What is a check digit?

A check digit is the twelfth and final number in a USPS bulk mail barcode string and is used by the USPS to detect barcode errors.

The first eleven digits depict three groupings: the delivery zone (your 5-digit ZIP code), the region within the zone (+4) and the exact location for the mail drop (+2 , the delivery point (DP), or the last two digits of a mailbox or flat).

The check digit is calculated by adding up the eleven digits, then subtracting the last digit of that result from 10. To illustrate this, the total, when adding up the 11 digits associated with the company I work for, is 34. Since 4 is the last digit in that number, I subtract it from 10 and get our check digit, 6.

You can also use an online ZIP+4 lookup that returns the check digit at the end of the barcode string. Try http://bit.ly/ZIPplus4

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