60-70 Liters (The Conventional Backpacker)These are the size packs that you are most likely to see when on a multi-day backpacking trip. Inside these you should be able to fit most conventional gear, plus some more creature comforts that will make your trip that much more enjoyable.
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Understanding Backpack SizesAs a rule of thumb, 25-30 liter backpacks are best for shorter weekend trips, while 30-45 liter backpacks are best for longer trips or long-term travel.
A 20-30 liter backpack is great for an overnight trip. The Small Knack Pack is only 15 liters when unexpanded - perfect for carrying your essentials with you during the day - but the expandable travel compartment allows you to easily pack a day's worth of clothing.
40-50 Liters (The Ultralight Thru-Hiker)Expect to fit everything you could fit in a smaller pack, plus: A food bag that can hold up to 5-6 days worth of food. A pot for cooking. A camp stove and fuel.
Their capacity lets you carry an extra layer, food and gear for day trips. 21–35 liters: This is the sweet spot for most hiking and travel daypacks. There's enough capacity to hold food, clothing and some extras, like a camera and a book.
If you buy a 55 liter to 65 liter back from a backpack manufacturer than only counts their closed storage in their volume spec, there's a pretty good chance you can carry all of your backpacking essentials inside your backpack, including a bulkier synthetic insulated sleeping bag and a two-person tent.
2-3 Day Packs: 35-50 litersA weekend backpack like a standard 40L backpack is big enough to carry a backpacking sleeping bag, small tent, and pad, as well as extra clothing and layers.
The one-size-fits-all approach – a 40L backpack. Yet all being said, there is a loose one-size-fits-all approach for most travellers. The truth is that if you're looking for a versatile backpack for travelling Europe, South East Asia or even South America, a 40L backpack will (probably) be the perfect size.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night.
I recommend a backpack around 40L-50L. Personally, I wouldn't go over 65L, but some people like a bigger bag. You can always go smaller, but I wouldn't recommend anything smaller than 35L — unless you're into minimalist travel.
70L is definitely too big. I believe the 55L is a 40L + a 15L daypack. This will probably work, but double check the length, width, and height, and make sure none of those dimensions are larger than allowed for a carry on.
A 30L – 50L backpack will be just the ticket. It will be able to hold all your sleeping, cooking, and camping gear you need for trips up to 5 or 6 days.
Size Matters (but Bigger Isn't Better)Like a few other things in life, size matters when it comes to choosing a backpack — but not in the way you might think. Based on many years of dragging excessively large backpacks around the world, I have to say bigger is not better. You do not need an 80 or 90 litre pack.
But bags larger than 65L are way too big for travel, even on extended trips. If you'll be traveling for more than a week or two, you'll need to do laundry anyway. You can't—or at least shouldn't—try to carry a month's worth of dirty clothes. Between these two extremes is the sweet spot of backpack sizes.
They're suitable for short breaks or extended trips. Most backpackers can travel for months, if not years, with a backpack in the 40-46-litre range! It's easy to fit in a week's worth of clothes, as well as all your travel essentials.