A box is the project almost everyone builds early on, and it’s a good one — four corners, a base, and a surprising number of decisions hiding inside something that simple. What size is it, inside or out? Which joint at the corners? Does the base sit inside the sides or under them?
Get any of those wrong on paper and you find out at the glue-up, which is the worst possible time. This tool turns the dimensions you want into the parts you actually need to cut.
Enter your box dimensions, material thickness, and corner joint, and it returns a complete cut list — every part, its finished size, and a note on how it fits. Alongside that you get a 3D preview you can rotate, an explode view that separates the parts so you can see how they meet, and a dimensioned plan drawing to mark out from.
The part sizes account for the joint you’ve chosen. That’s the bit worth having calculated rather than done by hand, because butt and mitre joints distribute the material thickness differently and it’s easy to subtract it from the wrong part.
The tool opens with a 300 × 200 × 150mm box in 18mm stock, five sides, butt joints, working from exterior dimensions. That produces:
| Part | Length | Width | Thickness | Note |
|---|---|---|---|---|
| Bottom | 300 | 200 | 18 | Full base panel — horizontal |
| Front | 300 | 132 | 18 | Full width — sits on bottom |
| Back | 300 | 132 | 18 | Full width — sits on bottom |
| Left Side | 164 | 132 | 18 | Fits between front and back |
| Right Side | 164 | 132 | 18 | Fits between front and back |
Two details are worth pulling out, because they’re exactly what gets miscalculated by hand.
The sides are 164mm, not 200mm. They fit between the front and back, so they lose 18mm at each end — 200 − 36 = 164. Cut them at 200 and the box comes out 36mm too deep.
The height parts are 132mm, not 150mm. The front, back and sides sit on the bottom panel rather than beside it, so the bottom’s thickness comes out of their height — 150 − 18 = 132. The resulting interior is 264 × 164 × 132mm, which the tool reports alongside the exterior so you can check the cavity is actually the size you needed.
Butt joints are the forgiving option — every cut is a square crosscut, with no angle to set and no compounding error around the box. Mitres show no end grain at all, which reads as more finished, but all four corners have to close at exactly 90° combined, so a small error in the saw’s angle multiplies.
Neither is the correct choice and the other a compromise; they suit different projects and different stages of learning. If you’re weighing it up, the butt joints vs mitre joints guide below goes into the reinforcement options for both, which is where the real strength difference gets decided…
Exterior is the outside size of the finished box, measured face to face. Interior is the size of the cavity inside it. They differ by twice the material thickness on each axis, so a 300mm wide box built from 18mm stock has a 264mm interior. Choose whichever one your project actually constrains — if the box has to hold something specific, work from the interior.
With butt joints the parts are different sizes: the front and back run the full width, and the sides fit between them, so the sides are shorter by twice the material thickness. With mitres — spelled 'miter' in American English — every corner meets at 45°, so each part runs the full exterior dimension and the joint is cut into the thickness rather than taken out of the length.
The same joint. 'Mitre' is the British spelling and 'miter' the American one, and you'll see both in woodworking writing depending on where the author is. This tool calculates identical geometry either way — a 45° cut on both meeting parts, with the all-edge bevel geometry a box needs.
On a five- or six-sided box the bottom meets the sides at a corner too, not just the four vertical corners. If you only mitre the vertical corners you get a clean look on four edges and a visible butt joint where the base meets. The tool calculates the all-edge 45° geometry so every visible corner closes the same way.
Five gives you an open-topped box or tray. Six adds a lid panel. If you're planning to cut the box open after glue-up to make a lid that matches perfectly — a common technique for small boxes — build it as six sides and saw it apart afterwards.
The cut list gives finished part sizes. Kerf matters when you're laying those parts out on a sheet or a board, which is what the Cut List Optimiser handles. Cut slightly oversize and trim to the final dimension if you want a margin for error.