Suitcase Packing Volume Calculator

Why the litres on the label are not the litres you can fill

Multiply the outside of a 55 × 40 × 23 cm cabin case and you get 50.6 litres. You will never pack 50.6 litres into it, and the manufacturer's own figure will not be that either. Three things stand between the two numbers.

• Wheels. On a four-wheel spinner the castors hang below the shell and are part of the height the airline measures — about 5 cm of a case you measured at 55. Two-wheel cases recess theirs and lose around 3.
• The shell itself. A polycarbonate wall is roughly 1.2–1.5 cm thick once you count the lining and the zip track, on both sides of all three dimensions. On a cabin case that is a fifth of the volume gone before anything is packed. Fabric cases lose less, which is the main reason a soft bag of the same external size holds more.
• The handle mechanism. Hard cases run the telescopic tubes through the inside of the packing space, taking roughly 6% and, worse, taking it as two ridges you cannot pack flat across. Soft cases usually hide the tubes in the back panel and lose about 4%.

This tool subtracts all three and gives you what is left. It is deliberately more conservative than the litre figure printed on the label, which is generally measured to the outer rim of the lid with the handle channel ignored — a useful number for comparing two cases from the same brand and an optimistic one for deciding what fits.

Rolling, folding and cubes

Rolling wins by about 10–15% on volume for soft items — t-shirts, underwear, jeans — because it leaves fewer air gaps, and it creases less than a bad fold. It does nothing for a jacket or a shirt you want to wear to a meeting. Compression cubes win another few per cent and, more usefully, stop the packed volume from expanding again as you live out of the case; they add 300–400 g of their own weight for the privilege. Nothing here beats simply taking less, which is why the packing list next door counts things.

Expansion zips

An expansion zip adds around 5 cm of depth, which on a mid-size case is 10–12 extra litres — a real gain. Two warnings come with it. An expanded cabin case is no longer cabin-sized on any airline, and gate staff measure depth. And an expanded case packed full is usually over 23 kg long before it is full, because the extra volume is at the end of the lever that your shoulder carries.

What the days figure does and does not know

It takes the usable volume, removes about 10 litres for shoes, toiletries and electronics, and divides the rest by a per-day figure that changes with the bulk of your clothing and the way you pack. That is genuinely useful for comparing a 40-litre case with a 70-litre one, and it is guesswork about your particular wardrobe. A person packing linen shirts and a person packing ski base layers get very different real answers from the same litres.

The rest of the set

Once you know the case, the luggage size checker tells you which airlines accept it in the cabin, the packing list generator works out what should go in it for a specific trip, and the camping and hiking gear checklist handles the trips where the answer is a rucksack.