The string of digits on the front of your debit or credit card might look random, but they’re actually telling a story. While each card brand (Visa, MasterCard, American Express, Discover) has a slightly different formula, here’s an overview of what the numbers signify.
A bank identification number (BIN) is the initial four to six numbers that appear on a credit card. BINs help ensure that transactions are routed through the proper card network and financial institution so that the transaction can be authorized.
The four most common credit cards in the US are Visa, MasterCard, Discover and American Express. Their starting BINs are as follows:
- Visa: 4
- MasterCard: 51-55, 2221-2720 (New)
- Discover: 6011, 622126-622925, 644-649, 65
- American Express: 34, 37
Numbers following the BIN signify everything from the currency being used, card type, bank processing the transaction and cardholder account number. The final digit is referred to as a check digit, which is a random number used to protect against errors and fraud.
The Luhn Formula
This low-tech formula can show whether a credit card number is valid. Try it for yourself.
- Enter your credit card number
- Double every other number, starting with the second number from the right. Write those digits under your original card number. Cross out any number you doubled.
- Look at the new line of numbers. If a number has two digits, add those together. Again, write those numbers down in a new row, and cross out the ones added together.
- Add together all numbers that are not crossed out.
- Is the sum’s last digit a zero? If not, the credit card number is not valid.
It’s important to note that the Luhn formula was designed to detect accidental data entry errors and not as a defense against fraud. And, it’s a pretty cool party trick.