Capacity is measured to the walls and you load it to the shelves

This looks at gross against usable capacity from the practical end — what holds up once conditions stop being ideal.

Two different ways to measure the same box

Gross capacity measures the whole internal cavity as if it were empty, which is a straightforward geometric calculation the maker can perform on a drawing. Usable or net capacity subtracts the fittings that occupy space, and defining which fittings count is where the two figures separate. Because gross is larger and easier to compute, it is the figure most likely to appear in a headline or on a shelf label.

The difference is not a small correction in most categories, since shelving, drawers, insulation thickness and mechanical assemblies all consume real volume. Comparing a gross figure for one product against a net figure for another produces a conclusion with no relationship to what you can fit inside.

Where the missing volume goes

Insulation is the largest hidden consumer in anything thermally controlled, and better insulation means thicker walls and therefore a smaller cavity for a given exterior. That produces the counter-intuitive result that a more efficient appliance can hold less than a less efficient one of identical external dimensions.

Mechanical assemblies such as compressors, fans, ducts and motors occupy corners that are then unavailable regardless of how the space is counted. Shelves, dividers, rails and drawer walls each take thickness, and their positions fix where the usable spaces begin and end. Door and lid mechanisms need swept clearance inside some designs, which removes volume that appears on a drawing as available.

What the test standards actually define

Most regulated categories have a published method describing how capacity must be measured, including which fittings are removed before measuring. Those methods differ between regions, so the same appliance can carry different capacity figures in different markets without anyone being dishonest.

Compared like for like, where a method exists and is cited, the figure is comparable between products tested to it, which is the only comparability you get. Where no method is cited, the number is whatever the maker decided to measure, and identical products can differ on paper alone. Reading the specification sheet for the phrase describing the measurement basis is more useful than reading the number itself.

Shape beats volume for what you can actually fit

Packing efficiency falls sharply when the objects being stored are rigid and the space is subdivided into awkward proportions. A tall narrow cavity and a short wide one of identical volume hold very different collections of real objects. Fixed shelf spacing sets a maximum item height, and an item slightly too tall renders an entire shelf useless rather than mostly useful.

Adjustable or removable fittings recover a large share of the theoretical volume, which is why adjustability is worth more than a few extra litres.

Measuring the largest thing you regularly need to store, and checking that dimension against the internal clearance, answers the question the capacity figure cannot.

Load ratings that are not volumes

Some categories rate capacity by mass rather than by volume, and the mass figure is defined by a standard test load rather than by your possessions. That test load has a defined density, so bulky low-density contents reach the volume limit long before they reach the stated mass limit. Manufacturers size the mechanism for the rated load, which means running at the rating routinely is a different duty from running below it.

On the bench, rated and recommended loads can also differ, with the recommended figure appearing in the manual and the rated one in the advertising. Where both a volume and a mass figure exist, whichever one you reach first is the one that governs your use.

Where a claim here has not been tested directly it is inference, and it should be read as inference.

Working out what you actually need

Start from the objects rather than the number, listing the largest and most awkward items that must fit and their real dimensions. Measure the internal clearances of a candidate rather than trusting the capacity figure, since manuals frequently publish internal dimensions.

Allow for the fittings you will actually use, because a cavity measured with everything removed is not the cavity you will live with. Consider access as well as volume, since space you cannot reach into is space you will not use. Where a maker publishes both gross and usable figures, the ratio between them is itself informative about how the interior is arranged.

The takeaway

Measure the largest thing you need to store and check it against internal clearances, not against a litre figure.

The question is rarely which is best. It is which is enough.