The most common finger joint mistake happens before any wood is cut. You pick a finger width you like, divide the board width by it, cut that many fingers — and end up with a thin, awkward sliver at one edge where a full finger should have been.
It’s an easy error to make, and the fix is arithmetic rather than skill. This tool does that arithmetic, and gives you the jig setting to cut from.
Enter your board width and material thickness, and it returns the exact finger width to set your jig to, how many fingers each of the two mating boards gets, and the glue area per corner. It shows the joint in 3D and as a marked-out layout with the segment boundaries measured from the board’s left edge.
Crucially, it only ever gives you odd finger counts — because that’s what makes the joint symmetric at both edges.
The tool opens with a 150mm board in 18mm stock and nine fingers:
Jig setting: 16.667 mm
Fingers: 5 + 4
Depth: 18 mm
Glue area: 2592 mm²
The 5 + 4 is the part that shows the symmetry working. Board A gets
five fingers — one at each edge and three between — and board B gets
four, which sit in board A’s gaps. Nine segments across 150mm gives
16.667mm each, and every one is identical.
The target-width route is worth demonstrating too, because it’s the feature that prevents the sliver problem. Ask for a 15mm finger on that same 150mm board and the tool reports:
Nearest odd count: 11 → fingers 13.636 mm
Ten fingers of 15mm would divide the board perfectly — but ten is even, so the joint would finish on a gap and leave an unsupported edge. Eleven fingers of 13.636mm is the nearest odd alternative. You give up 1.4mm of finger width you’d chosen fairly arbitrarily, and get a joint that’s symmetric and full-width at both edges. That’s a trade worth making without much hesitation.
The jig setting is the width of your indexing key as well as the finger width, so cut the key to that dimension and the spacing follows. Set the blade height to just over the mating board’s thickness — a whisker proud, so the fingers stand slightly above the surface and can be sanded flush after glue-up rather than finishing below it.
A backer board behind the cut is worth the thirty seconds it takes. Box joints are all end grain at the exit side of the cut, which is exactly where tearout happens, and a sacrificial backer removes the problem entirely…
A finger joint alternates finger, gap, finger, gap across the board. With an odd total you start and finish on a finger, so both edges of the board are full-width fingers and the joint is symmetric. With an even total you finish on a gap, which leaves a thin sliver at one edge — visually lopsided and structurally the weakest point of the joint.
Two conventions are in genuine circulation: finger width equal to the stock thickness, and finger width at half the stock thickness. Neither is wrong. Equal-to-thickness gives a bolder, chunkier look and fewer cuts; half-thickness gives a finer joint with more glue surface. Treat it as taste rather than rule, then let the odd-count requirement adjust the exact figure.
It's the exact finger width, which is also the width of your indexing key and the spacing on your box joint jig. Cut a test joint in scrap of the same thickness before committing to your actual workpiece — a jig that's out by even a fraction compounds across the board.
Because the board width has to divide exactly into an odd number of equal fingers. Rather than forcing a round finger width and leaving an awkward remainder at the edge, the tool finds the odd count closest to your target and divides the board width by it. The result is usually within a fraction of a millimetre of what you asked for, and every finger comes out identical.
Not meaningfully, for most furniture-grade work. Destructive testing has found box joints performing comparably to dovetails and sometimes slightly better, largely because a finger joint packs more glue surface into the same corner. Dovetails do have a genuine mechanical lock that resists being pulled apart without glue, which mattered far more in the hide-glue era.