Airline carry on rules can vary wildly. They each have their own specific guidelines for items that you can bring on-board a flight. Some even allow for a secondary “personal item” like a purse or laptop bag. If you do your research you’ll find lots of people saying a 45 L pack is good for carry on. As you’ll find out, this is not always the case. If you aren’t exactly within an airline’s requirements it’ll come down to the whim of the staff working on the day of your flight. Unfortunately people are sometimes unpredictable and inconsistent. So if someone tells you they were able to carry on a 45 L pack on say Ryanair, it doesn’t mean you’ll be able to do the same the next day.
In 2015 the International Air Transport Association (IATA) attempted to standardize carry on rules for its member airlines with advice from the aircraft manufacturers. This attempt didn’t last long as all members had their own agenda and so agreement between all proved impossible.