Laying out a perfect box joint follows a simple mathematical principle:

1. Measure Your Board: Get a precise measurement of the width of the board you intend to join.
2. Choose a Target Pin Width: Decide on an approximate size for your pins. This is often determined by the thickness of the material or the jig you are using (e.g., 1/4", 3/8", 1/2").
3. Calculate the Number of Pins: The calculator divides the board width by your target pin width to get an estimated number of pins. For a strong, symmetrical joint that starts and ends with a pin, this number must be odd. The calculator adjusts to the nearest odd number.
4. Determine the Actual Pin Width: The board width is then divided by the final odd number of pins. This gives you the *actual* pin width you must set your jig or fence to. This slight adjustment from your target width is the key to a perfect fit.

• Box Joint: A woodworking joint made by cutting a set of complementary, interlocking rectangular profiles in two pieces of wood. Also known as a finger joint.
• Pin: The rectangular "finger" that protrudes from the end of a board.
• Slot: The empty space between two pins, which accepts a pin from the mating board.
• Box Joint Jig: A device, typically used with a table saw or router table, that guides the workpiece to make evenly spaced, repeatable cuts for box joints.