What size travel backpack is best for carry-on?

Most airlines accept bags in the 35L to 45L range as carry-on, and the bulk of this list clusters around 40L. A 35L bag is the safest bet for strict carry-on limits and one-bag travel, while 40-45L expandable designs give more packing room but can exceed personal-item dimensions when fully loaded. Always check your airline's stated dimensions, not just liters.