The size of a backpack is often expressed in terms of capacity. The capacity of common backpacks is 30L, 40L, 50L and 60L. About 40L is enough for daily use. If it is used for long-distance travel, it is best to choose backpacks with more than 60L.
Airlines have size requirements for carry on backpacks, which must meet the boarding size before they can take them on the plane. However, now the implementation standards of various airlines are different, but if the backpack exceeds the foot standard, it needs to be checked in.
Generally speaking, the length, width and height of the backpack shall not exceed 55cm (21 inches), 40cm (15 inches) and 20cm (7 inches) respectively, and the capacity shall not exceed 44l, which is basically within the allowable range of boarding.
