If you're traveling by air with liquids, gels, nonflammable aerosols, creams or pastes in your carry-on luggage, the U.S. Transportation Security Administration and many international security departments restrict those items to a bottle size of 3.4 ounces (100 ml) or less. You're also restricted to only as many bottles as will fit into a single quart-size, zip-close plastic bag. Needless to say, there's an art to getting as much use as you can out of those limited quantities.
Finding Your 3-Ounce Bottles
If you're a frequent traveler, you can snag plenty of hotel-size toiletry bottles that fall under the 3.4-ounce limit. If you don't have a stash of hotel items, you can either buy travel-size toiletries at the drugstore or supermarket, or purchase travel-size bottles and fill them from your own larger bottles of toiletries.
Never fear – if you leave something behind or can't pack enough in a carry-on for your entire trip, once you're out of the United States you can find travel-size toiletries in pharmacies almost anywhere in the world. If you're focused on traveling light, it's often less hassle to replenish your stock of toiletries partway through the trip than to pack everything you need.
The Basic Strategy
Once you have your 3-ounce bottles of toiletries, the rest is simple. Just fill them up well in advance; make sure they fit into your quart-size plastic bag, and also make sure that the bag will close securely while the toiletries are inside. If you have long, thick hair, you might be concerned about how many shampoos you can get out of a small bottle. The best way to find out is by doing a trial run at home, so you'll know exactly how long it takes you to get through those bottles of shampoo or conditioner.
When you pack your baggie of liquids into your carry-on, place it somewhere easily accessible so that if the security staff asks to see it, you won't have to pull out everything else in your luggage.
Using Nonliquid Toiletries
If you just can't get enough toiletries into your bag and aren't comfortable with the idea of replenishing during your trip, you can squeeze more into your carry-on by using "hard" toiletries that don't fall under the liquids rule. This includes bar shampoo, hard deodorant and dry toothpaste in powder form.
Ultimately, the decision about what is and isn't allowed in carry-on luggage is up to the transportation security officer at the screening checkpoint. So if any of the items you've packed could be considered borderline, it's best to err on the side of packing them into your plastic baggie. When in doubt, consult the TSA's "What Can I Bring?" tool; restricted items that fall under the 3.4-ounce rule will be clearly marked.
An Exception for Duty-Free Liquids
There is an exception to the restriction on liquids in carry-on bags: If you've been traveling internationally and are on your way back to the United States, you can bring duty-free liquids back with you under several conditions: They must be packed in clear, tamper-proof bags by the retailer; you must have purchased them within the last 48 hours, and you must have the receipt with you.