Mortise and Tenon Joint Calculator

The mortise and tenon joint is the cornerstone of furniture and timber frame construction. Correct proportions ensure maximum strength without weakening the mortised piece.

Tenon thickness rule: Tenon thickness = 1/3 of the stock thickness (the piece being tenoned) This leaves 1/3 on each side of the mortise for strength.

Tenon width rule: For a single tenon: tenon width = 80% of the stock width (leave shoulders) For a double/haunched tenon: use where width exceeds ~100 mm

Tenon length (mortise depth):

• Blind mortise: tenon length = 2/3 of the mortised piece thickness (minimum 25 mm)
• Through tenon: tenon length = full thickness of the mortised piece + 3–5 mm for wedging

Shoulder size:

• Width shoulder: (stock width − tenon width) / 2
• Thickness shoulder: (stock thickness − tenon thickness) / 2

Worked example: Rail 40 mm thick × 80 mm wide, mortised into a 50 mm × 50 mm leg:

• Tenon thickness: 40 / 3 = 13.3 mm → round to 13 mm (use a standard chisel/bit)
• Tenon width: 80 × 0.80 = 64 mm
• Blind mortise depth: 50 × 2/3 = 33 mm
• Thickness shoulder: (40 − 13) / 2 = 13.5 mm each side
• Width shoulder: (80 − 64) / 2 = 8 mm each side

Fitting: A good tenon should fit the mortise with light hand pressure — no hammer required. The glue film (~0.1 mm) provides the final snug fit.