of both pieces

Stile Width (mortise piece)

Rail Width (tenon piece)

Joint Type

How We Calculate This

This calculator uses traditional proportions for mortise and tenon joints.

Rules applied

Tenon thickness = Stock thickness ÷ 3 (the one-third rule)
Shoulder width = (Stock thickness − Tenon thickness) ÷ 2
Tenon width = Rail width − 10mm (5mm shoulder each edge)
Through tenon length = Full width of stile
Stub tenon length = &frac23; of stile width
Haunch depth = &frac13; of tenon length

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.