They can only count the number of bills they find. That's because a dollar bill carries the same amount of responsive magnetic ink as a $100 bill — meaning somebody carrying $10 in ones would trigger the metal detector just as much as somebody carrying $1,000 in hundreds.

A standard metal detector cannot pick up traditional paper money because the "paper" (which is actually a blend of cotton and linen) contains no metallic properties. However, there is a common "urban legend" that high-denomination U.S. bills contain a "magnetic strip" that can be detected. In reality, modern currency—specifically the 100 bill—features a "Security Thread" that is made of a specialized plastic and metallic polyester, but the amount of metal is so microscopic that a standard airport or security metal detector will not be triggered by it. If you were carrying a massive, "movie-style" suitcase filled with millions of dollars, the high concentration of magnetic ink (which contains iron oxide) used in the printing process might theoretically cause a slight fluctuation in a highly sensitive industrial-grade sensor, but for an average traveler carrying a few thousand dollars in their wallet, there is absolutely no risk of setting off an alarm. In 2026, security "millimeter-wave" scanners can "see" the bulk of a large stack of cash through your clothing, but that is based on "object density" rather than "metallic detection."