Backpacks 27lt on board?

As a complement to the other answers, I'll share my experience: I've been using a 40-liter backpack for many years as a carry-on when traveling and no airlines has ever asked me to put my bag in some measuring cage. However, airlines sometimes (<10%) check the weight, and I wouldn't be surprised if some very low cost and/or stingy airline such as Ryanair check the backpack dimensions, especially if the backpack looks oversized.

Airline carry-on allowance varies slightly by airline but a 21" x 14" x 8" bag satisfies the majority of carry-on size limits set by airlines. This gives a total external volume around 38L (53 x 36 x 20cm).

The volume specified for a bag is the total internal capacity of all combined compartments, so a 27L bag will take more that 27L of space. It is unlikely to exceed 38L but, even so, it must fall within the limits specified by the airline in each dimension, or in total linear dimension (width + height + depth) which some airlines use. Since you do not have an airline in mind, it is not possible to say but you would be able to select the airline according to the bag or the bag according to the airline.

Given that most bags are not single-use, it would be best to select a bag that falls within the most common carry-on limit. Even though the product name might specify volume, most bag manufacturers publish specifications that let you know the exact dimensions. Unfortunately, it is very hard to search with such criteria, so you will have check each bag you are interested in one at a time. It is tedious. When I was traveling with strict airlines and had to carry plenty of stuff, I had a bag custom-made to exact measures because I could not find one the right size.

Airlines usually don’t give limits in litres but rather as width x depth x height.

Sadly, the limit is extremely variable, and can even vary for a single airline depending on the class of travel or purchased options.

For instance Easyjet allows everyone to take a small 45 x 36 x 20 cm bag, while only those buying the more expensive seats/tickets or holders of the Easyjet Plus card can take a 56 x 45 x 25 cm bag in the cabin.

The smaller size is about 32 litres, but what matters is that it fits in the given dimensions (they have metal sizers to check), so for soft bags it may depend on what you put into it and how you arrange things.

1. Each airline has their own set of rules and their own style of enforcement. The rules vary all over the place and so does the enforcement policy. You can only answer this for one specific airline at a time
2. Rules are expressed as maximum dimensions (width, length, height) and/or weight
3. Enforcement varies a lot: most airlines have a sizing cage at the gate and/or check in. The gate agents will decide whether to size your bag or not. In my experience, this happens mostly if the bag is obviously too large but is rare otherwise.
4. Example: US main line carriers are very relaxed about this (unless you have a basic economy ticket)
5. Example: Lufthansa Group (LH, Swiss, Austrian) was for a while very stringent about weight ad weighed every non-trivial bag. But they have eased up recently
6. Most budget carriers make a lot of revenue through bag fees, so they tend to be more thorough with checking weight and dimensions

Litres is a terrible, terrible way to measure a backpack. There's no agreement whatsoever what it even means. There was one backpack maker -- was it the Tortuga blog? Some Carryology intervew? Tom Bihn? Can't remember for my life -- who discussed this and said they fill their packs with little foam balls and that's how they get this number. But it's meaningless because organization vs your items will decide how much space gets wasted and how much you can actually pack in there. Those little balls will fill every nook and cranny unlike your items which, aside from clothes, are typically inflexible.

At the airport, they will put your backpack in a metal cage, if it fits, good, if not, not so good. Yours truly keeps a sheet of the sizes of these cages. For most airlines, we collected the sizes the airline tells you to keep the size under except for Ryanair and United which are the actual physical size which is slightly bigger than the airline "regulation". I would love to amend this with more actual sizers. And yes, backpacks sometimes get caged too, it's not only rollers.

So dimensions and organization are king and litres are just meaningless. There are 27L packs which can accommodate less clothing than a 22L backpack simply because it's split into several thin pockets. Good for documents, I guess? useless for travel. It is very hard to buy the ideal backpack and it depends on your loadout anyways. Personally I found the Greenroom136 Rainmaker by far the best but I guess this will vary but mostly only because people never heard of them, now you did, I just saved you years of searching. It's rectilinear, it's lightweight (in VX42 -- yet another reason, there are not many backpacks fully made form x-pac), it has sensible organization which doesn't waste space, it has customization through the roof (they have a bag builder on the website but if that's not enough, you can ask for more via Messenger, for example I asked for a removable laptop sleeve and more MOLLE in the main compartment), it has handles everywhere... The Large size happens to be 27L, actually :)