Dovetail Spacing Calculator: Even Tails and Pins

The dovetail joint is one of woodworking’s most iconic joints: strong, beautiful, and stubbornly resistant to being pulled apart. The interlocking tails and pins create mechanical strength that needs no glue to hold, though glue is always added for permanence.

Layout begins with the baseline, a line scribed across the board at the depth of the joint, equal to the thickness of the mating board plus 1/32 inch. Everything above this line is the joint; everything below is the shoulder.

The ratio (also called the angle) describes how steep the sides of the tails are. Common ratios:

• 1:6 (≈ 9.5°): for softwoods, steeper angle provides more mechanical resistance to the lower shear strength of pine, cedar, etc.
• 1:8 (≈ 7.1°): for hardwoods, shallower angle looks elegant and is sufficient for stronger wood

The spacing formula divides the board width into equal sections. For N tails, you create N+1 spaces, with the half-pins at each edge (half the width of a full pin space). The tail-to-pin width ratio is usually 2:1 to 3:1, so tails are wider than pins.

Worked example. A 6-inch board with 4 tails, counting in units where a tail is 2 units and a pin is 1:

• 4 tails × 2 units = 8, plus 3 full pins × 1 unit = 3, plus 2 half-pins × 0.5 = 1
• 12 units in total, so one unit is 6 ÷ 12 = 0.5 inches
• Tails are 1 inch wide, full pins 0.5 inch, half-pins 0.25 inch
• Check it: 0.25 + (4 × 1) + (3 × 0.5) + 0.25 = 6 inches exactly

That “check it” line is worth doing every time. The total always comes to 3N units for N tails, which is a quick way to catch a layout that will not fit before you cut anything.

Lay out on the face of the board, scribe the baseline with a marking gauge, then set a sliding bevel to your chosen ratio for the angled lines. Cut the tails first and transfer them to the pin board with a sharp knife rather than a pencil: the knife line is where the saw goes, and a pencil line is three times wider than the fit you are trying to achieve.