Dovetail Spacing Calculator

Dovetail Spacing Calculator settings turn board width, tail count and pin sizes into a divider step and pin marks, even or graduated, with a check for drawer grooves and lid kerfs.

Dovetail Spacing Calculator for Hand Layout

The Dovetail Spacing Calculator divides a tail board into equal or graduated tails and gives you a way to mark them without adding up fractions. It returns the divider step, the start and end marks, a tilted-rule setting and every pin center from both edges.

It is built for hand-cut work on drawers, boxes and carcasses. The slope, baselines and half-blind laps are handled by the Dovetail Calculator, so this page stays on spacing.

Enter the actual board width, since a nominal 1x8 is 7-1/4 in wide. Pins and half-pins are measured at the end of the tail board, the narrow end of the pin sockets.

Tail Width and the Divider Step

With even spacing, the pins and half-pins come off the board width first. Whatever width is left is shared equally by the tails, so every tail matches.

$$t=\frac{W-2e-(n-1)p}{n}$$

W is the board width, e the half-pin, p the pin and n the tail count. On the default 7-1/2 in board with six tails, 1/4 in pins and 5/16 in half-pins, each tail is 0.938 in wide.

The divider step is one tail plus one pin. It is the distance from one pin center to the next, and every step across the board is the same.

$$s=t+p$$

That gives 1.188 in here. The trick is where the dividers start. The calculator places the start mark half a pin outside the left half-pin line, at 0.188 in, and the end mark half a pin outside the right one, at 7.313 in.

Walking the dividers from the start mark then lands every step on a pin center, and the sixth step lands exactly on the end mark. Mark half a pin on each side of each tick and the layout is done.

Chris Hedges uses the same idea in his Woodcraft divider layout, marking in from both edges and adjusting the dividers until the steps come out even. The calculator hands you that divider setting up front instead of by trial.

Laying Out With a Tilted Rule

If you'd rather use a rule, the third card rounds the step up to the next 1/4 in and multiplies by the tail count. For the default layout that is 1-1/4 in marks across a 7-1/2 in span.

Tilt the rule until zero and 7-1/2 in touch vertical lines drawn through the start and end marks, which takes an 18.2° tilt here. Each 1-1/4 in tick, squared down to the board end, is a pin center.

This is the ruler trick Paul Sellers teaches, as members walk through it on the Woodworking Masterclasses forum. The rule's angle does the division, so no odd fractions ever need reading.

Checking Marks From Both Edges

The Pin Marks table lists the left edge, center and right edge of every pin, measured from the left and from the right edge of the board. Checking a mark from the far edge catches the error that builds up when small readings are added one after another.

For the default layout, pin 1 runs from 1-1/4 in to 1-1/2 in from the left, and pin 5 mirrors it from the right. The center pin sits at 3-3/4 in from both edges.

How Many Tails Suit the Board

Tail count is a design choice, but Paul Sellers gives a working range in his dovetail sizing guide. He keeps tails between 7/8 and 1-1/4 in wide, with pins of at least 1/4 in and mostly 3/8 in.

The Dovetail Spacing Calculator's fourth card shows the tail width one count up and one count down, and which counts land in that range. On the default board, 5 tails come out at 1.175 in and 7 tails at 0.768 in, so 5 or 6 fit.

The Tail Count Options table runs several counts side by side with their pin width and spacing. For a chest with graduated drawers, Sawmill Creek members describe keeping tails about the same size and changing the count, from as few as 3 on the smallest drawer to 7 on the tallest.

Sizing Pins by Ratio

Instead of a fixed pin, you can set a tail-to-pin ratio. The calculator then solves for the pin width that makes every tail that many times wider.

$$p=\frac{W-2e}{nk+n-1}$$

At 4 to 1 on the default board, the pins come out at 0.237 in and the tails at 0.948 in. A 1 to 1 ratio is offered too, and the tool notes when pins match the tails, since the joint then reads like a box joint.

Graduated Dovetail Spacing

Graduated spacing keeps every pin the same width and changes only the tails. Each tail is a set percentage larger than its neighbor, so the eye reads the steps as a pattern.

At 15% per tail on the default board, one-way graduation runs from 1.292 in at the wide edge to 0.643 in at the narrow edge, about twice the width. Both-ways graduation gives 0.810 in tails at the edges and 1.071 in tails at the center.

The divider step changes with every tail, so the third card switches to the first and last step instead of a rule setting. The Pin Marks table becomes the main layout guide.

Drawer Grooves and Box Lid Kerfs

A groove for the drawer bottom should run through a tail, so it hides inside the joint instead of showing on the face. Popular Woodworking locates the bottom slot in the space for the lowest tail.

Enter the groove's start and width and the tool reports whether it lands inside a tail and how much wood it leaves at each tail edge. When it cuts into a pin, it suggests a start position that keeps it inside the nearest tail wide enough to take it.

A box that will be sawn open for a lid needs the opposite. Fine Woodworking spaces the tails farther apart around the kerf, so that after sawing, the pieces left on the lid and box look like proper half-pins.

The tool checks that the kerf falls inside a pin and gives the pin width to aim for. That is the kerf plus two of your edge half-pins, so a 1/8 in kerf with 1/8 in half-pins calls for a 3/8 in pin at that spot.