The TSA rule says any liquids, gels, or aerosols in a carry‑on bag must be in containers of 3.4 ounces (about 100 ml) or less and must fit inside a clear quart‑sized bag.

Gary Leff, a travel industry expert, explains that peanut butter’s smooth consistency lets it conform to the shape of its container, so the TSA classifies it as a liquid.

Because of this rule, you can only bring small jars of peanut butter (3.4 ounces or less) in your carry‑on bag.

If you need more peanut butter, you must pack the larger amount in checked luggage.