How to use the box joint finger calculator
- Enter the board width, which is the width of the side being joined.
- Enter the finger width you would like, often the width of your router bit or dado set.
- Leave start and end with a finger ticked for a symmetrical joint with an odd number of fingers, or clear it to allow any count.
- Read the number of fingers, the exact width of each and the layout across the board. Open More options to check a fixed cutter width.
Formula
raw count = board width / wanted finger width
fingers = the whole number nearest the raw count (odd numbers only for a symmetrical joint)
finger width = board width / fingers
first board: fingers at positions 1, 3, 5 and so on; second board: the opposite pattern
The nearest count is the one whose finger width is closest to the width you wanted. A tie goes to the smaller count.
Two boards for a box
A 5.5 inch board with 1/2 inch fingers gives 5.5 / 0.5 = 11, which is odd, so the joint has 11 fingers of exactly 0.5 inch: 6 fingers and 5 slots on the first board.
A 7.25 inch board with 3/4 inch fingers gives a raw count of 9.67. The nearest odd counts are 9 and 11, and 9 gives fingers of 7.25 / 9 = 0.8056 inch (about 13/16), which is closer to the 0.75 you wanted than 11 fingers of 0.659. A 4 inch board with 5/8 inch fingers gives 6.4, so 7 fingers of 0.5714 inch.
When the fingers cannot match your cutter
A router bit or dado set cuts one fixed width. If the board width is not a whole number of cutter widths, the fingers on one board will not match the slots on the other. The best fix is to trim the board to a whole number of cutter widths. Enter your cutter width under More options to see the board width that fits.
If you must keep the board width, cut the fingers with a jig that moves in steps of the finger width, such as a table saw box joint jig, and cut one extra pass at the end.
Why an odd count
With an odd number of fingers, one board starts and ends with a finger and the other starts and ends with a slot, so every edge of the box has a clean finger on the outside corner. An even count leaves one board with a finger at one end and a slot at the other, which makes the corners differ.
Related woodworking tools
Price the lumber with the Board Foot Calculator, cut frames with the Picture Frame Stock Calculator, set angles with the Miter Angle Calculator, and add measurements with the Tape Measure Calculator.
Assumptions and limits
- Fingers and slots on a board are all the same width, and the joint is cut straight across the width of the board.
- The second board has the opposite pattern of the first.
- The width shown in fractions is rounded to the nearest 1/64 inch for reference only.
- Sawing tolerance and fit are tested on scrap, because a joint can be a little tight or loose.
Frequently asked questions
How many fingers should a box joint have?
Divide the board width by the finger width you want and use the nearest odd whole number for a symmetrical joint. A 5.5 inch board with 1/2 inch fingers has 11.
Why does the finger width change from what I entered?
The board width has to divide into a whole number of equal fingers, so the width adjusts slightly. A 7.25 inch board becomes 9 fingers of 0.8056 inch instead of 0.75.
What if my router bit is a fixed width?
Trim the board to a whole, odd number of bit widths. Enter the bit width under More options to see the board width that fits.
Can I use an even number of fingers?
Yes. Clear the odd count option. The corners of the box then differ, since each board begins with a finger at one end and a slot at the other.