We’ve all seen, “sorry, invalid credit card number”on the screen when ordering educational materials online, but how did they know?

They didn’t check your credit card number against every single credit card number on the planet. That takes too long and they’ve got important money to make.

Rather, credit card processors use a simple formula, called the Luhn algorithm.

Starting with the farthest right digit, they sum the double of every second digit. If the result is a double digit, both digits are added together. The results are summed. If the final result ends in zero, it’s a valid credit card number. If the final result doesn’t end in zero, it’s not valid.

So, the next time a website denies your education material purchase order, shout, “Damn you, Luhn!” — BEN POPKEN

Credit Card Validation – Check Digits [Beachnet]
Luhn Algorithm [Wikipedia]

Previously: What A Credit Card’s First Digit Means

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