POSTNET: What POSTNET Is and How It Worked

Overview

POSTNET (Postal Numeric Encoding Technique) was a barcode symbology developed by the United States Postal Service to represent ZIP Code and delivery-point information as machine-readable bars. Introduced in the 1980s, POSTNET helped postal facilities automate routing and sort large volumes of mail more quickly and accurately than manual methods. Although largely replaced by newer systems today, POSTNET played a major role in modernizing mail processing and is still a useful concept for understanding postal automation.

How POSTNET Encoded Information

At its core, POSTNET encoded only digits (0–9) plus a check digit. The standard POSTNET barcode represented either the 5-digit ZIP Code, the 9-digit ZIP+4, or the full 11-digit code that included a 2‑digit delivery point. Each digit was represented by a pattern of five bars: two tall (full-length) bars and three short (half-length) bars. The combination of tall and short bars across those five positions formed a unique pattern for each numeral.

The basic structure of a POSTNET barcode included a start frame bar, the series of encoded digits (including the check digit), and an end frame bar. The check digit was calculated so the sum of all digits, including the check digit, would be a multiple of 10—this helped scanners detect reading errors.

Typical Encodings

Common formats were:

• 5-digit ZIP (e.g., 02115)
• 9-digit ZIP+4 (e.g., 02115-1234)
• 11-digit delivery point code (ZIP+4 plus 2-digit delivery point)

For example, the ZIP+4 "02115-1234" would be converted into its numeric sequence, a check digit appended, and that full string translated into bar patterns between the frame bars.

Why POSTNET Was Useful

POSTNET offered several practical advantages:

• Speed: Machine-readable barcodes allowed sorting equipment to process mail far faster than manual sorting by human operators.
• Accuracy: The check digit and binary-style patterns reduced read errors and improved delivery precision.
• Cost efficiency: Faster sorting reduced labor and throughput costs for postal operators.

Postal users—businesses sending mass mailings, direct-mail marketers, and fulfillment centers—benefited from lower postage rates or discounts if they met USPS standards for barcode quality and addressing accuracy.

Scanning and Print Requirements

POSTNET barcodes required good print quality and proper placement on envelopes (typically the lower-right area). Scanners were designed to read the tall and short bars reliably, but smudges, incorrect dimensions, or placement too close to other markings could cause read failures. The USPS published specifications for bar height, quiet zones (blank margins), bar widths, and acceptable contrast to ensure scanners could decode barcodes consistently across different mail pieces.

Limitations and Why It Was Replaced

POSTNET encoded only numeric address elements. As mailing complexity and data requirements grew, the USPS developed the Intelligent Mail Barcode (IMb) to incorporate more data—such as tracking and extra service codes—into a single symbology. IMb provides higher data density and supports enhanced tracking features that POSTNET could not offer. As a result, POSTNET was gradually phased out in favor of IMb; the USPS stopped requiring POSTNET for automation discounts and encouraged migration to IMb.

Legacy and Practical Takeaways

Even though POSTNET is largely retired, understanding it is valuable for anyone learning postal automation history or working with legacy systems. POSTNET taught important lessons about standardized addressing, the value of check digits for error detection, and the importance of clear print standards for machine readability. If you handle old mail archives, maintain legacy printers, or study postal systems, recognizing POSTNET patterns—tall and short bars grouped into framed sequences—will help you identify and interpret older barcoded mail.

Real-world example

Picture a nonprofit preparing a large donor appeal in the 1990s: they would run their address list through address-verification software, append ZIP+4 codes and delivery points where available, calculate check digits, and print POSTNET barcodes on each envelope. A correctly printed POSTNET barcode allowed postal sorting machines to read and route the mail automatically, improving delivery speed and qualifying the nonprofit for lower bulk postage rates.

Conclusion

POSTNET was a simple, robust encoding system that revolutionized mail sorting by translating ZIP Code digits into easily scannable bar patterns. While superseded by more feature-rich codes like IMb, POSTNET remains an important part of postal automation history and a useful reference point for understanding how machine-readable addressing developed.