When selecting a new backpack you have to consider a number of factors that will affect its fit and performance. Much like your hiking boots having an ill fitting pack, or the wrong pack for the job, will result in a sore back, sore feet, and in extreme cases possible injury. Unlike a boot that doesn't fit well, the wrong pack will beat you death in a matter of minutes on the trail. I've seen strong healthy people brought to their knees in less than two miles, simply because they didn't do some homework before purchasing or selecting their pack.
The first thing to figure out is just what you will be using your pack for. Backpacks fall into three basic categories. The first of these are daypacks. These are used for short trips, where carrying a lot of gear or a lot of weight isn't a big issue. The next category is assault packs. An assault pack is used when climbing a mountain as a day trip; so you'll need extra capacity for all that gear. It can also be used on a weekend trip, or if you practice lightweight backpacking. The final category is called a full pack. These packs have lots of room to carry a lot of gear. These are used on longer treks, for distance hiking, or if you're carrying gear for a couple of people, say a child or an inexperienced friend on their first trip.
The next thing that separates packs is the type of frame, or if even has a frame. A burlap bag with two strings on it could be forced into being a backpack, but it isn't going to be very comfortable. The strings will dig into your shoulder and the weight of the sack will beat against you as you take each step. The frame allows a suspension to be placed on a pack, which allows the weight to be distributed, and supports your gear. Frames can be external or internal. An external frame simply means the frame is on the outside where you can see it. An internal frame means that the frame is integrated into the shell of the pack. Daypacks and some assault packs may not have frames at all.
Internal frame packs have become extremely popular. They allow the weight to be carried closer to your body, which helps with your balance, and offer more adjustments. External frame packs are still considered the best choice if you're going to carry a heavy load. The larger more complex frame bears the weight better and you have limitless attachment points for putting more gear on the outside of your pack.
If you're looking for a daypack or a frameless assault pack then the process is pretty simple for sizing up your pack. You should choose a pack with wide comfortable shoulder straps. Some straps are referred to as being, "cobra cut," that is wide at the shoulders and then tapering down. These allow the weight to be better distributed. The pack should have a waist belt that goes around your waist. This may seem simple but if you have a large torso (more on this latter) it may be difficult to find a pack that actually does that. The waist belt keeps the pack on your back, and helps prevent it from beating on your back with each step. Some daypacks have sternum straps. This is a strap located around your mid-chest. This helps keep the shoulder straps on your shoulders. Men find them very helpful, but women may find the tension across their chest uncomfortable.
When sizing up an assault pack or a full pack that has a frame, the first thing you need to do is measure your torso. This is very critical to get the best fit possible out of your pack because your overall height has very little to do with the frame size of your pack. You will need a friend to assist you and a cloth tape measure. First put a finger on the back of your neck and tilt your head back. You're looking for a bony point on your spine at the base of your neck and it is easier to find if you tilt your head back. Next, stand up straight and grab on to your hips, right at the hipbones, not at your waist or around your stomach. You should be able to feel the bones. Put your hands on your hips so that your thumbs are pointing towards each other behind your back. Now have your friend measure the distance from the bony point on your spine to the invisible line your thumbs are making. That is your torso length. Most people fall in a range between 14 and 21 inches.
The next thing you need to figure out is if you are small, medium or large framed. This is pretty easy to do. Take your thumb and forefinger and place them around your wrist. If your fingers touch easily, then you are small framed. If your fingers don't touch, but just barely, then you are medium framed. If your fingers don't touch at all and couldn't even if you squeezed as hard as you could, your large framed.
Read on to find out how to select the best pack for your frame and torso size NOW!