Paula Champagne All backpacks are basically a combination of a big fabric bag and a harness that attaches that bag to your body, but knowing what size you need makes all the difference.

This article was originally published in the 2017 edition of AMC’s Mountain Skills Manual.

Whether you’re heading out for one night, a week, or an entire thru-hike, choosing the right backpack is essential to a good experience in the outdoors. All backpacks are basically a combination of a big fabric bag and a harness that attaches that bag to your body. The harness, usually consisting of shoulder straps and a hip belt, is called the suspension. Pack volume comes from the dimensions of the bag (and any pockets), and pack size comes from the dimensions of the suspension.

Pack Volume

Selecting a pack involves choosing the optimal volume for your purposes, the right size for your body, and the optional features that meet your needs. Backpack volume is measured in liters or cubic inches, and you will see either or both, depending on the manufacturer. Pack volume categories are roughly based on the number of nights a backcountry traveler expects to be out on the trail and usually correlate with intended pack weight, as well. This system makes sense, as there will be a 1.5- to 2.25-pound increase in food weight for each person, each day on the trail.

The smallest packs are meant for a single night out. Packs in this category will be between 40 and 50 liters (2,500 to 3,000 cubic inches) and are designed to carry loads of less than 25 pounds. Even for one night out, these packs will require efficient packing and a minimal amount of compact equipment. Some experienced hikers consider them fair-weather or summer packs since there isn’t much room for extra layers or a tent. In cold, wet conditions, it will be challenging to fit all the required gear into a pack of this volume.

The next tier of packs is between 50 and 75 liters (3,000 to 4,500 cubic inches), designed to carry loads up to about 40 pounds. These versatile “weekender” packs are very functional for two to four days on the trail, or even a couple more if you are an extremely efficient packer. For trips of a week or longer, look for a pack between 75 and 90 liters (4,500 to 5,500 cubic inches),  designed to hold big loads up to 50 or 60 pounds. The biggest packs are 100 liters or more (more than 6,000 cubic inches). These monster load-haulers are designed for multiweek expeditions with bulky gear or for parents carrying supplies for the whole family.

When choosing a pack size, remember that an oversized pack will not feel comfortable or carry its load well if it is not sufficiently filled—knowing how to efficiently pack for a trip will help alleviate discomfort.

Pack Size and Suspension

Choosing a pack with the appropriate size and suspension is one of the most important factors in a comfortable, injury-free trip to the mountains. Padding on the shoulder straps and hip belt add a little comfort, but the way these pieces fit your body is most important. Correct torso and hip sizing will optimize weight distribution across the widest range of the body.

The most important dimension is the length of your torso (not your height), followed by the circumference of your hips (not your waist). Torso length is measured between two points on your spine. The top point is the seventh cervical vertebra (C7), the bony bump right at the base of the neck. If you look downward, it pokes outward and becomes obvious. The bottom point is the center of the line that connects the two bony tops of your hips (the iliac crests). If you rest your hands on top of your hips, fingers forward and thumbs backward, the line connecting your thumbs is the bottom point.

The hips should carry 75 to 80 percent of the pack weight, and it is the hip belt that transfers this load. An accurate measurement of hip circumference is less important than torso length because it’s easier to adjust the size of a hip belt. Since some packs offer different sizes of hip belt, though, it’s good to know your own dimensions. Hip circumference is measured right at the top of the hip crests described above, all the way around the body. It is a good idea to take this measurement wearing all the different combinations of clothing you will wear on the trail, from lightweight hiking shorts to a fleece jacket and rain shell. You will then have a range with lower and upper limits, which your pack’s hip belt should be able to accommodate.

Once you know your torso and hip sizes, you can choose your pack size. Manufacturers address pack sizing in two ways. One way is to offer sizes like clothing (e.g., XS, S, M, L, XL). Packs with this approach have a fixed torso length, so it is important to choose correctly.  The other method for pack sizing is to create an adjustable suspension, with a system of buckles and webbing that allows for substantial changes in torso length. Packs with this approach usually come in two overlapping size ranges. The adjustability makes it possible to get a custom fit, but the extra material will add some weight.

Extra Features and Options

After you find a pack with the appropriate volume and get it to fit your body like a glove, the rest will seem easy. There are a few standard features that all desirable packs will have: load-adjuster straps above the shoulders, a sternum strap, stabilizers connecting the  hip  belt  to  the  main  bag,  side  compression  straps,  and  small  pockets at the base of each side. If the pack you are considering is missing any of these features, it’s not likely to work for real mountain travel.

Once you’ve got all of those, a few personal preferences remain. These variables primarily are related to gender specificity, access methods, and compartments and external pockets. You can consult and expert at the store about how each of these work before buying.

When fitting a backpack, a 45-degree angle between the pack’s load-adjuster strap and the wearer’s neck is ideal. Make sure you have no less than a 30-degree angle and no more than a 60-degree angle to avoid the risk of injury.

Pack Weight

The weight of all this gear, from clothes to kitchen goods to the pack itself, begins to add up. Historically, wilderness-leadership organizations have used percentage of body weight as a sliding scale to find maximum pack weight. With a well-fitted pack, healthy adult hikers typically should be able to carry 35 to 40 percent of their body weight, while experienced and seasoned hikers can carry 40 to 50 percent of their body weight, if required.

A reasonable and affordable kit for three-season travel will weigh roughly 20 pounds, which assumes the following:

• Sleeping bag (2.5 pounds)
• Sleeping pad (1 pound)
• Sheltering items such as a tent, hammock, or tarp and bivy (4 pounds)
• Stove, fuel, and other cooking items (5 pounds)
• Remaining clothing and safety essentials (4 pounds)
• The pack itself (3.5 pounds)

On top of this, add two quarts of water (4 pounds total) and food (let’s say 6 pounds for a three-day trip). These bring the pack’s total weight to about 30 pounds with-out any luxury items. If you plan to hike for longer, your pack weight will increase.

A good way to reduce pack weight is to efficiently pack your backpack. You can learn more here.