Glossary
Litres (bag volume)
The measurement of how much a bag can hold, with most one-bag travelers using a bag in the 30 to 45 litre range.
Litres, or liters, measure the internal volume of a bag, essentially how much stuff it can physically hold when full. It is the most common way travel backpacks are compared to each other, though the number is measured differently by different brands, so two bags both listed at 35 litres will not always hold the same amount in practice.
For one-bag travel, 30 to 40 litres is a common range for warm weather or short trips, while closer to 45 litres suits colder climates, longer trips, or anyone bringing more tech and gear. Above roughly 45 litres, a bag starts to risk exceeding many airlines' carry-on size limits, even if it is still technically called a carry-on by the manufacturer.
When comparing volume, check whether the number includes external pockets and expansion zippers, since some brands count those and others do not. A useful gut check is to think in terms of what you can actually carry comfortably for a full day, not just what fits inside the bag. A common mistake is choosing volume based on worst case packing for the longest trip you might ever take, rather than the trip you are actually planning.