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Carry-on rules, in plain terms
Most US carriers allow roughly 22 by 14 by 9 inches for carry-on. A soft backpack compresses, which helps at the gate, but an overpacked 40L bag will not. If you fly budget carriers, measure your loaded bag, not the empty one.
Water resistance is not waterproof
Coated fabric sheds a shower. Seams and zippers do not. If you travel through monsoon season or ride to the airport, a rain cover matters more than the fabric rating printed on the label.
What to check before you buy
Padded laptop sleeve that sits off the floor, lockable zippers on the main compartment, and a luggage pass-through strap if you also travel with a roller. These three details separate a travel backpack from a school bag with a bigger badge.
