Wood Joint Calculator
Calculate mortise and tenon, dovetail, and box joint dimensions from board thickness.
Returns tenon length, mortise depth, and tail angle in inches and mm.
Wood joint dimensions determine both how strong a connection is and how it looks when finished. Getting the sizing right — especially for mortise-and-tenon, box joints, and dovetails — is the difference between a furniture piece that lasts generations and one that falls apart in a few years.
Key joint formulas:
Mortise-and-Tenon:
- Tenon thickness = 1/3 of stock thickness
- Tenon length = 2/3 to 3/4 of mortise member width
- Tenon width = 3/4 of stock width (leave 1/8" shoulder each side)
Box (Finger) Joints:
- Finger Width = Board Thickness (square fingers are strongest)
- Number of Fingers = Board Width ÷ Finger Width (round to nearest odd or even number as needed)
Dovetail Joints:
- Dovetail Angle: Softwood — 1:6 slope (9.5°), Hardwood — 1:8 slope (7.1°)
- Tail Spacing: typically 1/2 to 3/4 inch between tail centers
- Pin Width (at narrowest): 3–5mm for aesthetics, 5–8mm for strength
Worked example — mortise and tenon: Stock thickness: 1.5 inches, member width: 3.5 inches
Tenon thickness = 1.5 ÷ 3 = 0.5 inches Tenon length = 3.5 × 0.7 = 2.45 inches (use 2.5 inches) Tenon width = 3.5 × 0.75 = 2.625 inches (shoulder = 0.4375" each side)
Strength reference: Properly sized mortise-and-tenon joints can withstand 500–1,500 lbs of shear force, depending on wood species and grain direction. Oak and maple joints are significantly stronger than pine at identical dimensions.
Fit guidelines: Tenon should slide into mortise with light hand pressure — not loose, not requiring a mallet.