|It's entirely possible that I've done this before ...|
I keep seeing all these great articles about how to travel overseas for a few weeks using only carry-on luggage, and frankly, I don't think it's possible. I know that in the USA, people are allowed to bring much more carry-on luggage on board with them than in other countries - and thank heavens for that if you've ever seen me trying to fit all my shopping in after a trip back to the US - but in most other countries, carry-on luggage limits are very strictly controlled.
We fly a lot within Europe on Ryanair, who are well known for being astonishingly vigilant with their carry-on allowances - weighing, measuring, and generally making your life unpleasant. Easyjet have a strict size limit, but no weight limit, to their carry-on.
Click HERE to find out more about Ryanair's limits, and click HERE to explore how much you'll be able to lug on with Easyjet.
I can certainly do 3-4 days in another country with just one carry-on bag, but any longer than that and I think you're running out of clothing options, and you've certainly not got any room to buy anything to bring home with you. I think it'd be easier if you are a guy travelling with just carry-on luggage, but us girls have to heft around shampoo, conditioner, mousse, moisturiser etc., and all of that stuff has to be able to fit into the little resealable plastic baggie that has to be scanned separately at the airport.
That said, if you're determined to take only one carry-on bag to your destination, and have discovered that you've wedged your bag full and there's no room for your shampoo, conditioner etc., think about buying it at your destination and just leaving it behind when you depart. I know it's a bit of a waste, but if it's a case of paying $100 to check a bag in (and this is becoming normal on the cheaper airlines), or spending $20 on arrival at the nearest pharmacy to stock up on a few things, I know which way I'd be going. Pharmacy, here I come!
Remember that not everyone has the same level of income, so if you're going to leave things behind in your hotel room on departure that are still good, e.g. shampoo that's 50% full, make sure you put it next to the bin (not in it) so that the maid knows you've not forgotten it and that it's available for taking home if they wish (or are allowed to). Maids are paid appallingly in many countries, and I often leave clothing, books, etc., next to the rubbish bin (and a cash tip on the table with a "Thank You" note) in case they need it.
I've also read a couple of articles on rolling versus folding your clothes to go into your carry-on, to make more room. I was dubious about this practice, but since trying it, I think it's better. People say that it minimises creasing, but I'm not convinced about that. If you roll things up nice and tight, you can certainly fit more in, but it does get creased - there's no way around it.
The diagram below provides some handy ideas on how to wedge all your belongings into your carry-on bag, and I also recommend wearing a coat with lots of pockets for other random things that you might not be able to fit into your carry-on bag (e.g. I've come home with bacon, pottery, spices, Vegemite, camera, chocolate, paperback books etc. in my jacket pockets).