Equipped with all this information you are now ready to select a pack. In helping you decide on whether you should get a daypack, an assault pack, or a full pack, you need to look at the capacity of the pack you are interested in. If your planning to carry the essentials of day hiking for simple treks, then a pack that has 1,500 cubic inches or less capacity would be ideal. You might start running into problems if you get less than 750 cubic inches of capacity if part of your day hiking gear is warmer winter clothes or a rain suit.
If you're planning to do weekend trips or lightweight backpacking then you should look for a pack with 1,750 to 3,000 cubic inches of capacity. This will offer enough storage space to carry your gear for a couple of days, or if your gear consists of ice axes, ropes, and other mountaineering gear get you up the side of a mountain for an extended day. Another reason to consider a small assault pack is if you plan to do extended desert hiking. Water is very heavy, about nine pounds per gallon. If you're going on a long trek you may be carrying nine, fourteen, or even eighteen pounds of water. A frame is going to help you carry that weight.
If you plan to go on long trips of a week or more, or will be carrying gear for a couple of people, like a child or an inexperienced friend, then you will probably want the capacity of a full pack. Full packs range from 3,000 up to 6,000 cubic inches, or even more. They will have lots of room, lots of outside pockets, and lots of places to hang your gear. When looking at full packs if your plans don't include technical climbing you should seriously consider one with an external frame.
Now that you have selected what size capacity you ready to look at suspensions and frames, and they come in a wide variety of styles. The suspension is the network of shoulder and hip belts that allows you to carry the pack. Some packs have a fully adjustable suspension, like the Kelty Guide 5700 packs our editors use. The location of the shoulder straps and hip belt can be adjusted to accommodate people in the full range of 14 to 21 inches of torso size. Other packs have frames and suspensions specific to certain size ranges. They may come in small, with a typical range being 14 to 16 inches, medium with a range of 17 to 19 inches, and large with a range of 20 inches or more. Some may also have two ranges. If you're outside of the normal range with a very small or very large torso, you're not out of luck. You won't have as many choices but they are a number of packs designed for unusual torso sizes. If your torso is particularly small, there are a number of packs that have frames and suspensions designed for children, but are still up to the serious challenge of going into the outdoors.
The critical thing is to find out what torso size the pack you are considering is designed for. Get a pack that is too large for your torso, and the hip belt won't be able to bear any weight and the pack will beat on you as you walk. Get one too small and the shoulder belts will cut into you and the pack will pull at your back.
The next consideration is adjusting your suspension. This should be done in the store before you take the pack home. A good store will have pillows filled with weights to put a load on the pack, to simulate how it will feel. The most important thing is the hip belt. Notice on a daypack we called it a waist belt, on a pack with a frame we call it a hip belt. The belt should be resting on those hipbones that you were looking for when you were measuring your torso. With the belt on your hips, the weight of the pack can be transferred to that part of your body, and your ability to carry weight over a long distance will be greatly increased. Miss the hips by being too high and the weight will be on your stomach. Miss the hips by being too low and the belt will fit loosely on you so your shoulders will end up doing all the work. The hip belt should also fit snuggly around your waist, so that the pack isn't bouncing or moving.
The shoulder belts should fit comfortably on your shoulders. They should sit on the softer area that is about at the middle of your collarbone. A shoulder belt too close to your neck will be annoying and cause muscle strain. To far outward and the shoulders won't be carrying the weight, and they could slip off. If you use the sternum strap use it to aid in keeping the shoulder belts where they belong. The sternum strap shouldn't be up around your neck and it shouldn't feel like it is crushing or strangling you. The belts shouldn't be too tight; if they are they will cut into your armpits. If they are too loose the pack will bounce around on your back.
If all of this seems to confusing don't worry. Most major metropolitan areas will have stores with employees well versed in sizing a pack. It really isn't as hard as it seems. If you are buying a pack over the internet be sure they have a return and exchange policy, and have a more experience friend help you adjust the suspension when it arrives at your home. Put a load on your pack and try it out before you hit the trail. Anyone who has gone out with a poorly sized pack will tell you, it is not a fun experience. However if your pack is sized properly to your frame, and is the right pack for the job, even an average person in good shape can carry 70, 80 or even 100 pounds while out on the trail. It is just one more thing that will help you enjoy the great outdoor places.