A box joint (also called a finger joint) is a strong, decorative corner joint made from interlocking rectangular fingers. Use this calculator to determine exact finger spacing, count, and jig setup for any board width and desired finger size, then preview the interlocking layout before you cut.
Quick Presets
Box Joint Settings
Common Box Joint Sizes
| Finger Width | Best For | Typical Stock | Dado Blade |
|---|---|---|---|
| 1/4" (6 mm) | Small jewelry boxes, thin stock | 1/4" to 3/8" thick | 1/4" dado set |
| 3/8" (10 mm) | Medium boxes, drawers | 3/8" to 1/2" thick | 3/8" dado set |
| 1/2" (13 mm) | Standard boxes, cabinets | 1/2" to 3/4" thick | 1/2" dado set |
| 3/4" (19 mm) | Large chests, tool boxes | 3/4" to 1" thick | 3/4" dado set |
As a general rule, match finger width to material thickness for a balanced, attractive joint. Narrower fingers create more glue surface and a more refined look.
How to Use the Box Joint Calculator
Building a box with box joints (also called finger joints) requires precise spacing so the interlocking fingers on both boards mesh perfectly. This free calculator eliminates the guesswork by computing exact finger counts, widths, and jig setup measurements from your board dimensions.
Step 1: Choose a Preset or Enter Custom Dimensions
If you are building a common project, click one of the presets (jewelry box, medium box, or large chest) to pre-fill the board width and finger width. Otherwise, type your board width and select inches or millimeters with the unit toggle.
Step 2: Set the Finger Width
Enter your desired finger width or click one of the common-width buttons (1/4", 3/8", 1/2", 3/4"). The finger width typically matches the material thickness for a balanced look. Narrower fingers create more glue surface area and a finer appearance, while wider fingers are easier to cut on a table saw.
Step 3: Enter Material Thickness
The material thickness is used to render the 3D perspective view and calculate the jig fence distance. Enter the actual measured thickness of your stock, not the nominal size.
Step 4: Choose a Spacing Mode
In Adjust to Fit mode, the calculator tweaks the finger width slightly so that an odd number of fingers fits the board perfectly with no leftover gap. In Equal Spacing mode, the calculator uses your exact finger width and reports whether the last segment is wider or narrower. For most table saw jig setups, Adjust to Fit is recommended.
Step 5: Read the Results
Click Calculate Box Joint to see the full layout. The stats cards show the total finger count and how many fingers belong to Board A versus Board B. The SVG preview illustrates the interlocking pattern from the front and in 3D perspective. The spacing table lists every finger and slot with precise start and end positions.
Step 6: Set Up Your Jig
The Jig Setup section provides the dado blade width and the exact fence-to-blade distance for a table saw box joint jig. Set your dado stack to the calculated finger width, then position the indexing pin at the specified distance. Board A gets cut first (starting with a finger), then Board B is offset by one finger width to create the matching slot pattern.
Tips for Clean Box Joints
Use a flat-bottom dado blade for the cleanest cuts. Always make test cuts on scrap wood of the same thickness before committing to your project boards. A snug fit straight off the saw is ideal — the fingers should slide together with hand pressure. If they are too tight, micro-adjust the jig pin position. Apply glue to all mating surfaces and use cauls to distribute clamping pressure evenly across the joint.
FAQ
Is this box joint calculator free?
Yes, the box joint calculator is completely free with no limits, no signup, and no account required. Calculate as many layouts as you need. All calculations run locally in your browser.
Is my data private and secure?
Absolutely. Everything runs entirely in your browser. Your measurements and settings are never sent to any server. No data ever leaves your device.
Why does the calculator recommend an odd number of fingers?
An odd number of fingers ensures proper interlocking. Board A starts and ends with a finger (pin), while Board B starts and ends with a slot. With an even count, both boards would have the same pattern and would not interlock correctly.
What finger width should I use for box joints?
Common finger widths are 1/4 inch for small boxes, 3/8 inch for medium projects, and 1/2 inch or 3/4 inch for larger pieces. The width typically matches the thickness of the material for a balanced look.
How do I set up a table saw box joint jig?
Set the dado stack width to match your finger width. Position the indexing pin on the jig fence exactly one finger width from the blade. The jig fence distance shown by this calculator is measured from the near edge of the blade to the near edge of the pin.
What is the difference between box joints and finger joints?
Box joints and finger joints are the same thing. 'Box joint' is the more common term in North America, while 'finger joint' is widely used in the UK and other regions. Both refer to interlocking rectangular fingers cut into the ends of two boards.
Can I use this calculator for metric measurements?
Yes, toggle between inches and millimeters using the unit selector. All measurements in the output, SVG preview, and jig setup instructions will display in your chosen unit.
What does 'adjust to fit' mode do?
Adjust-to-fit mode slightly modifies the finger width so that an odd number of fingers fits perfectly across the board width. This ensures a flush edge on both sides without any leftover gap or overhang.