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Use our Box Joint Calculator to find finger width, segment count, slot count, and cut depth. Plan accurate wood joints with ease.
A box joint makes a strong link between two wood boards. It uses square fingers that fit into matching gaps. You can see this joint in wooden boxes, drawers, and shop projects.
Our Box Joint Calculator helps you plan each cut with less guesswork. Enter your board width, target finger width, and board thickness. The tool finds a suitable finger layout and shows the key results. It also supports mm, cm, inches, and feet.
A box joint uses equal-width parts across the edge of a board. The right count helps the joint fit within the full board width.
Raw segment count = Board width ÷ Target finger width
Lower count = The next lower odd whole number, with a minimum of 3.
Upper count = Lower count + 2
Lower width = Board width ÷ Lower count
Upper width = Board width ÷ Upper count
Lower error = Absolute value of (Lower width − Target finger width)
Upper error = Absolute value of (Upper width − Target finger width)
Selected segment count = The count with the smaller error. If both errors match, the tool picks the lower count.
Actual segment width = Board width ÷ Selected segment count
Finger count = (Selected segment count + 1) ÷ 2
Slot count = (Selected segment count − 1) ÷ 2
Cut depth = Board thickness
These formulas use the same unit for all values during the math. The tool converts your inputs to millimeters first. The cut depth is a starting value for a through joint. Your cutter and jig may need a small adjustment.
Enter the board width. Measure the edge where you plan to cut the joint.
Set the target finger width. Choose a size that suits your project and cutter.
Enter the board thickness. Use the real thickness, not just the label on the wood.
Choose your units. You can mix mm, cm, inches, or feet for your inputs.
Calculate and review the results. Check the segment count, actual finger width, finger count, slot count, and cut depth before you cut.
Think of the tool as a cutting plan. It helps you split the board into equal parts, much like sharing a cake into equal slices.
Suppose your board is 6 inches wide. Your target finger width is 0.375 inches, and your board is 0.75 inches thick.
First, divide the board width by the target width.
Raw segment count = 6 ÷ 0.375 = 16
The tool checks the odd counts around 16. These are 15 and 17.
For 15 segments, the actual width is 6 ÷ 15 = 0.4 inches. The error is 0.025 inches.
For 17 segments, the actual width is 6 ÷ 17 = 0.35294 inches. The error is about 0.02206 inches.
The tool picks 17 because its error is smaller.
The final results are 17 segments, 9 fingers, 8 slots, and an actual segment width of about 0.35294 inches. The nominal cut depth is 0.75 inches.
Test the setup on scrap wood first. A small jig error can affect the fit across the joint.
The Box Joint Calculator makes joint planning faster and easier. It helps woodworkers find a suitable segment count and work out the key sizes before cutting. Use its results as a guide, then test the fit with your own cutter and jig. A little care up front can save time, wood, and rework.
A box joint is a wood joint with square fingers and matching slots. It joins two boards at a corner.
Divide the board width by the selected segment count. The calculator picks an odd count that gives a width close to your target.
Yes. The calculator accepts mm, cm, inches, and feet. It converts the inputs to millimeters for the math.
For a through joint, start with the thickness of the mating board. Test the cut and adjust your jig as needed.
No calculator can account for every tool or setup. Check your cutter width, jig spacing, and wood size. Make a test joint before cutting your final boards.