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Dovetail calculator

Lay out a dovetail, exactly.

Give the boards, the number of tails and the pin width. Get every width on both faces, positions that add up to the board width to the hundredth, and a sheet to take to the bench.

Pin board below, tail board lifted above it. Drawn to scale from the numbers on this page.

With saw and chisel: you choose the pin width, the tails share the rest of the board.

With a dovetail jig: the bit and the template set the tail width, the pins are spaced evenly.

Boards

Tails and pins

Angle

Router

Fit the tails to sockets already cut

Measure one socket on both sides: its width on the inside face (narrow), on the outside face (wide), and its depth. You get tail widths that fit it.

LeftRightInside faceOutside faceDepth

Cut sheet

Marking out the tail board end

How the layout works

Everything is measured on the outside face of the pin board, where the pins are narrowest and the tails widest. Along that line: an edge pin, a tail, a pin, a tail, and so on, ending with an edge pin. Edge pins get the same narrow width as the others. The pieces add up to the board width.

Across the pin board's thickness each sloped side moves by the thickness times the tangent of the angle. That is how much narrower the tails get at their baseline and how much wider the pins get on the inside face. For through dovetails the tails are as long as the pin board is thick, and the pins as long as the tail board is thick.

Reading the angle

Hand-cut layouts use a slope: 1:8 means one across for eight along, which is 7.13° per side; 1:6 is 9.46°. Router bits are sold by the angle per side (7°, 8°, 14°). The included angle is twice that. Type the angle the way you think about it; the calculator shows it in all four.

Hand-cut gives the pin width and computes the tails. Router gives the tail width, spaces the pins evenly, and sizes the template: the bushing follows the template while the bit cuts the wood, so the template opening is wider than the tail by the difference between the two.

Use it from an AI agent

The same calculation is an MCP server. Add it as a custom connector in Claude, ChatGPT, Grok or any MCP client:

https://joinerycalc.com/mcp

One read-only tool, dovetail_layout. In clients that support MCP Apps, the drawings show up in the chat. No key, no account. What is logged is on the privacy page.

Or call the API

Schema at /openapi.json. Add "svg": true for the drawings.

curl -s https://joinerycalc.com/api/v1/dovetail -H 'content-type: application/json' -d '{
  "boardWidth": 140, "tailBoardThickness": 20, "pinBoardThickness": 20,
  "numberOfTails": 3, "pinWidth": 8, "angle": 6, "angleFormat": "slope_ratio"
}'