How To: Tell Whether a Credit Card Number Is Valid Just by Looking at It

Think credit card numbers are just a meaningless jumble of digits? A sort of number soup? Think again! Your credit card number isn't even number at all but rather a composite of four separate numbers, each of which plays an important, fixed role.

Because of these roles, you can figure out a lot about a card just by looking at its number. With a little bit of back-of-the-napkin math, you can, in fact, even determine whether a given credit card number is valid or not—which, if nothing else, is a great party trick.

Interested in cracking the credit card code for yourself? Here's how it works:

Tips

• Having trouble validating your card? A Mint.com forum user writes, "Some cards use a variation – 'Mod 10 plus 5', so they accept as valid if the sum ends with 0 or 5."

Want more? Explore WonderHowTo's collection of credit card videos.