How Many Litres Do You Need

What each size holds

The 40-litre row looks wrong at first glance — more litres, fewer days than the 30-litre row. It is not a mistake. Bulky clothing eats volume faster than days do, so the same bag covers a long warm-weather trip or a short cold-weather one.

The arithmetic

A folded t-shirt is roughly 1 litre. A pair of jeans is 3. A fleece is 5 before compression and about 2 after. A pair of shoes is 6 to 8 and cannot be compressed at all. Toiletries in a cabin-legal bag are 2. A 15.6-inch laptop with its charger takes 3 to 4 litres of the bag even though it sits in a flat layer.

Add those up for a real trip and the answer arrives quickly. Four days in summer: two pairs of trousers, five shirts, underwear, toiletries, laptop — around 22 litres, comfortably inside a 30-litre bag. The same four days in winter, with a coat and boots, lands near 38.

Why bags get over-bought

Capacity is the specification everyone compares because it is the one expressed as a number, and buying up feels like buying safety. What actually happens is that the space gets used. A 40-litre bag on a two-night trip comes home full, because packing expands to fill what is available and there is no penalty at the moment of packing — only at the moment of carrying.

Buying for the trip taken monthly rather than the trip taken once is the single change that improves this, and it usually points one size smaller than instinct.

Where compression changes the answer

Compression does not add capacity. It recovers volume that lofty items were wasting, which in practice moves a 40-litre bag from three days to five in a cold climate, and does close to nothing in a warm one where nothing being packed has loft. What it saves by item type is set out in the compression guide.

What the number does not tell you

Two bags of the same stated capacity can hold very different amounts, for three reasons that never appear on a listing.

Where the litres are. Stated capacity is usually total volume, including every external pocket. A bag with six pockets and a 30-litre rating has a smaller main compartment than a bag with two pockets and the same rating. For packing clothes, main compartment volume is what matters and it is never published.

The shape. Volume is not usable volume. A tall narrow bag and a squat wide one can both measure 30 litres, but folded clothing packs efficiently into a rectangular space and badly into a tapered one. Bags that open flat tend to use their volume better because the contents can be laid rather than dropped.

What the structure eats. A frame, a trolley sleeve, a padded laptop layer and a compression compartment all occupy space inside the stated envelope. A 40-litre travel bag with all four holds noticeably less clothing than a 40-litre hiking sack with none of them — and is far more pleasant to carry through an airport.

A practical way to choose

1. Pack for your most typical trip using the bag you own now.
2. Put everything into a bin bag and measure what it displaces, or simply note whether the current bag was full, half full or straining.
3. Add about 20 percent for the things that get bought or picked up on the way.
4. Choose the size that covers that, not the size that covers your longest trip of the decade.

This lands most people one size below where instinct puts them, and the bag is lighter every single day as a result.

Capacity FAQ