| The Luhn Check:
Counting from the check digit, which is the rightmost, and moving left, double the value of every second digit.
Sum the digits of the products together with the undoubled digits from the original number.
If the total ends in 0 (put another way, if the total modulo 10 is congruent to 0), then the number is valid according to the Luhn formula; else it is not valid.
As an illustration, if the account number is 49927398716, it will be validated as follows:
- Double every second digit, from the rightmost: (1 × 2) = 2, (8×2) = 16, (3×2) = 6, (2×2) = 4, (9×2) = 18
- Sum all the individual digits (digits in parentheses are the products from Step 1): 6 + (2) + 7 + (1+6) + 9 + (6) + 7 + (4) + 9 + (1+8) + 4 = 70
- Take the sum modulo 10: 70 mod 10 = 0; the account number is valid.
- Mod 10+5 Variant
Some credit cards use the "Mod 10 plus 5" variant to extend the space of valid card numbers.[citation needed] In this variant, if the sum ends in 0 or 5, the number is considered valid
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