Will putting two credit cards' magnetic stripes together ruin them?

At the convenience store this morning, I was putting my credit card back in my wallet when the cashier shrieked that I had put my credit card’s magnetic strip against the magnetic strip of another credit card in my wallet. She claimed that doing that would cancel out each of them and render them unusable.

I called her a filthy whore and left.

But she brought up an interesting question. Does putting the magnetic strips of two credit cards really cause them to lose whatever magnetic effect that they have?

“filthy whore”??? That’s a bit extreme, IMHO.

Might want to read Mythbusters Episode Guide

Near the bottom, they discuss Mythbuster’s attempts to demagnetize cards using various techniques.


Okay, I said “Thank you for the advice. Have a nice day.” But I was thinking “filthy whore.”

Thanks for the link.

You touched a sore spot. I think the magnetic strip as it is used on various cards is a marginal system. All retailers and customers should lean on the issuers to make improvements.

Merely sliding my card in and out of the holder wears mine enough that they always become unreadable before the expiration date. The card I use most expires in 2009. No way will it last that long.

The most common way they lose magnetism is by being laid down near the demagnetizing pad on the checkout counter. These are quite common in harware stores now, since many tools and things like faucets and even special lightbulbs will have a magnetic shoplifing tag pre-installed inside the packaging.

Somehow the warning message is only notice after you make the mistake.

Other things are accepting novelty business cards (like from pizza chains) that are also fridge magnets.
At trade shows I’ve been given keychains have little magnets on them. Fortunately I never used them, as my keys and credit cards always end the day in the same tray on the nightstand.

To answer the OP’s specific question: No, placing the magnetic stripe of one card against the magnetic stripe of another will not demagnetize or scramble the encoded data on either stripe. The reason is because the coercivity of the magnetic material in the stripes is high enough to resist the magnetizing force capable of being applied by another magnetic stripe. Simplistically stated, the coercivity is the amount of magnetizing force that a magnetic material needs to have applied to rotate the magnetic domains. It takes a significantly stronger field to do this in the credit card magnetic stripe material, such as that used in the card writers or the ESA tag deactivator pads mentioned earlier.

Related Straight Dope column (second one on the page)

Apart from the detailed answers given above, one could just think about how different sections of videotapes or cassette tapes (remember those?) are placed in direct contact with each other as they wind around the spindles, and how little that effects the encoded information on them.

Ain’t nothing like a record/erase head.

My ATM card managed to scramble itself one day. The people at the bank blamed my other magnetic striped cards in my wallet, and even though I found it unlikely I had no other explanation, so ever since I’ve kept my striped cards separated.

It’s possible to transfer information from one magnetic strip to another by heating one of them to its Curie point. This is how many pre-recorded tapes are made. It’s not likely to happen in your wallet.