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How To Validate Credit Card Numbers

how many digits are in a credit card number



This is the article which tells you how to validate “Credit Card Number”. This articles doesn’t guarantee that the Credit Card Number exist what it tells that it is the valid number or not.


Credit Card numbers are (most times) 13 to 16 digit numbers which are protected by a special numerical check, called Luhn check.

3 Example

4 Code

Informal Explaination

The formula generates a check digit, which is usually appended to a partial account number to generate the full account number. This account number must pass the following algorithm (and the check digit chosen and placed so that the full account number will):

Starting with the second to last digit and moving left, double the value of all the alternating digits. For any digits that thus become 10 or more, add their digits together. For example, 1111 becomes 2121, while 8763 becomes 7733 (from (1+6)7(1+2)3).

Add all these digits together. For example, 1111 becomes 2121, then 2+1+2+1 is 6; while 8763 becomes 7733, then 7+7+3+3 is 20.

If the total ends in 0 (put another way, if the total modulus 10 is 0), then the number is valid according to

the LUHN formula, else it is not valid. So, 1111 is not valid (as shown above, it comes out to 6), while 8763 is valid (as shown above, it comes out to 20).

In the two examples above, if a check digit was to be added to the front of these numbers, then 4 might be added to 1111 to make 41111, while 0 would be added to 8763 to make 08763. It is usually the case that check digits are added to the end, although this requires a simple modification to the algorithm to determine an ending check digit given the rest of the account number.


The algorithm proceeds in three steps. Firstly, every second digit, beginning with the next-to-rightmost and proceeding to the left, is doubled. If that result is greater than nine, its digits are summed (which is equivalent, for any number in the range 10 though 18, of subtracting 9 from it). Thus, a 2 becomes 4 and a 7 becomes 5. Secondly, all the digits are summed. Finally, the result is divided by 10. If the remainder is zero, the original number is valid.

The following is wikicode, a proposed pseudocode for use in many articles.

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