Dovetail calculator
Enter the board, the slope and how many tails you want. The layout updates as you type: tail and pin widths on both faces of the joint, where to mark each tail from the edge, and the slope as a ratio and an angle.
The depth of the pins: the baseline you scribe.
Pin widths are measured at their narrow end, on the outside face of the pin board.
Each tail, at its wide end
≈ 1 21/32 in
Slope 1:6 = 9.46° from square
Traditional rule, stated by Fine Woodworking (Taunton Press), Lee Valley Tools and Rockler
- Tails, outside / inside face
- ≈ 1 21/32 in / ≈ 1 13/32 in
- Pins, narrow / wide end
- 1/4 in / 1/2 in
- Half pins, narrow / wide end
- 1/4 in / 3/8 in
- Each side moves
- 1/8 in
Tail marks from the left edge (outside face)
- Tail 1: 1/4 in to ≈ 1 29/32 in
- Tail 2: ≈ 2 5/32 in to ≈ 3 27/32 in
- Tail 3: ≈ 4 3/32 in to 5 3/4 in
Why: 3 tails ≈ 1 21/32 in wide at 1:6 (9.5°), between 1/4 in half pins.
Why these numbers?
The situation
A 6 in wide board, 3/4 in tail board, 3 tails at 1:6 (9.5°).
From your input: width, thickness, tails and slope
What that means
Dovetails are traditionally cut at 1:6 in softwood and 1:8 in hardwood.
Traditional rule: Fine Woodworking (Taunton Press), Lee Valley Tools and Rockler , sources [1, 2, 3]
So
Each tail on the outside face is what is left after the pins, shared out: (6 in − 2 × 1/4 in − 2 × 1/4 in) ÷ 3 = ≈ 1 21/32 in.
Arithmetic: exact by definition
The number
Through the 3/4 in thickness each sloped side moves 3/4 in ÷ 6 = 1/8 in, so on the inside face the tails are ≈ 1 13/32 in and the pins 1/2 in.
Arithmetic: exact by definition
Sources
How the layout works
Every width is measured on the outside face of the pin board, where the pins are narrowest and the tails are widest. The half pins at the edges and the pins between tails take their share of the board first, and what is left is split evenly between the tails. That is plain arithmetic, so it needs no source.
The slope is written as 1:r: the side of a tail moves 1 unit across for every r units along. Through the thickness of the tail board, each sloped side therefore moves the thickness divided by r, which is why the inside face shows wider pins and narrower tails. If a tail would shrink to nothing on the inside face, the calculator says so instead of drawing a joint that can't be cut.
Which slope?
The usual advice is 1:6 for softwood and 1:8 for hardwood. That is a traditional rule, not a measurement, and the tag beside the slope names the references that state it. Any ratio flatter than 1:1 works here, so you can lay out whatever slope you prefer. A router bit's angle converts the same way: type the angle and the ratio follows. Common ratios in degrees are in the dovetail angle chart.