How a stripe stores card data
A magnetic stripe is not a printed barcode: it holds data in a thin layer of magnetic material. When a card is swiped, the reader senses a changing pattern of magnetization and converts it into information. A sufficiently strong nearby magnet can disturb that pattern, so the reader may no longer recognize the card’s data. The stripe can look perfectly normal even when its signal has been damaged.
Why the rest of the card may survive
The chip works differently. It communicates electronically with a reader through contacts or, for contactless cards, a short-range radio signal; neither method depends on the stripe’s magnetic pattern. So a swipe failure does not necessarily mean the entire card is unusable. A chip or tap may still work, although a merchant or machine that requires a swipe could reject it.
The risk depends on the magnet’s strength and how close it gets to the stripe. Ordinary decorative magnets are generally not a reason to panic, but unusually powerful magnets packed directly against a card are a poor bet. If a swipe stops working, try the chip or tap and contact the issuer for a replacement if needed. The practical travel nuisance is often not lost money, but discovering that the one terminal available can read only the stripe.
