How to measure a box: internal vs external dimensions and why it matters
The convention for stating box dimensions, how to measure a product for a new box, what board thickness adds, and the mistakes that lead to boxes that don't fit.
By Tradepack ·
More packaging quotes go wrong on dimensions than on anything else. A box arrives that the product will not fit into, or one so loose the product rattles, and the cause is nearly always the same: someone measured the outside when the manufacturer expected the inside, or wrote the numbers in the wrong order. Here is how the industry does it.
The convention
Corrugated box dimensions are stated as internal measurements, in millimetres, in the order length x width x height.
- Length is the longer of the two dimensions across the opening.
- Width is the shorter.
- Height is the distance from the opening to the base. On a standard 0201 case, this is the dimension the flaps close across.
So a box quoted as 400 x 300 x 200 mm has an opening 400 mm by 300 mm and is 200 mm deep inside. The product has to fit within those numbers, not the outside of the box.
Some styles complicate the height convention. On a 0427 mailer, height is usually taken as the depth of the base tray, since the lid hinges over it. On a book wrap with multiple crease lines, the height is a range rather than a single figure. When in doubt, state which dimension you mean.
Measuring a product for a new box
Start with the product, not the box.
- Measure the product at its widest points in each direction, including any protrusions, packaging it already wears, or a retail carton it sits in.
- Decide how it packs. One unit per box, or several? If several, work out the arrangement (two across, three deep, one high, and so on) and multiply.
- Add clearance. A few millimetres in each direction lets the product go in without forcing and come out without tearing the box. Three to five millimetres per dimension is typical for a snug case. Allow more if the product is fragile and needs void fill or an insert, or if it is packed at speed.
- Write it as L x W x H, internal. The largest opening dimension first.
That is the number a manufacturer needs. They will add board thickness and fold allowances to produce the blank size.
What board thickness adds
The gap between internal and external size is board thickness, twice over. Typical single-thickness figures for common grades are:
| Board | Approximate thickness |
|---|---|
| E flute single wall | 1.5 mm |
| B flute single wall | 3 mm |
| C flute single wall | 4 mm |
| BC double wall | 7 mm |
A 300 x 200 x 150 mm internal box in B flute will therefore be roughly 306 x 206 x 156 mm on the outside, and slightly more once flap thickness on the top and bottom is counted. That matters when the box has to fit a carrier size band, a shipping carton, or a shelf. Give the manufacturer both the internal requirement and any external limit, and they will choose a grade and construction that satisfies both.
Measuring an existing box
If you are replacing a box you already have, measure it erected, from the inside, with a steel rule or tape held flat against the board. Measure the opening across both directions and the depth to the base. Do not measure a flattened blank; the fold allowances make the numbers misleading. And check the board grade at the same time, since a like-for-like replacement needs both.
Mistakes that cost money
- External given as internal. The box arrives too small. This is the single most common fitting problem.
- Dimensions in the wrong order. The manufacturer builds the box with the flaps closing across the wrong dimension, which changes how the product loads and how the case stacks.
- No clearance. A box cut exactly to the product size is a box the packer has to force.
- Product measured without its own packaging. A retail carton or a protective sleeve adds size that the box has to accommodate.
- Inches and millimetres mixed. Corrugated is specified in millimetres in the UK. Convert before you write it down.
Sending it with the enquiry
When the dimensions are right, the rest of the quote follows quickly. Put the internal L x W x H alongside the style, the board grade if known, the print requirement and the quantities, and the manufacturer can return a price and a drawing first time. For anything new or unusual, expect a sample cut on a digital flatbed before tooling, which is the point at which any remaining doubt about fit is settled.
