Inside Windows Product Activation

Check digits

The rightmost digit in each of the groups is a check digit to guard against simple errors such as the call center agent's mistyping of one of the digits read to him or her. The value of the check digit is calculated by adding the other five digits in the group, adding the digits at even positions a second time, and dividing the sum by seven. The remainder of the division is the value of the check digit. In the above example the check digit for the first group (6) is calculated as follows.

position	1	2	3	4	5
digit	0	0	2	6	6

Step 1, add all digits:

0 + 0 + 2 + 6 + 6 = 14

Step 2, add digits at even positions:

0 + 6 + 14 = 20

Step 3, calculate remainder of division by 7:

20 % 7 = 6

The check digit is 6

Adding the even digits twice is probably intended to guard against the relatively frequent error of accidentally swapping two digits while typing, as in 00626 vs. 00266, which yield different check digits.