PostNet

What is a POSTNET barcode?

POSTNET (Postal Numeric Encoding Technique) is a linear barcode symbology developed by the United States Postal Service (USPS) for encoding ZIP codes and delivery point codes on mail pieces to enable automated sorting. Unlike conventional barcodes where data is encoded in bar width, POSTNET encodes data in bar height — each digit is represented by a combination of tall (full) and short (half-height) bars in a fixed 5-bar pattern. POSTNET was officially retired by USPS in 2009 and replaced by Intelligent Mail Barcode (IMB), though some legacy systems still reference it.

Symbol structure and height encoding

POSTNET uses a unique encoding method: each of the 10 digits (0–9) is represented by exactly 5 bars, of which exactly 2 are tall (full height) and 3 are short (half height) — a '2 of 5' pattern. The symbol begins and ends with a single tall frame bar. Data encodes the ZIP+4+delivery-point (11 or 5 digits) plus a correction digit. Because height (not width) carries data, bar width consistency is critical — all bars must be equal width.

Correction digit

POSTNET appends a single correction digit:

1. Sum all data digits.
2. Correction digit = (10 − (sum mod 10)) mod 10.

For a 5-digit ZIP: 5 digits + 1 correction = 6 encoded digits = 32 bars. For ZIP+4: 9 digits + 1 correction = 10 encoded digits = 52 bars. For ZIP+4+delivery point: 11 digits + 1 correction = 12 encoded digits = 62 bars.

POSTNET vs. Intelligent Mail Barcode (IMB)

POSTNET was retired by USPS in 2009. Intelligent Mail Barcode (IMB, also called OneCode) replaced it, encoding more information (including tray and container codes) in a single 65-bar symbol using four bar heights instead of two. IMB is required for all USPS automation discounts today. POSTNET documentation is retained for reference for legacy system operators.

When to use POSTNET

POSTNET should not be used for new implementations. USPS officially retired it on 28 January 2013 (after a transition period from 2009). Use Intelligent Mail Barcode (IMB) for all current USPS automation mail. POSTNET information is relevant only for:

• Legacy document archive reading.
• Maintaining backward compatibility in old mailing systems still being migrated.

Historical printing requirements

POSTNET bar height: full bars 0.125 in (3.18 mm); short bars 0.050 in (1.27 mm). Bar width: 0.020 in (0.508 mm) uniform. Bar pitch: 0.145 in (3.68 mm) centre-to-centre. Printed in black on white; placement on envelope bottom edge, 0.625–2.75 in from right, 0.125–0.625 in from bottom edge.

Historical scanning failures

POSTNET's height-based encoding meant that ink bleed or print smearing affecting bar height ratios was the primary failure mode. Envelopes with window film distortion and misaligned labels also caused misreads. These historical issues contributed to USPS's decision to adopt IMB's more robust four-height encoding.