Here is a table outlining the major credit cards that you might want to validate.

CARD TYPE	Prefix	Length	Check digit algorithm
MASTERCARD	51-55	16	mod 10
VISA	4	13, 16	mod 10
AMEX	34 37	15	mod 10
Diners Club/ Carte Blanche	300-305 36 38	14	mod 10
Discover	6011	16	mod 10
enRoute	2014 2149	15	any
JCB	3	16	mod 10
JCB	2131 1800	15	mod 10

Step 1: Double the value of alternate digits of the primary account number beginning with the second digit from the right (the first right--hand digit is the check digit.)

Step 2: Add the individual digits comprising the products obtained in Step 1 to each of the unaffected digits in the original number.

Step 3: The total obtained in Step 2 must be a number ending in zero (30, 40, 50, etc.) for the account number to be validated.

For example, to validate the primary account number 49927398716:

Step 1:

        4 9 9 2 7 3 9 8 7 1 6

         x2  x2  x2  x2  x2 

------------------------------

         18   4   6  16   2

Step 2: 4 +(1+8)+ 9 + (4) + 7 + (6) + 9 +(1+6) + 7 + (2) + 6

Step 3: Sum = 70 : Card number is validated

Note: Card is valid because the 70/10 yields no remainder.

Make sure that you:

1. have started with the rightmost digit (including the check digit) (figure odd and even based upon the rightmost digit being odd, regardless of the length of the Credit Card.) ALWAYS work right to left.
2. the check digit counts as digit #1 (assuming that the rightmost digit is the check digit) and is not doubled
3. double every second digit (starting with digit # 2 from the right)
4. remember that when you double a number over 4, (6 for example) you don't add the result to your total, but rather the sum of the digits of the result (in the above example 6*2=12 so you would add 1+2 to your total (not 12).
5. always include the Visa or M/C/ prefix.