Volume is generally measured by liters or cubic inches. Size is nothing but the dimensions of the bag. As volume provides a good indication of how things are rolled into the backpack, it is generally more preferable. Hence, it is measured in liters.

Bag sizes, especially for backpacks and luggage, are measured in liters because it is the most accurate way to represent internal volume (capacity) rather than just external dimensions. Since bags come in irregular shapes—curved tops, expanding pockets, and tapered bottoms—standard length x width x height measurements often fail to convey how much "stuff" actually fits inside. To determine literage, manufacturers traditionally used a method involving filling the main compartment and pockets with small plastic pellets or 20mm balls, then measuring the total volume of those pellets in a graduated container. This accounts for the "usable space" that a simple tape measure would miss. For example, a 40-liter bag is roughly the size of a standard carry-on, while a 20-liter bag is a typical daypack. Using liters also helps travelers compare across brands globally, as the metric system is the universal standard for volume. In 2026, as airlines become stricter with dimensions, liters provide a secondary "heft" check; a 70-liter bag is almost certain to be a checked item, regardless of its height or width.