USPS Intelligent Mail Barcode (IMb)

The USPS Intelligent Mail Barcode (IMb) is the current United States postal standard for encoding routing and tracking information on letter mail, replacing POSTNET with a more capable four-state (height-modulated) bar code that carries far more data in a similar footprint while adding individual piece tracking.

POSTNET, covered elsewhere on this site as a legacy US postal symbology, used only two bar heights (tall and short) to encode ZIP code, ZIP+4, and delivery point digits with a simple checksum, and carried no per-piece tracking capability at all. The Intelligent Mail Barcode was developed by the USPS specifically to replace POSTNET with a single unified symbology that could simultaneously handle routing (the same job POSTNET did) and unique piece-level tracking (something POSTNET was never designed for), consolidating what had previously required separate POSTNET and tracking barcodes into one combined symbol.

Unlike POSTNET's two bar heights, IMb uses four distinct bar states, each bar extending in one of four ways relative to a central tracking line: full-height (ascending and descending), ascender-only (extending upward only), descender-only (extending downward only), or tracker-only (a short bar touching neither extreme). Because each bar position can take one of four states rather than just two, IMb encodes roughly two bits of information per bar rather than one, allowing it to carry substantially more data — enough for a full tracking code plus routing information — within a comparable print width to POSTNET.

• Barcode Identifier — a short field distinguishing mail classes and special services.
• Service Type Identifier — indicates the specific mail service and any requested address-correction or tracking options.
• Mailer ID — identifies the business or organization that produced the mail piece.
• Serial Number — a unique per-piece (or per-mailing) sequence number enabling individual item tracking, the core capability POSTNET lacked.
• Routing Code (optional) — the ZIP code, ZIP+4, or full 11-digit delivery point code, exactly as POSTNET encoded, allowing IMb to fully replace POSTNET's sortation function.

The full field set is protected end-to-end by a mathematical error-correction scheme far more robust than POSTNET's simple checksum, allowing reliable decoding even with modest print defects.

IMb is used across nearly all USPS letter mail classes today, printed by mailers ranging from large-scale bulk mail operations to individual postage-metering systems, and is required for mailers seeking USPS discounted postage rates tied to automation compatibility and delivery-point sortation. It is defined and maintained by the United States Postal Service itself, with technical specifications published for mailing software vendors, printer manufacturers, and large-volume mailers to ensure consistent encoding and print quality across the mail stream.

Because IMb relies on precise measurement of bar height relative to the tracking baseline, print quality and dimensional accuracy matter more than for simpler two-state codes like POSTNET — mail processing equipment expects consistent bar positioning and height within tight tolerances. Mailers implementing IMb typically use USPS-certified mailing software or barcode generation libraries rather than hand-rolling the encoding themselves, since correct Mailer ID assignment, serial number management, and encoding-table conformance all require adherence to USPS technical specifications rather than general barcode knowledge alone.