Magnetic strips store data as a pattern of tiny magnetized particles embedded in a thin band of tape. When you swipe a card, a reader detects those magnetic patterns and converts them into the digital information needed to identify you, authorize a transaction, or unlock a hotel room door. The technology is elegantly simple, which is both its greatest strength and its biggest weakness.
What’s Inside the Stripe
The dark strip on the back of your card is made of tiny iron-based magnetic particles bound into a thin film of plastic tape. These particles can be magnetized in one of two directions, north or south, and that orientation is what encodes data. Think of it like a long line of microscopic compass needles, each pointing one way or the other to represent the ones and zeros of binary code.
A standard magnetic stripe contains three separate tracks running along its length. Track 1 holds alphanumeric data like your name and account number. Track 2, the most commonly used, stores your account number and expiration date in a numeric format. Track 3 exists but is rarely used on consumer cards. Each track has its own density of data, but together they hold only a few hundred bytes of information, roughly enough for a short text message.
How Swiping Generates a Signal
The reader head inside a card terminal is essentially a tiny electromagnet in reverse. As you slide your card through the slot, the magnetized particles on the stripe pass over a small coil of wire in the reader head. The changing magnetic field from each particle induces a tiny voltage in that coil, a principle described by Faraday’s law of electromagnetic induction. The pattern of voltage spikes corresponds directly to the pattern of magnetized particles on the stripe.
These electrical signals are extremely faint, so they get amplified and then decoded by the terminal’s processor. The speed of your swipe matters because the voltage depends on how quickly the magnetic field changes. Swipe too slowly and the signal is too weak; too fast and the reader can’t distinguish individual transitions. Most readers are designed to work across a reasonable range of swipe speeds, but this is why you occasionally get a “please swipe again” error.
Writing data onto a stripe works the same way, just in reverse. An encoding device sends electrical current through a write head, which generates a magnetic field strong enough to permanently orient the particles on the stripe. Each particle gets locked into its north or south position, where it stays until another magnetic field overwrites it.
High Coercivity vs. Low Coercivity
Not all magnetic stripes are created equal. The key difference is coercivity, which is how strongly the particles resist being remagnetized. High-coercivity (HiCo) cards are encoded at 2,750 Oersted and typically have a black stripe. These are your credit cards, bank cards, library cards, and employee ID badges. They’re designed for frequent swiping over months or years without the data degrading.
Low-coercivity (LoCo) cards use a weaker magnetic field of just 300 Oersted and usually have a brown stripe. You’ll find these in hotel key cards, theme park passes, and other short-term applications. They’re cheaper to produce but far more vulnerable to accidental erasure. That’s why your hotel key card sometimes stops working after sitting next to your phone, while your credit card almost never does.
What Erases or Damages Them
Because the data is stored magnetically, any sufficiently strong external magnetic field can scramble it. MIT’s magnet safety guidelines note that credit cards and magnetic tape can be irreversibly corrupted by exposure to strong magnetic fields. LoCo cards are especially susceptible since their particles were magnetized at such low intensity to begin with. Refrigerator magnets, phone case clasps, and purse clasps can sometimes carry enough magnetism to wipe a LoCo hotel key.
HiCo cards are far more resilient but not invincible. MRI machines, industrial magnets, and certain electronic article surveillance systems at store exits can generate fields strong enough to affect even high-coercivity stripes. Physical damage also plays a role. Scratches on the stripe surface remove or displace particles, creating gaps in the data that the reader can’t reconstruct.
Why They’re Easy to Clone
The fatal flaw of magnetic stripes is that they’re entirely static. The data on your card is the same every single time you swipe it. A skimming device, often a thin overlay placed on top of a legitimate card reader, simply reads and records that data as you swipe. Once captured, the information can be written onto a blank card with an inexpensive encoder, creating a perfect clone. A Tufts University security analysis put it bluntly: accessing information on magnetic stripe cards and creating cloned cards is trivial.
Chip cards solve this problem by generating a unique, encrypted code for every transaction. Instead of passively storing static data, the chip contains a small processor that communicates actively with the terminal. Even if someone intercepted the data from one transaction, it would be useless for the next one. Newer chip cards use dynamic encryption that makes man-in-the-middle attacks significantly harder than they were on earlier chip technology.
The Stripe Is Being Phased Out
Mastercard announced a detailed timeline for retiring magnetic stripes entirely. Starting in 2024, newly issued Mastercard credit and debit cards in markets like Europe, where chip adoption is already widespread, no longer need to include a stripe. U.S. banks will follow in 2027, when they’ll no longer be required to issue chip cards with a magnetic stripe. By 2029, no new Mastercard cards worldwide will include one. The final deadline is 2033, when magnetic stripes will disappear from all Mastercard products entirely. Prepaid cards in the U.S. and Canada are currently exempt.
The transition has been gradual by design. Billions of terminals and ATMs worldwide still rely on swipe capability, and replacing that infrastructure takes time. But the direction is clear: the magnetic stripe is a 1960s technology being retired in favor of chips and contactless payment.
How the Stripe Was Invented
The magnetic stripe exists because of an IBM engineer named Forrest Parry and his wife’s household iron. In the early 1960s, Parry was trying to create more secure identity cards for CIA officials by attaching a strip of magnetized tape to a plastic card. He couldn’t find a reliable adhesive that would bond the tape without damaging it. His wife, who was ironing clothes at the time, suggested he try using the iron to melt the stripe onto the card. It worked, and the method became the foundation for billions of cards produced over the next six decades.