How to Measure a Box: Internal vs External Dimensions

Internal is what fits, external is what ships

Every box has two sets of dimensions, and confusing them is the most common ordering mistake buyers make.

• Internal dimensions are the usable space inside. This is what your product has to fit into.
• External dimensions are the outside of the box. This is what the carrier measures for size bands and what decides whether the box fits on your shelf or in a mailer.

For choosing a box around a product, always work from internal dimensions. For working out postage bands and storage, use external.

Why the gap between them matters

The wall of a corrugated box takes up real space. A single wall box adds roughly 3 to 7mm to each dimension depending on flute; double wall adds more. So an item that is exactly 300mm long will not fit a box quoted at 300mm internal, because there is no clearance, and it definitely will not fit one quoted 300mm external.

As a rule, most suppliers quote boxes by internal dimensions, because that is what buyers need. But not all do, and some list external. When you compare boxes, check which measurement is quoted before you compare prices, or you are comparing different products.

The standard order: length, width, depth

UK and international convention lists box dimensions in a fixed order:

1. Length: the longest side of the opening.

2. Width: the shorter side of the opening.

3. Depth (or height): the distance from the opening down to the base.

So a box written as 305 x 220 x 100mm has a 305mm long opening, 220mm wide, and is 100mm deep. Length and width describe the mouth of the box, depth is how far down it goes.

Getting the order right matters when you order. A 300 x 200 x 100 box and a 300 x 100 x 200 box use the same amount of board but behave very differently. One is a shallow tray, the other is a tall carton.

How to measure an existing box

To measure a box you already have and want to reorder:

1. Open it flat or stand it up square.

2. Measure the internal length across the longest side of the opening, ruler flat against the inside wall.

3. Measure the internal width across the shorter side.

4. Measure the internal depth from the inside of the base to the top edge.

5. Record in millimetres, in length x width x depth order.

Measure the inside, not the outside, and not the flattened blank. Corrugated board can vary a millimetre or two between batches, so round sensibly rather than obsessing over a single mm.

How to measure a product for a box

To find a box for a product:

1. Measure the product at its longest, widest and tallest points, including any protrusions.

2. Add clearance. For a snug fit with a little void fill, add roughly 10 to 20mm to each dimension. For fragile items that need cushioning on all sides, add 40 to 60mm to allow for padding.

3. That gives your target internal box size.

4. Round up to the nearest available stock size rather than paying for a bespoke box.

Too tight and the product will not go in, or the box bulges and fails the carrier gauge. Too loose and the product rattles, needs more void fill, and you pay for air.

A worked example

You sell a boxed candle measuring 90 x 90 x 110mm. It is glass, so it needs padding.

• Add 40mm each side for cushioning: target internal roughly 130 x 130 x 150mm.
• The nearest common stock size might be 150 x 150 x 150mm, which gives room for a corrugated insert or paper void fill.
• External will be a few mm larger, so check it still sits in your intended postage band.

Comparing sizes and prices

Once you have your internal dimensions in length x width x depth, you can search that size across UK suppliers on Packaging Marketplace and compare live prices, making sure you compare like for like on internal measurements rather than accidentally pricing a smaller external box against a larger internal one.