The 40L will max out carry on dimensions on most airlines so you never have to check a bag. For stricter airlines, like RyanAir, 30L will be a better choice.
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Most airlines allow backpacks as carry-ons, but size and weight restrictions vary depending on the carrier. Typically, the bag must be within the cabin baggage dimensions of around 22 x 14 x 9 inches (56 x 36 x 22 cm), commonly considered the standard size for carry-on.
Some airlines also publish weight limits. Volume: A 45L backpack is the maximum-sized carry on allowed on most US airlines. For international or budget airlines, choose a bag under 40L. If you want to pack ultralight, anything 35L or under can be carried on virtually any airline.
Will a 40L backpack fit under an airplane seat? Likely such a bag would be 21 -22 inches long, and so be 3–4 inches too long. A maximum under seat bag would be about 18 x 14 x 8 inches, 2016 cubic inches. With 61 cubic inches per liter, maximum under seat size would be about 33 liters.
A 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. Other things you'll need to carry include a small stove, food and basic cookware, plus a water filter/purifier, first aid kit, and navigation tools.
Note that an Allpa 28L will fit in an overhead bin even if it's stuffed to the gills, but the 35L and 42L are where things get a bit murkier. While the 35L will fit in most (but not all) overhead bins, the 42L generally cannot be fully packed out and still fit in an overhead bin.
A 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. Other things you'll need to carry include a small stove, food and basic cookware, plus a water filter/purifier, first aid kit, and navigation tools.
The 40L dimensions are 21 x 14 x 10.5 (53 x 35 x 27 cm). The depth is a little over max on many airlines which tends to be about 9”. But this bag has no frame, so the actual packed dimensions is largely determined by what you put in the bag. And if you don't pack it too full it will work fine as a carry on.
Luckily, Tortuga makes backpacks that work as underseat carry ons. 30L or 40L backpacks are great options for primary carry ons but are usually too big to be considered personal item that need to be small enough to slide easily under the seat in front of you.
This is why most new backpackers think they need a larger backpack to travel, but with some clever organising and good packing cubes, a 40L backpack could just be the perfect size. You'll be incredibly surprised at just how small of a space you can fit everything you need for 3, 6 or even 12 months.
Backpacks, totes, and crossbody bags are typically considered personal items too. The key to remember is it must be able to fit underneath the seat in front of you. A carry-on is a type of hand luggage that you bring into the plane cabin with you and is larger than a personal item.
While personal item size restrictions vary across airlines, most airlines consider a small backpack, purse, briefcase, or laptop bag to be a personal item. These must always fit under the seat in front of you. Many airlines restrict passengers to one personal item and one carry on bag.
Dimensions for carry-ons vary by airline, but a good rule of thumb is that any backpack less than 45 liters should fit most airlines (but be careful with ultra-budget airlines — especially in Europe). Any bigger than that, and you risk extra charges.