The capacity of the backpack you will buy is totally determined by its use. It’s important to be cautious of the space capacity of the backpack so that you don’t put too much that will end up straining the bag and you also. Also, excessive space is not something you want to have when running. When checking the capacity make sure you look at all the pockets and the compartments of the backpack. Make sure the compartments suite your need. Of late there have been backpacks that the space capacity can be enlarged by extra zippers. You can consider these options also.