The Intelligent Mail barcode, for one letter at a time

What the bars are

An Intelligent Mail barcode, or IMb, is the row of 65 short and tall bars that sits above the address on most business mail. USPS's Domestic Mail Manual describes it as "65 vertical bars, each representing one of four possible states: full bar, ascender, tracker, and descender." Those bars carry up to 31 digits, which USPS splits into a 20 digit tracking code and a routing code of up to 11 digits.

USPS's sorting machines read the barcode on every pass, which is how a letter that carries one gets a scan at each plant. The bars are printed at about 22 to the inch, so the whole barcode is roughly 3 inches long.

What the 31 digits hold

The first four fields make up the 20 digit tracking code, and together they point to one mailpiece. The encoder then turns all of the digits into bars along with an 11 bit check value, and it spreads every digit across the whole barcode. That is why one misread bar makes the check fail instead of quietly changing a digit, and why a decoder needs all 65 bars to read any of it. Our free Intelligent Mail barcode decoder reads the bars or the digits and names each field.

How one stamped letter gets a barcode USPS reports on

A barcode only turns into scans you can see when four things line up, and the same four apply to one letter as to a million.

1. The barcode carries a Mailer ID that belongs to you or to the service printing it. USPS gives one 9 digit Mailer ID to a business location with no volume requirement, through its Business Customer Gateway.
2. The Mailer ID is signed up for Informed Visibility, USPS's free mail tracking service, which the Domestic Mail Manual says "is available at no charge without a subscription."
3. The Service Type ID asks for tracking. For a stamped First Class letter with no address corrections, that is 310 in USPS's table.
4. The routing code matches the address. The manual says an outgoing piece "must bear an Intelligent Mail barcode that correctly corresponds to the delivery address," so the ZIP+4 and delivery point come from USPS's own address data, and the serial must be one no other piece is using.

The barcode goes either in the address block, just above the name, or in the barcode clear zone along the bottom right edge, and the letter has to be machinable so the machines can read it. With all of that in place, a single envelope with a Forever stamp is scanned and reported the same way a bank statement is. PigeonTrail handles the Mailer ID, the serials, the address check and the printing, so a seller only adds the stamp.

Why a free barcode generator won't show you scans

A free generator draws bars for whatever digits you type, and USPS will happily read them, but the scans go to the owner of the Mailer ID in the barcode. USPS's release notes from September 2026 say that only "mailers who own a particular" Mailer ID, or who were given access to it, can receive its piece data through Informed Visibility. A made up Mailer ID sends the scans nowhere, and someone else's sends them to that business. The barcode works for tracking only when you can receive the data behind it.

What the scans show, and what they don't

Each time a sorting machine reads the barcode, USPS records the time and the plant, so you can follow a letter from the plant near you to the plant near the person you wrote to. USPS records no scan when you drop the letter in a box or when the carrier delivers it. A letter's delivered event is USPS's estimate from the carrier's route, and the manual says this service "does not provide a delivery scan or proof of delivery and not every piece is guaranteed such data or complete data." Our guide to what each USPS scan means walks through every event.

The barcode isn't a USPS tracking number either. USPS's tracking page doesn't take it, because, as USPS puts it, "First-Class Mail postcards, letters and flats are not eligible for USPS Tracking service." The scans reach the barcode's owner through Informed Visibility instead.