From the rightmost digit, which is the check digit, moving left,double the value of every second digit; if the product of thisdoubling operation is greater than 9 (e.g., 7 * 2 = 14), then sumthe digits of the products (e.g., 10: 1 + 0 = 1, 14: 1 + 4 =5).

Take the sum of all the digits.

If the total modulo 10 is equal to 0 (if the totalends in zero) then the number is valid according to the Luhnformula; else it is not valid.

Assume an example of an account number "7992739871" that willhave a check digit added, making it of the form 7992739871x:

Account number	7	9	9	2	7	3	9	8	7	1	x
Double every other	7	18	9	4	7	6	9	16	7	2	x
Sum of digits	7	9	9	4	7	6	9	7	7	2	=67

The check digit (x) is obtained by computing the sum of digitsthen computing 9 times that value modulo 10 (in equation form, (67* 9 mod 10)). In algorithm form:

Compute the sum of the digits (67).

Multiply by 9 (603).

The last digit, 3, is the check digit.

Credit Card error detection check -> Luhn Algorithm, (http://www.freeformatter.com/credit-card-number-generator-validator.html#cardFormats)