An editorial baggage question separating a backpack's measured exterior from its advertised liter capacity.
1 answer
The allowed size comes from the airline and the baggage allowance attached to your journey. A backpack’s capacity in liters does not settle it, because two bags with the same volume can have different external shapes.
For a concrete example, American Airlines currently lists a carry-on maximum of 22 by 14 by 9 inches, including handles and wheels. Its personal-item limit is smaller at 18 by 14 by 8 inches, and that item must fit under the seat. American also notes that some airports and aircraft have additional restrictions.
Decide which of those roles your backpack needs to fill before shopping. If your suitcase already uses the main cabin-bag allowance, the backpack may need to meet the personal-item rules instead. A product described as carry-on is not necessarily suitable for that smaller role.
Measure the packed bag across its longest external points, including filled pockets and any expansion you intend to use. For a connecting itinerary, check each operating carrier. Save the relevant official allowance with your trip details so your packing decision is based on your flights rather than a generic capacity recommendation.