Inner dimensions versus outer dimensions
Almost every box generator on the web asks for the outside size. That is the number the finished box occupies on a shelf, but it is not the number you care about. What you care about is whether your object fits inside — and converting between the two means subtracting wall thickness on every axis, correctly, in your head, before you cut.
It goes wrong often enough that it is worth writing down. With wall thickness t:
- Outer length = inner length + 2t
- Outer width = inner width + 2t
- Outer height = inner height + t
The height only gains one thickness because the bottom panel sits under the walls rather than between them. This generator uses that layout, so the top opening is exactly the inner length by the inner width — which is why the inset lid is cut to the internal footprint.
Worked example
Suppose the thing you need to box up measures 100 × 60 × 40 mm and you want 5 mm of clearance on the length and width, and 5 mm on the height for a cable.
| Material | Type in | Finished outer size |
|---|---|---|
| 3 mm plywood | 105 × 65 × 45 | 111 × 71 × 48 mm |
| 6 mm MDF | 105 × 65 × 45 | 117 × 77 × 51 mm |
| 9 mm plywood | 105 × 65 × 45 | 123 × 83 × 54 mm |
Same input, three very different boxes. If you had instead typed the same numbers into a generator that expects outer dimensions, the 6 mm MDF box would have come out with 93 mm of usable length — 12 mm too small, and you would not find out until assembly.
How much clearance to add
Type the inside size you want, not the size of your object. The gap between the two is your clearance:
- 0–1 mm — the object drops in and never moves. Good for a snug case around something rigid.
- 2–3 mm — a little room to lift things out. Works for most electronics and hand tools.
- 5–10 mm — comfortable access, foam or dividers, fingers. Costs material, not much.
Add extra height if anything connects at the top, and remember that a lid with a lip takes a couple of millimetres out of the interior.
Three mistakes this avoids
- Subtracting the wrong number of walls. It is two walls on each horizontal axis, one under the bottom. Getting this wrong by a single thickness is a 6 mm error at 3 mm material — enough to scrap the cut.
- Measuring the object instead of the space. A 100 mm ruler is not a 100 mm box. Measure the item plus whatever it sits in.
- Forgetting the joint. Finger joints do not change the outer size, but they do consume the corners of the internal volume at the wall thickness. A box that is exactly the size of a square object will pinch at the four vertical corners.
Other box generators
- Laser cut box generator — kerf measurement, laser settings and import notes.
- 3D printed box generator — the same dimensions logic, exporting an STL.
- 3D box generator — the main page, with both laser and print output.