Wall thickness is the depth of expanded polystyrene (EPS) foam between the inside of the box and the outside, measured in millimetres. It is the main reason two boxes that look the same size from the outside can behave very differently once they are packed and left standing.

Why thickness changes how long things stay cold

Heat moves through the foam at a fairly steady rate for a given material, so the only way to slow it down is to put more foam in its path. A thicker wall gives heat further to travel before it reaches whatever is inside, whether that is fish on ice, a tray of pastries, or a batch of vaccines. A thinner wall lets that heat through faster, which shortens the time your ice pack or gel pack has to do its job.

This is also why wall thickness has a knock-on effect on how much a box actually holds. Two boxes with the same external dimensions but different wall thicknesses will not take the same volume inside, which is worth checking before comparing products on outer measurements alone (see internal vs external dimensions).

What else affects the hold time

Wall thickness sets the ceiling on how well a box insulates, but it is not the only thing at play. How much ice or how many gel packs you pack, whether the box has drain holes to clear meltwater, how full it is, and the temperature it sits in on the day all change the result in practice (see drain holes, ice packs and dry ice). Two boxes with identical wall thickness can still perform differently depending on how they are loaded and where they are left waiting for collection.

None of this is really about how sturdy a box feels in your hand. It comes down to how much foam sits between the contents and the outside air, which is why the millimetre figure is worth checking before you choose a box for anything time-sensitive.

A supplier's product page will usually give you three figures: the internal size, the external size and the wall thickness in millimetres. If all three are there, you can work out the wall thickness yourself by comparing internal and external dimensions on each side, which is worth doing as a check even when a figure is stated separately.

What to look for on the page

• Internal dimensions - the space you actually pack into, always given before the external size
• External dimensions - the outside footprint of the box, useful for working out whether it fits inside a courier's outer carton
• Wall thickness in millimetres - stated as a single figure for the sides, though some suppliers give the base and lid separately, since a lid is sometimes moulded thinner than the walls
• Whether the figure is confirmed on the supplier's own page

If the wall thickness is not stated

Some supplier listings leave this field blank, and where that is the case it is shown as not stated. A missing figure is not the same as a thin wall. It usually just means the supplier has not published it, and the way to settle the question is to ask them directly before you order, quoting the product code or the dimensions of the box you are looking at.

If you already know roughly what internal size and hold time you need, the product directory lets you compare boxes by these fields side by side. The difference between internal and external size is covered in more detail in the guide on internal vs external dimensions, if you want to check how the two figures relate before you order.