The bottoms of envelopes of some first class letters mailed with the USPS have a printed barcode on them, what does it encode
Executive summary
The barcode printed across the bottom of many First‑Class envelopes is the USPS Intelligent Mail barcode (IMb), a 65‑bar, 4‑state barcode that encodes a 20–31 digit numeric payload used to route, sort and track individual mailpieces; its fields include identifiers for the mail class/service, the mailer, a per‑piece serial number and a routing ZIP code, plus error‑detection bits [1] [2] [3] [4].
1. What the bars actually are: the Intelligent Mail barcode explained
The IMb replaced earlier systems like POSTNET and PLANET and is sometimes called OneCode or the 4‑State Customer Barcode; visually it’s a sequence of 65 bars whose four vertical “states” encode binary information that machines read to extract a numeric string of up to 31 digits [4] [5].
2. The fields inside the barcode: what each part represents
The IMb is constructed from two main components: a 20‑digit tracking code (Barcode Identifier, Service Type Identifier, Mailer ID and Serial Number) and a Routing Code of up to 11 digits that can hold 5‑, 9‑ or 11‑digit ZIP information (ZIP+4 and the 2‑digit delivery point) — together these fields identify the class of mail, requested services, the sending business and a unique per‑piece serial to facilitate tracking [3] [2] [6].
3. How unique and traceable is a single envelope?
When mailers use Full‑Service IMb or Origin IMb Tracing, the Mailer ID combined with the Serial Number can make a barcode unique to an individual envelope, enabling downstream tracking and visibility across the mailstream; the USPS explicitly documents that these fields can uniquely identify a mailpiece for tracing and reporting purposes [6] [5].
4. Routing, error checking and technical constraints
Beyond identification, the IMb effectively incorporates routing information previously carried by POSTNET: the Routing Code can include the full 11‑digit delivery point code so automation equipment can sort pieces to the carrier delivery sequence, and the IMb includes an 11‑bit CRC for error detection — leaving about 103 bits of payload in the specification [4] [5].
5. Operational notes and related marks (FIM and placement rules)
The IMb is required for automation prices on many letter classes and is normally printed in the barcode clear zone or address block; a Facing Identification Mark (FIM) on some letters tells mail processing machinery whether postage and/or an IMb are present and what type of postage is used, which affects downstream handling [1] [7] [8].
6. What this means practically and where reporting varies
Practically, the bottom‑of‑envelope barcode is a machine‑readable packaging of delivery and commercial metadata — routing instructions, class and service flags, and potentially a sender ID plus per‑piece serial that enables tracking for mailers and the Postal Service [2] [3] [6]. Popular explanations sometimes conflate POSTNET visual styles or over‑simplify the content as merely “ZIP codes”; official USPS documentation makes clear the IMb carries broader tracking and service data and includes technical safeguards like CRC bits [4] [1].