It is necessary to first determine the size you require based on your requirement. A bigger isn’t always better since you would tend to include a lot of stuff you would not need. It is best to always check out backpack size in a physical store to get an idea, since different manufacturers have different size nomenclatures.

The second decision is around the kind of frame. An internal frame is better at distributing the weight across the body, from the shoulders to the lower back, at the cost of a snug fit. An external one will allow you some space between the bag and body, hence, it is ideal for hot weather conditions.