The capacity of a backpack is measured in liters. Most manufacturers disclose the capacity of their respective backpacks. You will usually see a number in the name of a product (for example Osprey Talon 44, Kelty Revol 65 etc.) which defines the capacity of the backpack in liters. In case that this doesn’t apply to the product you are interested in, check the manufacturer’s website to find the technical specifications of that particular backpack.