Well, it turns out a very common backpack measurement, liters, is one that varies based on who’s calculating. You can learn more about how backpack liters are calculated in the video above or read on. Exactly how this is done isn’t standardized – although a lot of people use the word standard when talking about backpack liters – there’s no governing body or backpack liters organization issuing a set of specific guidelines that everyone follows. Understanding LitersThe best way to think about backpack liters is in generalities. You know now that liters can vary – a 25 liter backpack by one brand doesn’t’ necessarily have the same capacity as a 25 liter bag from another brand.