How many digits are usually in a credit card, and what is the usual formatting (e.g.: xxxx-xxx-xxxx-...)??

Question

Nooni: The reason I want to know is because there are annoying telemarketers calling my house (after being asked not to 5 times!) and, just for my own personal satisfaction, I would like to waste their time, as they have been wasting mine by stringing them along, saying "Oh yes, I want to sign up for your long distance", and then giving them completely made up personal information. (I WILL tell them what I'm doing after I've wasted enough of their time!)

Since I personally don't use credit cards, I want to be prepared when they ask for that info. Now, Chris mentioned I might be required to provide an expiration date. Are there any other things they might ask for relating to "my" credit card (I seem to recall something about a 3-digit # on the back of the card, or something)? And no, I'm not trying to commit credit card fraud. For that I would need a VALID credit card#/expiration and the cardholders name, so Yahoo please don't remove this like you did with my weiner question. (What, I just wanted to know if
anyone likes hot-dogs, they're the ones reading into things!! ;)

Answer 1

I don't wish to address your card question, since in Canada card#'s have different rules - but since I once worked at a telemarketing centre, I thought I would provide some useful information to you.

When you get an irritating call - ask the person what CENTRE they work in. They will evade the question and say they represent X company, but persist and ask again "what CENTRE do you work in?" If they still evade and refuse to answer, ask for their supervisor. Call centres, by law, must identify themselves to the individuals they are calling if asked. WHEN they have identified themselves, THEN ask to be removed from the calling list. This will accomplish a few things:

1. The client they represent (for example, Providian) cannot call you.
2. NO clients from that call centre can call you
3. This exclusion lasts for a period of 7 years
4. It may take 60 days to go into effect.

Answer 2

Major Industry Identifier
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:

MII Digit Value Issuer Category
0 ISO/TC 68 and other industry assignments
1 Airlines
2 Airlines and other industry assignments
3 Travel and entertainment
4 Banking and financial
5 Banking and financial
6 Merchandizing and banking
7 Petroleum
8 Telecommunications and other industry assignments
9 National assignment


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.

Issuer Identifier
The first 6 digits of your credit card number (including the initial MII digit) form the issuer identifier. This means that the total number of possible issuers is a million (10 raised to the sixth power, or 1,000,000).

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

Issuer Identifier Card Number Length
Diner's Club/Carte Blanche 300xxx-305xxx,
36xxxx, 38xxxx 14
American Express 34xxxx, 37xxxx 15
VISA 4xxxxx 13, 16
MasterCard 51xxxx-55xxxx 16
Discover 6011xx 16


If the MII digit is 9, then the next three digits of the issuer identifier are the 3-digit country codes defined in ISO 3166, and the remaining final two digits of the issuer identifier can be defined by the national standards body of the specified country in whatever way it wishes.

Account Number
Digits 7 to (n - 1) of your credit card number are your individual account identifier. The maximum length of a credit card number is 19 digits. Since the initial 6 digits of a credit card number are the issuer identifier, and the final digit is the check digit, this means that the maximum length of the account number field is 19 - 7, or 12 digits. Each issuer therefore has a trillion (10 raised to the 12th power, or 1,000,000,000,000) possible account numbers.

If we consider the large number of potential customers and usurious interest rates charged by issuers, there is obviously a lot of money to be made in the credit card industry. In more civilized ages, people believed that usury was a grievous offense contrary to nature or a mortal sin, not an acceptable business practice (Aristotle, Politics 1.10; St. Thomas Aquinas, De Malo 13.4; Dante, Inferno 11.94-111; etc.).

Check Digit
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. For details about Luhn's life, see

Biography on the American Society for Information Science and Technology's Web site, at http://www.asis.org/Features/Pioneers/luhn.htm.
Notes compiled by Susan K. Soy on "H.P. Luhn and Automatic Indexing" at http://www.gslis.utexas.edu/~ssoy/organizing/l391d2c.htm

Thanks to Aleksandar Janicijevic for directing me to information about H.P. Luhn.

The most succint description of the Luhn algorithm I have found comes from the hacker publication phrack 47-8: "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."

The bit about even and odd is a little confusing. The main point is that you don't want to double the check digit, and this can easily be done by starting with the check digit, going backwards, and doubling every other digit. See the source code below for details

Answer 3

there are four sets of 4 numbers...
like 1234 5678 9012 3456
and then an expiration is also
usually required.

Answer 4

16 digits, usually 4 sets of 4 numbers xxxx-xxxx-xxxx-xxxx

Answer 5

Why do you want to know?

Answer 6

xxxx-xxxx-xxxx-xxxx
16