USPS Intelligent Mail

The Intelligent Mail barcode (IMb) is a Postal Service™ barcode with 65 bars. According to the USPS, it is used to sort and track letters and flats. It allows people mailing things to use a single barcode to simultaneously participate in multiple Postal Service programs. It also increases a user’s ability to track individual items and provides mail stream visibility. For these reasons, the Intelligent Mail barcode has been referred to as a One Code Solution.

Intelligent Mail barcode Specifications and Imaging Geometry

The Intelligent Mail barcode is a height-modulated barcode. Each code has 65 vertical bars, and there are four distinct symbols.

USPS Intelligent Mail

Each bar contains a central tracker portion and may also contain an ascender, descender, neither, or both. For this reason, Intelligent Mail barcode has also been referred to as 4-State Customer Barcode, abbreviated 4CB, 4-CB or USPS4CB.

In almost all cases Intelligent Mail® barcodes must contain a valid Mailer Identifier (MID). This MID is required for an IMb® tracking code. Both are provided by the USPS.

Data Payload

The Intelligent Mail barcode encodes up to 31 decimal digits of information. There are five IMb fields: Barcode Identifier, the Service Type Identifier, the Mailer ID, the Serial Number, and the Routing Code (AKA Zip Code).

USPS Intelligent Mail
  • The Barcode Identifier field is almost always a double zero.
  • The Service Type Identifier (STID) identifies the class of mail, ACS service desired, and if there is IMb tracing.
  • The Mailer Identifier (MID) is a six- or nine-digit field that identifies a mailer. A MID is required for an IMb® tracking code and these are provided via the USPS.
  • The Serial Number can be used alongside a MID and class of mail to identify mail.
  • The Routing Code often contains a 5-digit ZIP Code™, 9-digit ZIP+4® code, or 11-digit delivery-point code.

Error Detection

The 65 vertical bars represent 130 bits that are grouped as ten 13-bit characters. Each character has 2, 5, 8, or 11 of its 13 bits set to one. The Hamming distance between characters is at least 2. Consequently, adding or deleting one bit results in an invalid character, i.e., single-bit errors in a character can be detected. And 27 of the 130 bits are devoted to error detection.

The Intelligent Mail barcode does not have error correction encoded.