The TSA 3-1-1 rule means each traveler may bring liquids, gels, aerosols, creams, and pastes in containers of 3.4 ounces or 100 milliliters or less, and all of them must fit inside one quart-size bag. That bag goes in your carry-on and gets screened at the checkpoint.
The rule is why toothpaste, shampoo, lotion, and similar items have to be packed in travel sizes. If the container is larger than 3.4 ounces, TSA treats it as too big even if there is not much product left inside.
So the easy way to remember it is 3 ounces per container, 1 quart-size bag, 1 bag per passenger. The official number is 3.4 ounces, but the shorthand gets people through security without guessing.
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