Mortise & Tenon Calculator
Calculate mortise and tenon dimensions using the traditional one-third rule for strong frame joints.
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The one-third rule states that the tenon thickness should be one-third of the stock thickness. This provides equal shoulder thickness on both sides. The tenon length should be 2/3 to 3/4 of the width of the receiving piece. Always make the mortise 1/16" deeper than the tenon length to leave room for excess glue. Four-shouldered tenons (haunched) resist racking better than two-shouldered ones.