You may have noticed that while most credit cards have 16 digits of numbers, some have only 15 digits, for example, American Express credit cards. You may have wondered what credit card numbers really mean.

Credit card numbers share a common numbering scheme and have an internal structure specified by the International Standards Organization (ISO/IEC 7812-1:1993) and the American National Standards Institute (ANSI X4.13). Every credit card number contains a single-digit Major Industry Identifier (MII), a six-digit Issuer Identification Number (IIN), an account number, and a single digit check sum calculated using the Luhn algorithm.

The first digit of your credit card number is the Major Industry Identifier (MII), which represents the category of entity which issued your credit card. Different MII digits represent the following issuer categories:

For example, American Express, Diner’s Club, and Carte Blanche are in the travel and entertainment category, VISA, MasterCard, and Discover are in the banking and financial category, and SUN Oil and Exxon are in the petroleum category.

The first 6 digits of the credit card number (including the initial MII digit) are known as the Issuer Identification Number (IIN). These identify the institution that issued the card to the card holder. Cards are issued by the issuer through an issuing network. Theoretically, 6 digits of IIN means that the total number of possible issuers is a million.

Some of the better known issuer identifiers are listed in the following table:

The final digit of your credit card number is a check digit, akin to a checksum. The algorithm used to arrive at the proper check digit is called the Luhn algorithm, after IBM scientist Hans Peter Luhn (1896-1964), who was awarded US Patent 2950048 (“Computer for Verifying Numbers”) for the technique in 1960. The most succint description of the Luhn algorithm is: “For a card with an even number of digits, double every odd numbered digit and subtract 9 if the product is greater than 9. Add up all the even digits as well as the doubled-odd digits, and the result must be a multiple of 10 or it’s not a valid card. If the card has an odd number of digits, perform the same addition doubling the even numbered digits instead.”

For American Express, digits three and four are type and currency, digits five through 11 are the account number, digits 12 through 14 are the card number within the account and digit 15 is a check digit.

For Visa, digits two through six are the bank number, digits seven through 12 or seven through 15 are the account number and digit 13 or 16 is a check digit.

For MasterCard, digits two and three, two through four, two through five or two through six are the bank number (depending on whether digit two is a 1, 2, 3 or other). The digits after the bank number up through digit 15 are the account number, and digit 16 is a check digit.

Let’s analyze a sample number of 4408 0012 3456 7890 to see whether it is a valid credit card number.

The Major Industry Identifier (MII) is 4 (banking and financial), the issuer identifier is 440800 (a VISA partner), the account number is 123456789, and the check digit is 0. Let’s apply the Luhn check to 4408 0012 3456 7890.
(4*2)+4+(0*2)+8+(0*2)+0+(1*2)+2+(3*2)+4+(5*2-9)+6+(7*2-9)+8+(9*2-9)+0
= 8+4+0+8+0+0+2+2+6+4+1+6+5+8+9+0
= 63, which is not a multiple of 10. Therefore we conclude that the number 4408 0012 3456 7890 is an invalid credit card number.

Try to use a valid credit card to verify the numbering, and you will see that the first 6 digits match the card issuer and the final digit always makes the result from Luhn algorithm to be a multiple of 10.