Editing POSTNET (section)

==Checkdigit algorithm==
The check digit is chosen so that the [[summation|sum]] of all digits in the bar code is a multiple of 10.  Equivalently, the [[Modulo operation|modulo]]-10 sum is 0.

To calculate the check digit:
#Add up the digits.  For example, if a letter is sent to [[Young America Township, Minnesota|Young America, Minnesota]], it might be sent to 55555-1237, which would have the sum of '''38'''.
#Find the remainder of this number when it is divided by 10, in this case '''8'''.  This is also known as the sum modulo 10.  A simple way to combine the two steps is to sum the digits without a tens column at all, but discard all carries.
#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 will always be a multiple of 10.  Continuing with the example above, (5+5+5+5+5+1+2+3+7+2) = 40, and 40 mod 10 = 0.

Note that the [[delivery point|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.