Chances are, you're going to need something a little bit bigger than what is in the picture unless it’s for a day trip. When it comes to hitting the road, the key is to find a bag that isn’t too small or too big.
In fact, most packs that fall squarely somewhere in between the 30L and 40L range should suit most travel needs, no matter the length of the trip. These bags are often big enough to contain everything you need, while being compact enough to fit in overhead compartments. They also give a greater degree of mobility. Most seasoned backpackers agree this is the magical volume limit; some people I know have managed year-long, round-the-world trips on them.
If you’re not used to backpack terminology, the very first thing you’ll notice is that the L stands for liters; it denotes how much volume, and not weight, it can hold. Most readers should be aware that hiking backpacks get very often used as travel backpacks, which means it's not being used it for its original purpose. There’s nothing wrong with that, but you should realize this means your needs will be slightly different unless you plan to go up Mount Kilimanjaro.
Some guides, like REI’s official guide, suggest an 80L bag for extended trips but I can tell you right away to ignore that because that’s for people who don’t know how to pack. Most backpackers love to pack every single item they’ve ever owned in their life for their big round-the-world trip, but it’s also their first. You’re going to sacrifice a high degree of mobility, lose things and are going to have to check that monstrosity on every flight. The weight on your shoulders will add up.
People are natural overpackers; very few people underpack, and that’s often because they’re packing at the last minute and forgot something. That means if you’ve got a 40L bag, you’ll probably stuff it to the brim. That’s about the same with a 50L and 60L bag too. There will be a tendency to shovel what you don’t need in, and the truth is, you’d be surprised at what little you could get away with.
I forego some conveniences in exchange for others—I can tell you straight away my hair dryer always makes it on, but my heels may not—but it is a very delicate balance of picking your priorities. If I can’t fit everything into my bag, I know that more likely than not, that means I’ve put too much of something in there.
Unless you’re carrying serious camping gear or going into below-freezing temperatures, there’s really no need to spring for a bigger backpack. I’m a fairly tiny 5’3 girl, so I can squeeze by with a 30L about 95 percent of the time just fine. I once managed to impressively fit a hairdryer and heels in my bag for a month-long excursion to Europe during the dead of winter. Big dudes can probably draw the line at 40 or 45L.
From the beginning, I have always used a 30L bag. At first, I really didn’t know what I was doing. My friend Andy at the time just threw me a knockoff Quechua bag and told me to go travel Southeast Asia alone for three weeks. I went with it, mostly because I didn’t know any better. When I returned from that trip, I went straight to REI and bought a Traverse 30L, which they sadly don’t make any more in its former incarnation.
Things are a little different now. I’m pretty much a seasoned traveler at this point, with 20 countries and 300,000 miles on my last count and that Traverse 30L has been with me on every single trip since then. Even then, I must give respect where it’s due: Andy is the original Yoda before this Yoda. He’s seen 32 countries and every continent except Antarctica. And he also only keeps two bags, a 30L and 40L backpack.
If you can’t take my advice, at least listen to Melissa Arnot, who has climbed Mount Everest a total of five times—just one short of Lakpa Sherpa's Guinness World Record—and who agrees the 40L mark is a good delineator:
For multiple night forays, overnight mountaineering trips or general backpacking, I go with a 40+-liter pack. I am a size XS or S, so my clothes are a little less bulky then those of an XL male, so my stuff fits neatly into a 40-50L pack for almost any trip I do.
In fact, the only time that she suggests that you should even consider bringing a 70-80L bag is when you’re carrying “all of [your] food, fuel and gear” for a full-on mountaineering trip. Do you plan on climbing Everest too? Cause that’s pretty damn hardcore.