Have you ever looked at USPS tracking codes and noticed the first several digits look the same?

That’s by design. The United States Postal Service (USPS) doesn’t (fully) randomly generate tracking numbers. There’s a logical reason for it.

What’s going on with the first 4 digits?

The first five digits comprise the channel and the service level. Per USPS Publication 199 , the Intelligent Mail package barcode guide, the first two digits are the channel application identifier: 92 or 93 when a commercial mailer printed the label, 94 for online, PC Postage and postage-meter labels, and 95 for a Post Office counter. The next three digits are the service type code, and the various services for USPS each have unique codes. Below is a list of example tracking numbers

USPS Tracking: 9400 1119 0238 4716 5209 31
Priority Mail: 9205 5903 6152 7418 4630 24
Certified Mail: 9407 1119 0238 4716 7041 81
Registered Mail: 9208 8903 6152 7420 9385 18
Signature Confirmation: 9202 1903 6152 7433 1764 09

These are illustrative numbers with valid check digits. In each one, digits three to five are the service type code from USPS’s IMpb service type code list: 001 is USPS Ground Advantage with USPS Tracking, 055 is Priority Mail with USPS Tracking, 071 is First-Class Mail with Certified Mail, 088 is Priority Mail with Registered Mail, and 021 is USPS Ground Advantage with Signature Confirmation. Here is how one number breaks down:

Anatomy of a USPS tracking number

As printed: 9205 5927 4813 5604 1826 36

  1. 92 Channel application identifier. Who printed the label: 92 or 93 for a commercial mailer (nine- or six-digit Mailer ID), 94 for online, PC Postage and postage-meter labels, 95 for a Post Office counter. Every one starts with 9, which is why USPS tracking numbers do.

  2. 055 Service type code. The mail class plus any extra service. 055 is Priority Mail with USPS Tracking.

  3. 927481356 Mailer ID. The shipper's USPS-assigned ID. Nine-digit Mailer IDs always start with 9.

  4. 0418263 Serial number. The package's own number, unique for at least 120 days.

  5. 6 Check digit. Catches typos: weight the first 21 digits 3, 1, 3, 1 from the right, add them up (154), and the check digit is what it takes to reach a multiple of 10.

Illustrative number with a valid check digit. Layout from USPS Publication 199 (IMpb); the barcode also carries 420 and the destination ZIP Code in front, which USPS leaves off the printed tracking number.

How does package tracking work?

As packages make their way to the destination, the barcodes are scanned and thus updated in the USPS database. All labels created through I’d Ship That are trackable, so you can monitor your package’s progress in real time.

What about letters and cards?

Although letters and cards are assigned tracking numbers, they are not always able to be scanned properly so tracking these types of mail can be hit or miss. For example, a letter that has a tracking status of “in transit” may in fact have already been delivered.