Specifying · 5 min read
How to measure a box so the die comes out right
Almost every box we have ever had to remake was measured correctly and written down wrongly. Three numbers, one convention, and a decision about which side of the board you mean.
Always give internal dimensions
A box is described by the space inside it, because that is the space your product has to fit into. External dimensions vary with the board — a 3 mm B-flute box is 6 mm wider on the outside than an E-flute box with the same internal size.
If you measure an existing box, measure the cavity, not the outside. If you are working from a product, add your clearance to the product and quote that. When you build a spec in our box tool, every number it asks for is internal.
Length, width, height — in that order
Length is the longer of the two dimensions of the opening. Width is the shorter one. Height is the depth from the opening down. It sounds obvious until someone sends 6 × 8 × 3 and means a tall narrow box rather than a shallow wide one.
The convention matters because a die is cut from the numbers, not from the intention. Two boxes with the same three numbers in a different order are different boxes and cost different money.
If there is any chance of ambiguity, write it as L × W × H and say which face opens.
How much clearance to add
Corrugated relaxes after it is cut, so a fit that is perfect on the sample is snug in the run. Folding carton board does not move as much and can be built tighter.
| Rigid product, hand-packed | 1–2 mm per side |
|---|---|
| Soft or compressible product | 2–3 mm per side |
| Product with a lid or pump | Add 3–4 mm to height, not to the footprint |
| Multiple units side by side | 1 mm between units plus 2 mm at the walls |
The measurement people forget
Board thickness eats into height twice — once at the base and once under the lid. On a 25 mm deep tray in 2 mm grayboard, you lose 4 mm before anything goes in. For anything shallow, measure the product and tell us its height rather than telling us the box height you think you need.
This is why we send a dieline with every quote. It shows the cavity as it will actually be, and it is far cheaper to argue with a drawing than with a pallet.
Two more things people ask
Internal or external dimensions?
Internal, always — that is what the product has to fit inside. External dimensions include board thickness and are what the carrier measures.
In what order do I list length, width and height?
Length is the longest side of the opening, width the shorter, height is the depth. Say which is which and mix nothing.
How do I measure a curved or irregular product?
Across its widest point in each direction, including any cap, handle or protrusion. A photograph beside a ruler removes most ambiguity.
How much clearance should I add?
Two to three millimeters on each side for a snug fit; more where void fill is needed. A box cut exactly to the product is hard to pack.
Should I measure the product or the container?
The product, filled and assembled, with its cap or lid on. A container measured empty is not what has to fit.
What if my product varies between batches?
Measure several and build to the largest. Glass, textiles and molded parts all vary more than people expect.
Can you measure it for me?
Yes. Send the product, or a sample of your existing box, and we measure and redraw it — faster and more accurate than a description.
Does board thickness change the box size?
Yes. Heavier board on four walls takes internal space from the product, which matters most on small boxes.
Should I send you a sample box?
If you have one, yes. Measuring a physical box takes us five minutes and removes every ambiguity about which face opens and how deep the cavity really is.
Metric or imperial?
Either, as long as you say which. We work in both and quote in the units you send. Mixing them in one spec is where mistakes happen.