The Tool Mill

Dovetail Layout Calculator

Space through dovetails evenly across a board. Choose the number of tails and the width of the pins at their narrow end, and the calculator gives the tail width, the half-pins at each edge, and the slope angle to set your bevel gauge.

Laying out before you saw

Through dovetails are laid out as a row of tails with pins between them and a half-pin at each edge. The proportions are a matter of taste — narrow pins and wide tails read as hand-cut and refined; wider pins look sturdier — but the spacing has to add up to the board width exactly, and doing that with dividers or a ruler is fiddly.

Decide how many tails you want and how wide the pins should be at their narrowest point. The calculator shares the rest of the width equally among the tails.

How the spacing is calculated

With N tails there are N − 1 full pins and two half-pins, which together equal N pin widths. Whatever is left is divided among the tails. The slope is the angle whose tangent is 1 ÷ ratio.

total pin = N × P · half-pin = P ÷ 2 · tail = (W − N × P) ÷ N · angle = atan(1 ÷ ratio)

Pin width P and tail width are both measured at the same face — the narrow end of the pins, on the face where the tails are widest.

Slope angles

Ratio	Angle	Typical use
1:5	11.31°	bold, decorative
1:6	9.46°	softwood, general
1:7	8.13°	a common compromise
1:8	7.13°	hardwood, fine work

The difference between 1:6 and 1:8 is mostly visual. Steeper slopes resist pull-out a little better in soft wood; shallower slopes are less likely to chip the short grain at the corners in brittle hardwood.

Spacing tables

Metric, with 12 mm pins:

Board width	Tails	Tail width	Half-pin
100 mm	3	21.3333 mm	6 mm
150 mm	4	25.5 mm	6 mm
200 mm	5	28 mm	6 mm
250 mm	6	29.6667 mm	6 mm
300 mm	7	30.8571 mm	6 mm

Inch, with ⅜ in pins:

Board width	Tails	Tail width	Half-pin
4 in	3	0.9583 in	0.1875 in
6 in	3	1.625 in	0.1875 in
6 in	4	1.125 in	0.1875 in
8 in	5	1.225 in	0.1875 in
10 in	6	1.2917 in	0.1875 in

Examples

A 150 mm drawer side, four tails, 12 mm pins, 1:6. Pins total 48 mm, so each tail is (150 − 48) ÷ 4 = 25.5 mm. Half-pins are 6 mm at each edge, and the bevel gauge goes to 9.462°.

A 6 in hardwood box side, three tails, ⅜ in pins, 1:8. Each tail is (6 − 1.125) ÷ 3 = 1.625 in, half-pins 0.1875 in, angle 7.125°.

When the numbers look wrong. A 100 mm board with six tails and 12 mm pins leaves only 4.666667 mm per tail — narrower than the pins. That is a valid layout mathematically but not a good-looking or strong one. Drop to three or four tails.

From numbers to marks

Mark the baseline on both boards at the thickness of the mating board, plus a hair if you plan to plane the joint flush.
On the tail board, mark the half-pin at each edge, then step off tail, pin, tail, pin across the end. A pair of dividers set to tail + pin is quick and accurate.
Set a sliding bevel to the slope angle, or use a dovetail marker, and draw the sloped lines on the face. Square them across the end grain.
Mark the waste clearly with an X. Most mistakes in hand-cut dovetails are sawing on the wrong side of the line.
Saw the tails, then transfer them to the pin board with a sharp knife and saw the pins to match.

FAQ

Can I have the half-pins wider than half a pin?

Yes. Many makers use slightly wider half-pins at the edges for strength. This calculator uses exactly P ÷ 2; for wider edges, subtract the extra from the board width before you enter it.

Does the calculator handle half-blind dovetails?

The spacing across the width is the same; the depth of the sockets is different and is not modelled here.

What if the tails come out with a long decimal?

Set dividers from the result rather than measuring each one. Spacing evenly matters more than hitting a round number.

Should I cut tails or pins first?

Either works. Tails first is more common because the tails are easier to transfer accurately to the pin board.