Box Joint (Finger Joint) Spacing Calculator

What Is a Box Joint? A box joint, also called a finger joint, is a row of interlocking rectangular pins and spaces cut into the ends of two boards. Meshed together they present an enormous area of long-grain-to-long-grain glue surface, which is why a well-cut box joint on a drawer will outlast the drawer. It is also honest joinery: every finger is visible, so the quality of your setup is on display forever.

The layout rule, and the arithmetic everyone gets wrong

A box joint should begin and end with a pin, so that both edges of the board are solid rather than a fragile half-open socket. That gives you N pins and N minus 1 spaces, which means the board is divided into 2N minus 1 equal segments. The segment count is always odd.

So the number you divide the board width by is the segment count, not the pin count. Confusing the two is the classic error, and it is off by roughly a factor of two.

segments = board width ÷ pin size, rounded to the nearest ODD number pin size actually cut = board width ÷ segments pins = (segments + 1) ÷ 2

Worked example. A 6" board with 3/8" pins in mind: 6 ÷ 0.375 = 16 segments, which is even, so you round to 15 or 17. Seventeen gives 6 ÷ 17 = 0.3529" per segment, 9 pins and 8 spaces. Fifteen gives 0.400", 8 pins and 7 spaces. Seventeen is 22 thousandths off what you asked for, fifteen is 25 thousandths off, so the calculator picks seventeen. Either is fine. Neither is 6" worth of 3/8" fingers, because that does not exist.

Adjusting for width When the pin size does not divide the board evenly, you can nudge the pin size or rip the board. Most people nudge the pin size, and a difference of 1/64" is genuinely invisible in the finished joint. If you are cutting with a fixed dado stack rather than a router bit, ripping the board to a width that divides evenly is often easier than shimming the stack.

Jig setup Box joints are cut on a table saw with a dado stack, or on a router table, using a sled with an indexing key. The key width and the gap between key and blade both equal the segment size, and this is the whole ball game: a setup a few thousandths wide compounds across every segment, and all of that accumulated error arrives in the final pin. Cut a test joint in scrap of the same thickness first, every single time.