Box joint calculator
Box joints (finger joints) look best when the fingers come out even across the board. Tell the calculator the board width and the cutter you have, or the finger width you are aiming for, and it works out the count, the leftover and the widths that fit.
Whole fingers across the board
14
1/4 in left over: 1/8 in at each edge if you keep the width.
- Width for 14 fingers
- 5 1/4 in
- Width for 15 fingers
- 5 5/8 in
- Matching edges, 13 fingers
- 4 7/8 in
- Matching edges, 15 fingers
- 5 5/8 in
Why: 14 fingers of 3/8 in, with 1/4 in over.
Why these numbers?
The situation
A 5 1/2 in board and a 3/8 in cutter.
From your input: board width and finger width
So
5 1/2 in ÷ 3/8 in = 14.67, so 14 whole fingers fit.
Arithmetic: exact by definition
The number
14 × 3/8 in = 5 1/4 in, leaving 1/4 in: 1/8 in at each edge if you keep the width.
Arithmetic: exact by definition
Two ways to fit the fingers
With a fixed cutter, such as a dado stack or a straight router bit, the finger width is set by the tool. The calculator divides the board width by it, counts the whole fingers, and shows what is left over. You can then rip the board to a width that fits exactly, or keep the width and leave a narrow part-finger at each edge.
With an adjustable jig, you can set any finger width, so the calculator picks the odd number of fingers whose width is closest to your target. An odd count puts the same thing at both edges: one board has a finger at the top and the bottom, its mate a notch at both, so the corner looks the same either way up.
All of this is arithmetic, exact by definition. Cut a test joint in scrap first: the fit of a box joint depends on the jig's pin and the cutter's real width more than on the numbers.