POSTNET — Postal Height-Modulated standard

Overview

POSTNET (Postal Numeric Encoding Technique) is a legacy, height-modulated barcode symbology developed by the United States Postal Service to encode ZIP Code routing information for automated mail sorting ^{1, 2}. It is a two-state symbology — every bar is either a tall (full) bar or a short (half) bar — and encodes each digit with a two-of-five code of five bars per digit ^{1, 2}. POSTNET was replaced by the Intelligent Mail Barcode in the fall of 2009 and lost automation-discount eligibility on January 28, 2013 ¹.

History

POSTNET was developed by the United States Postal Service as a barcode symbology to encode ZIP Code data for automated routing ¹. One barcode-history source states the concept emerged during the 1980s USPS modernization effort and that POSTNET was officially adopted by the USPS in 1982 ⁶.
The format evolved to encode progressively more precise routing: a 5-digit ZIP Code, a 9-digit ZIP+4 Code, and finally an 11-digit Delivery Point Barcode (DPBC) combining the ZIP Code, ZIP+4 Code, and delivery-point code ¹.
The Intelligent Mail Barcode (IMb) was introduced and POSTNET replaced in the fall of 2009 ¹. The USPS allowed mailers to keep receiving automation discounts using POSTNET until January 28, 2013, after which the Intelligent Mail Barcode was required for those reduced rates ¹.

Technical specification

POSTNET is a two-state, height-modulated 1D postal symbology ^{1, 2}:

Parameter	Value
Symbology family	1D height-modulated, two-state (tall / short bars)
Per-digit encoding	5 bars, two-of-five code (2 tall + 3 short)
Framing	starts and ends with a full bar (guard rail / frame bar)
Tall bar height	0.125 in ± 0.010 in
Short bar height	0.050 in ± 0.010 in
Bar width	0.015–0.025 in
Bar+space pair width	0.045–0.050 in
Density	~21 bars per inch (±1), ~4 characters per inch
Check digit	modulo-10
Character set	numeric only

Digit encoding — each digit is represented by five bars, two of which are tall ². The five bar positions carry weights of 7, 4, 2, 1, 0 from left to right; the two tall bars sum to the digit's value (with "0" encoded as the 7+4 combination, i.e. pattern 11000) ¹.

Variants and bar counts ¹:

Variant	Data	Total bars
5-digit "A" code	ZIP Code only	32
6-digit "B" code	(obsolete)	37
9-digit "C" code	ZIP + ZIP+4	52
11-digit DPBC	ZIP + ZIP+4 + delivery point	62

Check digit — POSTNET appends a single modulo-10 check digit chosen so that the sum of all digits in the barcode is a multiple of 10; it is computed by summing the data digits, taking the remainder modulo 10, and subtracting from 10 ^{1, 2}. This provides error detection only.

Use cases

ZIP Code routing for automated letter sorting — POSTNET's sole purpose was encoding ZIP, ZIP+4, or delivery-point routing data so high-speed USPS sorting equipment could direct mail ^{1, 2}.
Pre-printed business reply and bulk mail — mailers printed POSTNET on envelopes, business reply mail, and bulk mailings to qualify for automation-discount postage rates prior to the 2013 cutover ¹.
Delivery Point Barcode (DPBC) — the 11-digit DPBC variant identified mail down to an individual delivery point, the finest routing granularity POSTNET supported ¹.

Implementations

C — zint/zint — 771★, active 2026. Encoding library supporting POSTNET among 50+ symbologies.
JavaScript — metafloor/bwip-js — 2,381★, active 2026. Pure-JavaScript barcode writer supporting POSTNET and PLANET among 100+ types.
JavaScript decoder — Douglas Boynton Quine publishes a browser-based POSTNET / PLANET / IMb decoder ⁵.
Online encoder/decoder — dCode provides a POSTNET encoder/decoder/solver ⁷.

Comparison

vs. PLANET — PLANET is POSTNET's tracking companion and its direct geometric inverse: where a POSTNET digit uses two tall and three short bars, the corresponding PLANET digit uses three tall and two short bars ^{1, 6}. POSTNET encodes routing (ZIP) data; PLANET encoded tracking data for the USPS Confirm service ⁶. Both share the same two-state, two-of-five, modulo-10 framework and were used together on the same mailpiece.

vs. Intelligent Mail Barcode — The IMb is a four-state symbology (Tracker / Ascender / Descender / Full bars) with a fixed 65 bars and up to 31 digits, carrying both routing and tracking data plus an 11-bit CRC; POSTNET is two-state with at most ~14 digits, a single modulo-10 check digit, and routing data only ^{1, 6}. The IMb replaced POSTNET's routing role in 2009 ¹.

When to choose it — for new mail, never: POSTNET is deprecated and no longer qualifies for automation pricing. It remains relevant only for reading or reproducing legacy mailpieces and historical archives ^{1, 6}.

Fun facts

The "0" digit in POSTNET is encoded by the 7+4 weighted positions (pattern 11000) rather than by any zero-valued bars — a quirk of the two-of-five weighting scheme where the four numeric positions are 7, 4, 2, 1 and the fifth carries weight 0 ¹.
POSTNET and its inverse-twin PLANET were frequently printed on the same envelope at once — POSTNET for routing and PLANET for Confirm-service tracking — until the IMb collapsed both into a single symbol ⁶.

Status

Deprecated / superseded. POSTNET was replaced by the Intelligent Mail Barcode in the fall of 2009 and lost automation-discount eligibility on January 28, 2013 ¹. Dynamsoft's comparison lists POSTNET and PLANET as legacy barcodes that "should not be used," with the IMb as the replacement ⁶. Its successor is the Intelligent Mail Barcode ^{1, 6}.

Sources

POSTNET — Wikipedia — Wikipedia, accessed 2026-06-03
USPS POSTNET (Zip / DPBC) — Seagull Scientific Barcode Guide — Seagull Scientific, accessed 2026-06-03
zint/zint — GitHub — GitHub, accessed 2026-06-03
metafloor/bwip-js — GitHub — GitHub, accessed 2026-06-03
USPS POSTNET, PLANET, and IMB Barcode Decoder in JavaScript — Douglas Boynton Quine — quine.org, accessed 2026-06-03
Difference Between Intelligent Mail barcode, Postnet and Planet — Dynamsoft — Dynamsoft, accessed 2026-06-03
Barcode POSTNET Cipher — Decoder, Encoder, Solver — dCode — dcode.fr, accessed 2026-06-03

Deployments

No country reports mention this standard by name.