40L is perfectly within the size limits for carry on as long as it has the correct dimensions. Many bags, even 40L bags, don't however.
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Size and Weight RestrictionsTypically, 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. Any bag within this dimension can fit most aeroplanes' overhead compartments.
Your backpack will generally be allowed as a carry-on if it's under 50 liters, or about 22 x 14 x 9 inches. Get a 45-liter backpack if you want to carry as much stuff as possible on the plane with you.
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.
TSA Guidelines for Checkpoint Friendly BagsIn a nutshell, the current requirements are as follows: Your laptop bag has a designated laptop-only section that you can lay flat on the X-ray belt. There are no metal snaps, zippers, or buckles inside, underneath, or on top of the laptop-only section.
It is the perfect size for travel, just don't pack it full. A 40L backpack half empty weighs essentially the same as a 20L backpack stuffed full. I did 4 months abroad with a GORUCK GR2 34L a few years ago.
Generally, you're allowed one carry on and one personal item, such as a purse, suit bag, briefcase. As long as your backpacks conform to the airline size specifications for hand luggage, it's not a problem that both your carry-ons are backpacks.
Airlines calculate carry-on luggage size measurements using two methods: Height by width by depth: Most – but not all – airlines measure bags by inches or centimeters using this formula: height by width by depth, such as 22 x 14 x 9 inches.
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.
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.
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.
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.
Which of your packs are carry-on compatible? In our Travel series, the Meridian 22, Sojourn 22, Ozone 18, Ozone 22, Contrail 22, Shuttle 22, and Porter 46 all meet maximum legal carry-on requirements.
It is rare for airlines to weigh your carry on or hand luggage. So long as your bag is not clearly overweight, and you're able to carry it with ease – you shouldn't have any issues with a slightly overweight bag.