What is POSTNET?
POSTNET is the USPS postal barcode made of tall and short bars that encodes an address's ZIP code: either the 5-digit ZIP, the 9-digit ZIP+4, or the 11-digit ZIP+4 plus delivery point. Each digit is a pattern of exactly two tall bars among five, with a check digit framing the code. It powered USPS automation sorting for decades and still appears on legacy mail pieces, internal mail systems and mailroom software that was built around it.
Common Uses for POSTNET
- Bulk mail pieces sorted with legacy mailroom software
- Envelope and label barcodes encoding ZIP+4 addresses
- Direct-mail templates that still specify POSTNET
- Teaching and archival reproductions of classic US mail barcodes
POSTNET vs Intelligent Mail (IMb)
The Intelligent Mail barcode replaced POSTNET across USPS mail. IMb encodes far more - a 31-digit payload combining a tracking code with the routing ZIP+4+delivery point - in 65 height-modulated bars, and it merges what used to require POSTNET plus the PLANET barcode. If you are preparing new bulk mail, generate IMb through a USPS-approved mail preparation workflow; use POSTNET only when an existing system or form explicitly requires it.
The POSTNET check digit
Every POSTNET symbol ends with a check digit before the closing frame bar. The rule: add all the data digits, and the check digit is the amount needed to bring the total to the next multiple of 10 - the same modulo-10 idea the EAN/UPC family uses. The generator computes it automatically, so entering 12345, 123456789 or 12345678901 always produces a complete, valid symbol.
Printing POSTNET on envelopes and labels
Print the barcode in the address block area, typically 1/4 to 1/2 inch above the bottom edge of the mail piece, at the height the original specification intended - roughly 1/8 inch between bar tops and bottoms. Keep the surrounding area clear of text and graphics, print in plain black at best quality, and test-scan one piece with your mailroom reader before running the batch.
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