USPS tracking numbers start with a 9 because the first two digits are not random. They are a channel code that says who printed the label, and the channel codes USPS uses on domestic package labels are 92, 93, 94 and 95. The digits after it identify the service, the shipper and the individual package.
The layout comes from USPS Publication 199, the implementation guide for the Intelligent Mail package barcode (IMpb), which is the barcode printed on USPS package labels. The details below were checked against version 35 of that guide, dated July 2, 2026.
The five parts of a USPS tracking number
Publication 199 (section 4.2) says a USPS tracking number contains a channel application identifier, a service type code, a Mailer ID, a serial number and a check digit. Here is one number split into those parts:
Anatomy of a USPS tracking number
As printed: 9205 5927 4813 5604 1826 36
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.
055 Service type code. The mail class plus any extra service. 055 is Priority Mail with USPS Tracking.
927481356 Mailer ID. The shipper's USPS-assigned ID. Nine-digit Mailer IDs always start with 9.
0418263 Serial number. The package's own number, unique for at least 120 days.
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.
| Part | Length | What it tells you |
|---|---|---|
| Channel application identifier | 2 digits | Who produced the label |
| Service type code | 3 digits | The mail class and any extra service |
| Mailer ID | 6 or 9 digits | The shipper or label provider, assigned by USPS |
| Serial number | varies with the label layout | The individual package |
| Check digit | 1 digit | A calculated digit that catches typing and scanning errors |
The example above is 22 digits long. Publication 199 (Appendix J) also defines label layouts that produce 26 and 30 digit tracking numbers, so a longer number is not by itself a sign that something is wrong.
The first two digits: who printed the label
The channel application identifier is the reason for the 9. Appendix J of USPS Publication 199 lists four values:
| First two digits | Channel |
|---|---|
| 92 | Commercial mailer with a nine-digit Mailer ID |
| 93 | Commercial mailer with a six-digit Mailer ID |
| 94 | Online and PC Postage labels, USPS web tools and APIs, postage meters |
| 95 | USPS retail, including self-service kiosks |
A label bought online or through shipping software falls under 92, 93 or 94. A label produced at a Post Office falls under 95.
On a 94 label, the two digits after the service type code are a source identifier. USPS assigns one to each PC Postage or meter vendor, so those two digits point to the system that produced the label.
Digits three to five: the service
The next three digits of a USPS tracking number are the service type code. USPS publishes the full list as a spreadsheet, the IMpb Service Type Code list. The edition checked here is effective June 24, 2026. A few codes that sellers see often:
| Service type code | Meaning |
|---|---|
| 001 | USPS Ground Advantage up to 15.999 oz, USPS Tracking |
| 346 | USPS Ground Advantage one pound or more, USPS Tracking |
| 055 | Priority Mail, USPS Tracking |
| 490 | Media Mail, USPS Tracking |
| 021 | USPS Ground Advantage up to 15.999 oz, Signature Confirmation |
| 071 | First-Class Mail, Certified Mail |
| 088 | Priority Mail, Registered Mail |
Put the first two parts together and the familiar prefixes explain themselves:
9400 1… an online label (94) for a USPS Ground Advantage package under a pound (001)
9205 5… a commercial mailer’s label (92) for Priority Mail (055)
9407 1… an online label (94) for Certified Mail (071)
The service type code describes the service on the label. Two packages sent on the same day by the same seller can have different prefixes if one went Priority Mail and the other went USPS Ground Advantage.
The Mailer ID and serial number
The Mailer ID identifies the shipper or the label provider. USPS assigns it, and Publication 199 (section 1.4) sets a simple rule: a nine-digit Mailer ID begins with 9, and a six-digit Mailer ID begins with 0 through 8. On a 92 label, that puts a second 9 at digit six.
The serial number is the part that belongs to one package. USPS requires the serial number to stay unique for 120 days (Publication 199, section 4.1). After that window a mailer may use the same number again, which is one reason an old tracking number can bring up a shipment that is not yours.
The last digit: a check digit
The final digit of a USPS tracking number is calculated from all the others with a MOD 10 formula (Publication 199, section 4.6). Take the number without its last digit. Starting from the right, multiply every other digit by 3, add the remaining digits as they are, and total everything. The check digit is the amount that brings the total up to the next multiple of 10.
The guide’s own example is 9212 3912 3456 7812 3456 70. The weighted total is 180, already a multiple of 10, so the check digit is 0.
For a seller, the practical point is that one mistyped digit produces a number that fails this test. If a marketplace or a carrier site rejects a tracking number as invalid, compare it digit by digit with the label before assuming the label is bad.
What the barcode holds that the printed number leaves out
The barcode on a USPS label carries more than the printed digits. It begins with 420 and the destination ZIP Code, which USPS uses for routing. Publication 199 (sections 4.1 and 4.9.3) says the 420 and the ZIP Code are left out of the human-readable number, and the rest is printed in groups of four.
If you scan a label and get a number that starts with 420, you have read the whole barcode. Drop the 420 and the ZIP Code that follows it. The remaining digits, starting with 9, are the tracking number.
What about letters and cards?
Ordinary letters are not eligible for USPS Tracking. The USPS Domestic Mail Manual says USPS Tracking is available for all products except Periodicals and First-Class Mail letters and flats (DMM 503, Exhibit 1.4.1). A letter gets a number when an extra service is added, such as Certified Mail, which has its own service type code (071 for First-Class Mail).
USPS numbers that do not start with 9
International shipments use a different format. Publication 199 (section 3.1.3) refers to a 13-character customs barcode for international items, and the USPS claims page describes tracking and label numbers as between 13 and 34 characters.
Related reading
- Where to find a tracking number and how each carrier’s format differs: tracking number, defined
- What each scan on a tracking page means: USPS tracking statuses
- What a tracking record can and cannot prove in a dispute: What a tracking number proves
Sources
USPS Publication 199, version 35 (July 2, 2026), sections 1.4, 3.1.3, 4.1, 4.2, 4.6, 4.9.3 and Appendix J; USPS IMpb Service Type Code list, effective June 24, 2026; USPS Domestic Mail Manual 503. All checked October 12, 2026. The example numbers in this post are illustrative, with valid check digits, and do not belong to real packages.