Calculate finger width and spacing for perfectly fitted box joints at any box width.
Board A — dark = finger, light = space · Board B is the inverse
Set your box-joint jig or router table fence to 3/8". Cut all fingers on board A, then shift the jig by one finger width to cut board B. Sand the fingers for a snug slip-fit before gluing.
Box joints (finger joints) divide the board width into equal sections that alternate finger and space. The calculator rounds your target finger width to the nearest integer number of sections, then ensures an odd total count so both boards start and end with a finger — giving the cleanest corner appearance.
Finger width is usually ¼″–½″ for small boxes, and up to ¾″–1″ for larger carcasses. Thinner fingers create more glue surface and a finer visual texture; thicker fingers are faster to cut and more forgiving.
Set your router table fence or box-joint jig, make a test joint on scrap, and aim for a slip-fit — fingers should slide together with light hand pressure, no hammer required.