Credit Card Account Number Specifications

Although credit card account numbers may seem like a string of 15 to 19 digits with little or no rhyme or reason, they actually contain a great deal of information including the type of the card, name of issuer and even a hidden mathematical element to make it harder for people to make up a fake card number.

Credit card numbers can have up to 19 digits, although most have 15 or 16. They contain four specific pieces of information--the type of card, the issuer of the card, the account number and a security checksum digit. Like the physical size of the card and the way in which its magnetic strip is encoded, the format of the card number is defined by a standard from the International Standards Organization, or ISO, which ensures that every credit card number follows a predictable format.

The first digit of every credit card number is called the major industry identifier. Cards which begin with 4 and 5, like Visa and MasterCard, are typically issued by the banking or financial industries, while those starting with 6 are either banking or store cards. Cards which begin with a 3 are tied to travel and entertainment, such as American Express and Diner's Club. Finally, gasoline cards without a Visa or MasterCard logo would start with a 7.

The first six digits, including the MII, make up the issuer identifier. These numbers not only identify the broad type of card, but also who is actually issuing the card. For instance, both American Express and Diner's Club cards begin with a three, but Amex cards start with 34 or 37, and Diner's Club cards start with 30, 36 or 38. Every Discover card, on the other hand, starts with 6011.

Credit card issuers are free to set account numbers however they wish. While American Express cards embed a number of codes in the account number area, many other issuers just assign a number to each account. With a theoretical maximum of 12 digits in an account number, it is possible for each issuer to have 1 trillion different account numbers, although most use only eight or nine digits for the actual account number portion of the card number.

The last digit of a credit card number is reserved for a checksum digit using the Luhn algorithm. The Luhn algorithm works on the basis of adding up every number on a card's face while doubling half of them and then adding one additional digit so that the final sum is an even multiple of 10. This checksum digit makes it challenging for someone to make up a card number at random since only one in 10 numbers will actually be a valid number.