Construction & DIY · Woodworking
Box joint finger calculator.
Enter the board width and the finger width you want to get an odd finger count and the exact finger width that divides the board evenly.
Inputs
Finger layout
Enter a board width and finger width.
How many fingers does a box joint need?
A box joint alternates equal-width fingers and notches across the board. Both boards are the same width, so an odd number of equal segments makes board A start and end with a finger and board B start and end with a notch, and the two interlock. The number of fingers is the board width divided by the finger width, rounded to the nearest odd whole number, and the exact finger width is the board width divided by that number.
Worked example. A 5.5 in board with 0.5 in fingers divides into 11 segments of exactly 0.5 in: board A has 6 fingers and 5 notches, board B has 5 fingers and 6 notches. A 6 in board with a 0.5 in target gives 12, which is even, so it rounds to 11 or 13 segments and the exact width changes slightly; set the jig to the exact width the tool shows.
This is layout arithmetic. Joint strength and the best finger size depend on the wood and the use, which this tool does not claim to judge.
Frequently asked questions
Why use an odd number of box joint fingers?
With an odd count of equal segments, one board begins and ends with a finger and the other begins and ends with a notch, so the corners interlock cleanly and the outside edges of both boards are symmetrical.
How do I get a box joint to fit a board width that does not divide evenly?
Adjust the finger width slightly so the board width divides into an odd number of equal parts. This tool shows the exact width and how far it is from your target.
How many fingers on each board?
With n segments (odd), one board has (n + 1) / 2 fingers and (n - 1) / 2 notches, and the other has (n - 1) / 2 fingers and (n + 1) / 2 notches.
What is the minimum number of fingers?
This tool requires at least three segments, which means the finger width cannot be more than one third of the board width.
Sources
Checked
No published data table is used; this is arithmetic on the board width, checked in the test suite for the odd-count rule, the finger and notch counts and the exact-fit property. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Layout arithmetic for woodworking, not a strength or design recommendation. Test on scrap and use your saw and router guards.