Lay out pins and sockets along the jointed edge for a tablesaw or router box joint. Enter the board width and a finger width. This returns the space count, leftover, and a pin width that lands a full finger at both ends if you want that.
A box joint (finger joint) tiles the jointed edge with equal-width pins and sockets. Along width W with pin width P:
n = floor(W / P)
R = W − nP
n = number of full pin-or-socket spaces · R = leftover that is not a full finger.
Starting with a pin: an odd n puts a pin at both ends of this board (sockets at both ends of the mate). An even n puts a pin at one end and a socket at the other. That even count is not wrong — it just makes the two long edges of the box look different.
If you want pins on both ends and a zero leftover, pick the odd integer k nearest to W/P and set P* = W / k. Then n = k and R = 0. You can use either neighbouring odd count; the closer k keeps P nearer to the width you typed.
Pin width is the finished finger. It equals the saw kerf or bit diameter only when you cut each socket in one pass with a dado or bit of width P. A thin-kerf blade (~2–3 mm / ~⅛ in) nibbles a wider socket in several passes — do not set P to that kerf unless that skinny finger is what you want.
| Width | Pin | Spaces n | Remainder | Start pin → pins / sockets |
|---|---|---|---|---|
| 3 in | 0.25 in (¼) | 12 (even) | 0 | 6 / 6 |
| 3.5 in | 0.25 in | 14 (even) | 0 | 7 / 7 |
| 4 in | 0.375 in (⅜) | 10 (even) | 0.25 in | 5 / 5 |
| 6 in | 0.375 in | 16 (even) | 0 | 8 / 8 |
| 100 mm | 8 mm | 12 (even) | 4 mm | 6 / 6 |
Those five rows were computed independently in Python and match this page. For the 3 in / ¼ in case, pins on both ends with no leftover means 11 spaces at 3/11 ≈ 0.273 in, or 13 spaces at 3/13 ≈ 0.231 in. The 3.5 in case is 13 spaces at ≈ 0.269 in, or 15 at ≈ 0.233 in.
Only if you cut each socket in one pass with a dado stack or router bit equal to the finger. Otherwise the pin is the wood you leave between sockets, and the kerf is just the cutter. Measure the actual slot in scrap; printed kerf on the blade pack is a starting guess.
Odd n starting with a pin puts a pin on both ends of board A and sockets on both ends of board B. The top and bottom of a box then match. Even n is fine for drawers or production runs where every board can be the same (pin on one end, socket on the other) and you flip the mate end-for-end.
Leave it as a skinny last sliver (usually ugly), split it onto the two end fingers (those two no longer match the dado), or change P so R = 0. This page shows R first, then a suggested P* that divides W into an odd count if you ticked “same feature on both ends.”
Same layout math. On the saw, a miter-gauge box-joint jig with an indexing key equal to P is the usual setup; stack a dado to P or nibble with a single blade. On the router table, a straight bit of diameter P (or a smaller bit with two passes) and an indexing pin does the same job. Cut a test corner in scrap before the project stock.
Two of this start and two of the opposite start, if all four sides share this width (a square box). A rectangle uses this layout on the pair that shares this width; the other pair is a separate ticket with its own W. Adjacent corners always mate pin-to-socket.
A 1:1 finger-to-thickness ratio is a common shop habit (½ in fingers on ½ in stock) but it is not a structural law. Finer fingers add glue area and look lighter; coarse fingers cut faster. This calculator does not take thickness — it only tiles the jointed edge.