You might think it's completely random how they come up with your credit card numbers. But, oh, how wrong could you be! There's a very complex, and very exact method that credit card companies use to come up with those 10 digit numbers. It's called the Luhn Formula, or the modulus 10, or mod 10 for short.
The Luhn formula not only helps the credit card companies come up with your numbers. It also allows them to double-check, verify, and validate your card numbers, so they know that cards are actually real when people try to use them for a purchase. Sneaky, right?
Not actually. The Luhn Formula may be complicated, but it's no secret. As anybody in the credit card industry will tell you (maybe except your customer service rep), one of the most basic rules of the Luhn Formula is the so-called check digit, the last digit in your number. The first digit, on the other hand, simply identifies whether a card is a Visa, a Discover Card, a MasterCard, etc. Meanwhile, the middle digits are dedicated all to you, the customer.
Those first and last digits are used in the Luhn Formula to come up with your control number, and your total 10 digit credit card number. On the other hand, when a company wants to verify your card, they take the control digit, plug it into the Luhn Formula, and see if the other nine digits pop out.
Who came up with the cockamamie scheme? The Luhn Formula was actually created by a bunch of mathematicians back in the 1960s. While others were listening to Jimi Hendrix and going to Woodstock, these guys were figuring out a way to make the credit card companies, and themselves, rich.
Believe it or not, the Luhn Formula is also used by the Canadian government to come up with Canadian Social Insurance Numbers. It goes even further than that. Basically, any institution that uses a long primary account number uses the Luhn Formula to create those account numbers, and figure out if your account number is true.
Finally, here's how it works. Starting with the second to last digit and going left, double the value of it and all alternating digits. Then take all of the resulting digits, and the unaffected digits, and add them together. When you get to double digits, add those together (e.g., 12 becomes 1+2), and keep going to the end. The grand total must end with a zero to be true.
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