Once you have decided whether or not you want an internal frame pack or an external frame pack, you will need to choose the size of your pack. Here is a brief overview of the most common sizes of packs and how much gear they hold so that you can make an informed decision about what size pack will be the best one for you:
Half day packs
Half day packs are fairly small and can only really carry the essentials like water, a lunch or a snack, dry socks, rain gear, an extra jacket or sweater and a first aid kit. If you’re going for a short hike with the kids or if you’re not going very far from your campsite, then this size pack will be ideal.
Full day packs
Full day packs are more substantial and are the perfect choice for a full day of hiking or if you have to carry gear for yourself and your family. These packs come with waist straps and other support plus external loops and straps to hold a GPS navigator, water bottles, lunches, and other necessary items that you want to be able to reach quickly. Inside there is plenty of space to keep all the extras that you need to take with you for everyone in the family.
In general if you’re not going on a full hiking trip where you are carrying tents and all your equipment with you, then you won’t need a pack bigger than a full day pack. But if you are ambitious and are planning a full back country hiking vacation where you’ll be carrying all of your gear with you, then you will definitely need a multi-day pack.