Unless you're an experienced backpacker with positive experience using a particular brand, it's not a good idea to buy your pack via mail order. You want to wear the backpack, fully packed, to determine if it's right for you.

2torial #0825: Learn2 Choose the Right Backpack (Continued) Choose the pack capacity How much you need to carry and how much you're willing and able to carry will determine the pack size you need. Packs are measured in cubic inches, but even this precise-looking measurement is interpreted differently among manufacturers. Some companies measure the volume of their packs by filling them to the brim with a substance, emptying the pack, and measuring the volume of the stuffing. Depending on the materials chosen, e.g. marbles or styrofoam peanuts, the capacity of the same pack will be measured differently. What does this mean? The c.i. numbers are good guidelines, but realize that 3,000 c.i pack of one company may be smaller than a 3,000 c.i. pack of another company. For example, an average bookbag or daypack is usually 1200-1500 cubic inches (c.i).   If you're carrying a sleeping bag, you'll probably require a minimum of 3000 c.i. This size (and up to 5000 c.i.) is good for two to five-day hikes.   Backpacks for multi-week expeditions can get as large as 7000+ c.i. Step 4