# How To: How Credit Card Algorithms Work: The Anatomy of Credit Card Numbers

Have you ever wondered how credit card numbers work? I mean, how they really work? How do they come up with the numbers? Credit cards actually follow a very specific pattern. Let's take a look at how they're set up.

Example of a Credit Card Number

4485 3151 5882 2849

These numbers are split up into various parts that identifiy different things about the credit card (who issued it, etc). These things are: major industry identifier, bank issuer ID, user ID and check number. They correspond to these parts of the number:

• Number 1 is the MII.
• Numbers 2-6 are the issuer ID, telling which bank the card is from.
• Numbers 7 to n-1 are the user ID.
• The last number is a check number to make the number valid against the checking algorithm.

## MMI

The major industry identifier tells you what the card is for. Here are the numbers and corresponding meanings.

MMI / Digit Value 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
= Merchandising and Banking
7
= Petroleum
8
= Telecommunications and other industry assignments
9
= National assignment

## Issuer ID

The issuer ID is for identifying which bank owns the card. Here are the bigger companies numbers.

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

As you can tell from this chart, my example number is a VISA card.

## User ID

This number is pretty self-explanatory. The user ID is the identification for who owns the card. With VISA, each bank has 999,999,999 possible card holders. These are given based on availability.

## Check Number

To ensure that credit card numbers are valid, a final checking number is appended to each number to ensure that it passes a checking algorithm, otherwise known as the Luhn check. Let's check to see if my fake number above is a valid credit card number.

#### Checking Number Validity

1. Let's split up the number I made so that we have an easier time managing the number check.
4 4 8 5             3 1 5 1              5 8 8 2                 2 8 4 9
2. From the back, double every other number.
8 4 (16) 5        6 1 (10) 1         (10) 8 (16) 2       4 8 8 9
3. After, we have to de-concatenate the double-digit numbers and then add them to make one single digit.
8 4 7 5             6 1 1 1              1 8 7 2                 4 8 8 9

If you add all of the numbers together and they add up to a multiple of 10, you have a valid credit card number! If you have any questions, be sure to leave them in the comments below.

Follow and Chat with Null Byte!

Just updated your iPhone? You'll find new features for Podcasts, News, Books, and TV, as well as important security improvements and fresh wallpapers. Find out what's new and changed on your iPhone with the iOS 17.5 update.

Photo by Andres Rueda

Hi Alex - this is a great post. I have often wondered what the numbers mean on Credit & Debt Cards.
This tutorial explains it well. Can I ask though whether this applies to the United States only, or does it apply to other countries also? Chris.

To be honest, I'm not sure. If I had to make an educated guess, I would say that this is only inclusive to the US. Anyone out of country want to take a VISA card and throw the algorithm against it to check?

This is awesome!

It's the same princip as IMEI numbers on your phone :D Shows Fabric place, brand and so on :D

This is interesting, and a neat trick to know… A number of immediate applications to jump to mind, but it is difficult to make use of any of them isn't it?

so this is how CC numbers are 'faked'....so the scanner thing is the CC number 'encoded' (readable by machine)?.

The black strip on a CC contains the number magnetized into it :). I'm assuming it works via binary converted to decimal, as that would be easiest for someone to create a device to read the numbers.

The magnetic strip on the back of a credit card contains more than just the number on the front of the card though. Especially on modern credit cards that work with a pin.

So, my friend is telling me.. if you are calculating AmEx you can pad the front with zeros.

The other thing he said was that although adding up all the numbers end up in 0, if don't add up the checksum but multiple your total 9, the last digit of your answer will be the checksum digit. So, in your example, if we add up all the numbers except last one... it will be 71. 71x 9 is 639. Checksum digit is 9. On my card, the total was 56. 56x9 is 504. The last digit on my credit card is 4.

of course this stuff is interesting to know but if you're doing this to get free stuff and not paying for it. I have to pay for your cheating the system by higher prices and higher interest rates etc..... and if there's a person attached to the credit card you're using then thats even more a fuked up thing to do. I think punishment for those who rip people off by duplicating or just plane using numbers off their debit and credit cards needs to start at first offense min of ten years prison without any chance of early release. Whether it for a buck or ten grand. Stealing is Stealing. \$500.00 isn't the magic number. Any amount is wrong if your name isn't on that credit card or assigned along with that account. PERIOD! Hey, I like free stuff too. But it just doesn't happen like that. If I get lucky and come up on something thats a great deal I find before not too long its got its price. There's no such thing as a free lunch without paying a price somewhere later down the line. Trust me on that! All you brilliant people turned shady, you are too smart seriously for taking the easy way (to you) and ending up in jail. Think about all the legal money you could be making just by thinking up the things that we humans need on a daily basis. Then cash in on that!

i tried it on a master card in egypt and it worked succesfully

please can someone drop to me a credit card before in finish learning how to activate one myself..... thanks

I have a old set of visa credit card numbers that a new card was issued to replace it. I have the first 6 and the new last 4 but something changed in the middle 6. How do I figure out the middle six ?

How do I get a number? And would it work on Xbox live.

Unfortunately people with no morals or empathy for others who won't work hard like the people they steal from off their credit cards use this info in a bad way . Anything useful is made useful to bad people who run with it. And I have been the one they targeted. But now I have horrible credit thank you very much. There's nothing more to get. I want to tell you all you are shit. Your day is coming. Until then I'm happy I live a different life now. I have to because I won't let such things defeat me. What you get from people you don't even have 6 months later.

• Hot
• Latest