POSTNET - Checkdigit Algorithm

Checkdigit Algorithm

The check digit is 10 minus (the digit-sum mod 10). That is, the check digit is calculated by:

1. Add up the digits, including the delivery point code if it is being used. If you are sending a letter to somewhere in Young America, Minnesota, you might be sending to 55555-1237, which would have the sum of 38.
2. Find the remainder of this number when it is divided by 10, in this case 8. This could also be called the sum modulo 10. (Note that when summing the digits, any leading 1 in the 10's column of the current sum may be discarded.)
3. Subtract the sum modulo 10 from 10. Continuing with the example, 10 - 8 = 2. The check digit is therefore 2.

If calculated correctly, the sum of the ZIP, ZIP+4, or ZIP+4+delivery point digits and the check digit, modulo 10, will always be zero. Continuing with the example above, (5+5+5+5+5+1+2+3+7+2) mod 10 = 0.

Note that the Delivery Point is often added after the ZIP+4 and before the check digit, in which case the computation of the check digit includes the ZIP+4 and the Delivery Point.