Dovetail Calculator

Dovetail Calculator layouts start from your board width, tail count and angle, then give every mark at the end grain and at the baseline for a through, half-blind or sliding joint.

Dovetail Calculator for Hand and Router Work

The Dovetail Calculator lays out woodworking dovetails from your board width, tail count and slope. It gives the tail and pin widths at the end grain and at the baseline, every layout mark, the marking gauge settings and the chisels that fit the sockets.

It handles through dovetails for boxes and carcasses, half-blind dovetails for drawer fronts and sliding dovetails for shelves. The dovetail calculators machinists use measure metal dovetail slides over pins, which is a different job.

Enter the actual board width, not a nominal size, so a 1x6 goes in as 5-1/2 in. Pins and half-pins are measured along the end of the tail board, where the pins are at their narrowest.

Tail Width From Board Width and Pin Size

The calculator takes the pins out of the board width first and splits what is left equally between the tails. That keeps every tail the same size, with a half-pin at each edge.

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

W is the board width, e is the half-pin width, p is the full pin width and n is the number of tails, all measured at the end of the tail board. A 6 in board with 4 tails, 1/4 in pins and 3/8 in half-pins gives 1.125 in tails at the wide end.

Paul Sellers explains in his dovetail sizing guide that he never makes a tail wider than 1-1/4 in and rarely narrower than 7/8 in. He uses two tails on boards up to about 3-1/2 in wide, three on 4 in and four on 5 to 6 in.

When your tails fall outside that range, the insight box suggests a tail count that brings them back inside. Sellers aims for pins of at least 1/4 in and mostly 3/8 in. For spacing alone on a wide case side, the Dovetail Spacing Calculator works out the centers.

How Tails and Pins Change at the Baseline

Every tail narrows from the end grain down to the baseline, and every pin widens by the same amount. The change on each side of a tail is the tail length times the tangent of the angle.

$$o=L\times \tan \theta$$

On a through dovetail, L is the pin board thickness plus the proud allowance. A 3/4 in pin board with 1/32 in allowance at 1:8 gives an offset of 0.098 in per side.

So the 1.125 in tail comes down to 0.930 in at the baseline, and the 1/4 in pin grows to 0.445 in. That is why the chisel rows read 3/4 in for the tail sockets and 3/8 in for the pin sockets, since each chisel has to fit the narrowest part of its socket.

The Layout Marks table lists both edges of every tail from the left edge of the board, at the end and at the baseline, to the nearest 1/64 in. The drawing shows the same layout to scale, and it stretches only the depth when the tails would be too short to see.

Picking a Slope in Ratio or Degrees

Hand-tool slopes are written as ratios, while router bits and jigs are sold in degrees. Converting a ratio to degrees is the arctangent of one over the ratio.

$$\theta =\arctan \left(\frac{1}{r}\right)$$

A 1:8 slope is 7.13° and 1:6 is 9.46°. The hardwood and softwood pairing of those two is a trade tradition, not a standard. In a Woodcraft expert answer, Chris Hedges notes that antique furniture runs from 7° to 14°.

Hedges also cites laboratory tests on dovetails cut between 7.5° and 17.5° that found no difference in strength. Rockler describes the 1958 pull tests behind that finding and reports that the angle made no difference to joint strength.

Fine Woodworking still sets a working range of 7° to 15°. Below 7° the joint loses its wedge, and above 15° the tail tips are left with too much short grain. For stock under 1/4 in thick, it suggests at least 10°.

The Dovetail Calculator's angle chart gives the offset for every common ratio and bit over your actual tail length. It warns below 7°, adds a note past 1:4, and warns again past the 17.5° that was tested.

Marking Gauge Settings for the Baseline

The baseline sets how deep the tails and pins go. The Tail Board Gauge is the pin board thickness plus the proud allowance, and the Pin Board Gauge is the tail board thickness plus the same amount.

With a 3/4 in pin board and 5/8 in tail board, the default gauges are 0.781 in and 0.656 in. The parts need to be 1/16 in longer overall, since each end gets 1/32 in extra.

Rockler and Popular Woodworking both set the gauge about 1/32 in past the mating board, so the pins and tails stand proud and get planed flush. Woodsmith suggests 1/64 in for a first joint.

The Shy option works the other way. A Fine Woodworking forum thread credits Ian Kirby with setting the gauge 1/32 in under the board thickness, and the tool offers 1/64 in shy. The board faces then stand proud and get planed down to the joint ends.

Half-Blind Dovetails on a Drawer Front

A half-blind joint stops short of the drawer front's show face, so the front keeps a lap of solid wood. The tail length is the front thickness minus that lap.

$$L=t_{front}-\text{lap}$$

Christian Becksvoort explains in Fine Woodworking that he leaves about a 1/8 in lap on a 3/4 in front, with 5/8 in tails. He calls it a 5-to-1 ratio and warns that a thicker lap looks clunky while a thinner one risks chopping through.

An older joinery text reprinted by Lost Art Press keeps the lap between a quarter and a third of the front. The calculator's 3/16 in default on a 3/4 in front is 25%, and it warns below 16% or above 34%.

With that default the tails are 0.5625 in long and taper to 0.984 in at the baseline. The Front Inside Gauge of 0.656 in marks the pin baseline on the inside face of the front, just over the 5/8 in drawer side, as Becksvoort does.

Sliding Dovetail Groove and Neck

A sliding dovetail groove is widest at the bottom, where the bit is at full diameter, and narrowest at the board face. That narrow neck is the weakest section of both the groove and the tail.

$$w_{neck}=D-2d\tan \theta$$

D is the bit diameter and d is the groove depth. A 1/2 in bit cutting 1/4 in deep gives a 0.4375 in neck at 1:8, but only 0.375 in with a 14° bit.

Leigh's sliding dovetail instructions ask for a cutter of at least 12° and limit a full-width groove to one-third of the board. Canadian Woodworking allows one-third to one-half, which is 1/4 to 3/8 in on a 3/4 in gable.

Canadian Woodworking also says the bit should be no wider at its tip than the shelf is thick. The tool checks both limits, then sizes the shoulder trim on each face of the shelf and the largest straight bit that can plow out the waste first.

Tail Order and Router Bit Angles

Should you cut tails or pins first?

Both work, and the calculator's marks suit either order. Becksvoort cuts tails first on through dovetails and knifes the pins from them, which is the approach the insight box describes.

Woodsmith notes that many craftsmen once cut pins first, since they are easier to square to the baseline. Whichever comes first becomes the template for the other, so only the first board is laid out from these numbers.

Is a 14 degree router bit 7 degrees per side?

No. Router bit angles are given for each side of the cutter, so a 14° bit cuts a 14° slope on both walls. That makes it about the same as a 1:4 ratio, which is 14.04°, and the calculator treats a bit angle the same way.

The mix-up comes from the included angle, which is twice the bit angle. In a Sawmill Creek thread, one member reads a 14° bit as 7° per side, and another corrects it by matching the bit to 1:4.