OpticsPlanet.com recently published an article about How to Choose a Backpack.  The very in depth article goes into detail on every aspect of properly selecting a backpack to hike with.   If you are looking into purchasing a backpack, it’s definitely worth looking into.

Measuring Your Body and Sizing

Length of Trip Size Liters For Use Back Packs
Single Day/Overnight 20-30 Hike a small mountain with friends or climb a volcano like Mt. Hood in Oregon (7.2 miles, but takes all day), perhaps a morning and evening hunt in the George Washington National Forest or climbing Pike’s Peak. small backpack for day hikes
1-3 Nights 35-50 Weekend camping trip with lots of hiking and trail walking. Stop to enjoy the waterfalls, steep canyons, caves, and wildlife. A good Bugout Bag is just what you need to take your kids on hike to your secret fishing spot – accessible only by foot. medium sized backpack
3-5 Nights 50-70 A 4 day hike through the Grand Canyon from rim to rim, many people take multiple days to do it – on purpose. Also, mountaineering, trekking, multi-day ski touring, and alpine climbing. large backpack for hiking
5 Nights or More 70+ Whether you’re Cameron Hanes on another trip to Colorado to fill an Elk tag or you’re looking to up your game by dropping into Alaska via parachute and kill a Brown Bear with your buddies – you’re going to need a bigger pack. extra large black backpack

Backpack Sizing: How To Determine Your Torso Length

In the normal course of a day or week, most people do not give much thought to the size of their torso. The torso is not usually measured in any capacity for choosing clothes, winter coats, a new suit or that custom fit touring bicycle.

However, your torso size is extremely important when it comes to choosing the appropriate backpack. After all, there are at least 3 points of security for most backpacks today. Sternum support and strap, Load lifter straps which go over the shoulders and the sternum straps that go across the chest.

All three of these must work in concert to keep your gear riding comfortably on your back. We encourage women to try backpacks specifically for women. While it is true that many unisex backpacks will fit women just fine, 75%-80% of the weight of your gear should be supported by your hips (for men and women). Every woman has a different hip shape, so it is especially important that a woman’s hips match the backpack.

When measuring your Torso, you want to place a flexible tape measure at the base of the bottom of your neck where the top of your shoulders meet the neck. Run the tape measure down to the small of your back between both hips. This measurement is your Torso Length.

The Torso length does not have to be exact like a shoe-size, as you can see from the size chart below.

Extra Small Small Medium/Regular Large/Tall
Up to 15 1/2 in 16 – 17 1/2 in 18 – 19 1/2 in > 20 inches
* Many Backpacks are adjustable, so exact size is not necessary

You can read a lot more about backpacks as well as shop for one on their page.