Wood Joint Calculator
Calculate dimensions for dovetail, finger, box, half-lap, mortise and tenon, and dowel joints with SVG diagrams. Enter board dimensions to get joint measurements.
Joint Settings
Classic interlocking pins and tails
Affects dovetail angles
Joint Dimensions
Notes: Create 5 tails with 25mm width. Pins are 25mm wide. Slope angle: 9.5°
Joint Diagram
Dovetail, Mortise, Box: the Maths Behind Each
Each traditional joint has a proportional rule of thumb. Dovetails use a slope ratio of 1:6 for softwoods and 1:8 for hardwoods, where the steeper ratio resists the higher tear-out tendency of softwood. The default 150 mm board with 1:6 slope gives around 5 tails at roughly 25 mm wide each. Mortise and tenon traditionally uses a tenon thickness of one-third the board thickness; an 18 mm board (the default) gives a 6 mm tenon, with a mortise depth of two-thirds board thickness, so 12 mm.
Box and finger joints are simpler: equal-width fingers across the joint, with finger width usually equal to gap width. The calculator divides the board width by 20 mm to get a rough finger count for a 150 mm board, returning around 8 fingers. Dowel joints use a dowel diameter of half the board thickness with a length of roughly board thickness plus 20 mm to give grip on both sides.
Why the Numbers Are a Starting Point
These ratios are rules of thumb refined over centuries, not absolute laws. A drawer pull will look better with 6 evenly spaced dovetail tails than with 5 unevenly spaced ones, even if the maths suggests 5. A small jewellery box might use 1:5 dovetails to make the slope visually obvious. The point of the calculator is to give a sensible default that fits the wood type and dimensions, then you adjust for aesthetic preference.
Joint strength depends as much on glue surface area and grain orientation as on proportions. A well-cut mortise and tenon glued with PVA on long-grain to long-grain surfaces is essentially permanent. The same joint cut sloppy, with end-grain trying to take load, fails in months. For the wood pricing and project costing side, [Wood Cost Calculator](/wood-cost-calculator) handles the timber bill of materials.
Frequently Asked Questions
What is the strongest wood joint?
Mortise and tenon is the strongest traditional joint for frame work, especially when reinforced with a wedge or pin. Dovetails are strongest for box corners under tension (drawer fronts pulling out). Box joints have the most glue surface area and are often the most practical for cabinet construction with modern adhesives.
What slope angle for dovetails?
1:6 for softwoods (pine, fir): around 9.5 degrees from vertical. 1:8 for hardwoods (oak, ash, walnut): around 7.1 degrees. Hardwoods need less slope because the wood resists tear-out better and the steeper slope can leave fragile short-grain at the corners.
How thick should a tenon be?
One-third of the board thickness is the traditional rule. An 18 mm board gives a 6 mm tenon. The mortise width should match exactly. Tenon length is typically two-thirds of the joining piece's thickness, so a 6 mm thick tenon goes into a 12 mm deep mortise.
Can I cut these joints with hand tools?
Yes. All the joints in the calculator have been cut by hand for centuries. Dovetails by tenon saw and chisel, mortise and tenon by mortise chisel and tenon saw, box joints by saw and chisel or a router with a jig. A bench plane and a sharp paring chisel handle the fitting.
Related Tools
Wood Cost Calculator
Build a cut list and calculate timber costs for your woodworking project. Supports pine, oak, walnut, plywood and MDF with waste factor.
Sawhorse Angle Calculator
Calculate the leg length and cut angles for a sawhorse based on your desired height and base spread. Includes an SVG diagram with labelled dimensions.