POSTNET is a barcode symbology that is used by the United States Postal Service to assist in directing mail. The ZIP Code or ZIP+4 code is encoded in this unique symbology that encodes data in half- and full-height bars. Most often, the delivery point is added, usually being the last two digits of the address or PO box number.

The barcode starts and ends with a full bar (often called a guard rail) and has a check digit after the ZIP or ZIP+4. The check digit is calculated as follows:

1. Add up all the data being encoded. If you are sending a letter to somewhere in Young America, Minnesota, you might be sending to 55555-1234, which would have the sum of 35
2. Find the number that would need to be added to this number to make it evenly divisible by 10, in this case 5, which is your check digit.

The encoding table is shown on the right. | denotes a full bar and ╷ denotes a half bar.

Each individual digit is represented by a set of five bars, two of which are full bars. The full bars represent "on" bits in a pseudo-binary code in which the places represent, from left to right: 7, 4, 2, 1, 0. (Though in this scheme, zero is encoded as 11, or "binary" 11000.)

Example

The above-mentioned example of 55555-1234 yields:

1. Sum is 35
2. The check digit is therefore 5
3. The data encoded will be 5555512345

Together with the initial and terminal guard bars, this would be represented as:

|╷|╷|╷╷|╷|╷╷|╷|╷╷|╷|╷╷|╷|╷╷╷╷||╷╷|╷|╷╷||╷╷|╷╷|╷|╷|╷|

Note that the delivery point is often added after the ZIP+4 and before the check digit.