Order a box in the wrong dimension order and you can end up with a tall skinny carton when you wanted a wide flat one. Box sizing follows a convention, and once you know it, it is hard to get wrong.

The standard order

Box dimensions are almost always listed as length x width x height (L x W x H), measured as the inside dimensions:

Length is the longest of the two edges of the opening (the top face)
Width is the shorter of the two edges of the opening
Height (or depth) is the distance from the opening down to the base

So a 12 x 9 x 4 inch box has a 12-inch long opening, a 9-inch wide opening, and 4 inches of depth.

Why length and width are measured at the opening

For a standard box, you take length and width across the open top face, then height as how deep it goes. This matters for shipping boxes because the flaps meet on the longest sides, which affects how the box folds and seals.

Measure inside, not outside

Always use the internal dimensions when you are fitting a product. The board has thickness, so the outside is slightly larger. For corrugated, the difference is small but real, and it is the inside space your product actually sits in.

Getting the size right

A box cut to your exact product protects better and ships cheaper, because carriers bill by dimensional weight and an oversized box costs you on every order. If a stock size wastes space, order a box cut to your three measurements: see our custom L x W x H boxes.

To recap: length is the longest top edge, width the shorter top edge, height the depth, measured inside. Send those three numbers in that order and the box comes back the shape you pictured.