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 45L 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 45L pack on say Jetstar or 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.
There are things you can do to work with the airline carry on rules to improve your chances of not having to check anything. Wear your bulky and heavy clothes and shoes for the flight as the airlines don’t weigh passengers. Yet . Also stuff your pockets with heavy items like your camera, USB battery pack, phone etc. Taking this to the extreme are clothing items designed specifically to help you do this with tons of pockets such as the SCOTTeVEST line. Where airlines allow for it, make use of your secondary personal item. When you’ve got everything arranged cinch down your pack so it looks as small as possible and try not to appear loaded down.
When planning a big trip it’s a good idea to make a list of your gear and it’s weight. You could use your own spreadsheet or use one of the sites specifically designed for this such as lighterpack.com. You can use this to play with combinations of items to arrive at your best gear list before you even pack. Once you get your gear you’ll likely have to do multiple test packs.
Sometimes there is no way your baggage will meet the requirements on a flight that you need to take. For these cases it is much cheaper to pay for checked in luggage when you book. Paying this fee at check-in can sometimes be exorbitant so try to plan ahead. We use a USB travel scale to make sure we meet the carry on weight allowances.
Here’s a great resource from luggage maker Minaal, for finding what rules many airlines have: Airline Carry On Bag Sizes.