Bag Size

Size is the one detail that decides whether a bag works at all. Bags are banded on width, the measurement straight across the widest point of the base. That single number is what a listing means by small or large. Height, depth and strap drop are quoted separately, and none of them appear in the band. Depth is what actually moves volume. Adding 2 inches of depth to a 12 inch bag gains more usable space than adding 2 inches of width. That is because you multiply the change across the whole footprint.

Judge a size against the largest thing you carry every day, then add an inch. A 13 inch laptop measures roughly 12 by 8.5 inches, so it needs 13 inches of internal width to slide in without forcing. A letter-size folder is 8.5 by 11 inches and stands upright only if the bag is 11.5 inches tall. Interior dimensions always come in smaller than the exterior ones quoted, usually by half an inch to a full inch. Lining, padding and a stiffened gusset take up the difference.

Read the five bands as a ladder of contents rather than a ladder of liters. Each step up lets in one new class of object and adds empty weight you carry all day. Stop at the first band that takes your real daily load.