It's a key question if you've got an international trip coming up: how much stuff should you cram in your hand luggage? Learn how to strike the right balance without crippling yourself or being forced to repack at the check-in desk.
Hand luggage onboard is essentially a trade-off between two conflicting agendas. On the one hand (sorry), you've only got a limited amount of space, and you don't want to be carrying lots of stuff in and out of transit airports and fighting for onboard overhead bin space with your bag. On the other, if the airline loses your bags (and there's a good chance it will), or you get stuck somewhere in transit without access to your cases, you want at least the essentials with you.
All airlines have guidelines for the size and weight of economy travel luggage (a sum of height, width and depth less than 120cm and a 6kg limit is pretty common), but they vary in how strictly they enforce these. However, it still makes sense to respect them: some airlines will weigh your hand luggage and make you repack at check-in, which is a lousy way to start the trip and will make everyone in the queue behind hate you.
Below, I've listed some of the commonly suggested items for carry-on luggage, and discussed just how necessary they really are. In a slow year, I'll make half-a-dozen international trips, so I make no apologies for the fact that this listing is heavily based on personal experience. With that said, everyone's requirements (and paranoia levels) will differ, so share your disagreements and extra thoughts in the comments. I've also deliberately concentrated on international flights here: for domestic flights, where you may well not want to check any luggage, the agenda is quite different.
Passport, return flight details and accommodation information, cash and cards, pen, home house keys: Even if you're a Zen traveller with minimal possessions, these are all essential, compulsory, and pretty much unarguable. The passport is self-explanatory. The return flight and accommodation details and cash are for the benefit of immigration overseas: many countries won't let you in if you can't prove that you're solvent, will have a roof over your head and will be leaving. You'll inevitably have to fill in some kind of landing card, and the cabin crew never have enough pens. And putting your house keys in checked luggage is just plain stupid and asking for troble.
Key electronics (Computer, phone, iPod or other entertainment device). Even if you charitably assume baggage handling crew have the delicacy of ballerinas, your luggage will get royally bashed just dropping off the conveyer belt onto your carousel. Anything breakable and valuable -- like your PC -- needs to be with you. And why have an iPod if you can't use it while travelling? You'll inevitably make a call or send a text when you land, so bring the phone.
Supporting electronics (PC adaptor, international power plug, mic/headset, destination details). Since your power brick is pretty resilient, it can be tempting to put it in your checked luggage. However, if you have an unexpected stopover or delay, you're unlikely to have enough power to make your PC useful, which is teeth-gnashingly annoying. A compact mic/headset can also be useful for making Skype calls. (I always carry a USB backup drive of crucial work files in my pocket for the vanishingly unlikely event of the plane crashing at sea, but you might not want to get paranoid at that level.)
Emergency bathroom supplies and clothing. If your luggage goes missing, having some deodorant, a toothbrush and spare underwear can seem like a handy idea. I've abandoned this policy myself in recent years, though: if there's time or necessity for the airline to put me up somewhere for the night (or even leave me sitting in an airport for a few hours), there'll be time to purchase those items for myself. Restrictions on travelling with liquid also make it harder to bring much in the way of personal grooming products. But if you're not heavily gadget-laden, it might be worthwhile.
Reading material. In-seat entertainment is great, but it doesn't work in transit lounges, during takeoff and landing, or when the onboard PC crashes. Having a book to hand will help pass the time, but don't make it too thick or it'll take up precious at-seat room. I like travelling with magazines, which I can more easily ditch during flight stages, but you could take the same approach with books and some BookCrossing labels.
A cloth bag for your at-seat essentials. As a firm proponent of Russian doll packing, I always pull out the things I'll actually want access to during the flight -- normally a pen, reading material, iPod, headphones and seat headphone adaptor, and my BlackBerry -- and put them in a separate bag inside my main carry-on so I can pop them under the seat in front of me while the main bag goes overhead. (Sitting them in the seatback pocket greatly reduces your seating comfort.) Having these ready to go means you can get seated quickly, which is good travel karma. A bag with a zip, or which you can tie closed, stops you panicking about stuff falling out under the seat mid-flight. You can use a plastic shopping bag, but they rustle annoyingly.
What else have I missed? What are your travel packing essentials for global trips? Share all in the comments.
Lifehacker Australia editor Angus Kidman actually has a scarily specific onboard luggage packing list, but didn't want to scare y'all too much. His Road Worrier column, looking at technology and organising tips for travellers, appears each week on Lifehacker.