A service type code sits near the front of a USPS number and identifies the mail class and any extra services attached to the piece.
What Ground Advantage bundles in
The reason this service dominates small-parcel searching is that the extras are not extras.
USPS puts the class at a 2-5 day expected delivery with a 70 lb weight ceiling and $100 of coverage included, from usps.com/ship/ground-advantage.htm retrieved 2026-07-27. On the tracking side its basics article is explicit: there is no additional cost for the tracking mechanism on products that automatically include USPS Tracking, and you pay the product's postage. The one carve-out it names is USPS Marketing Mail parcels, where tracking is a paid add-on.
The same article rules a category out entirely: First-Class Mail postcards, letters and flats are not eligible for USPS Tracking service, though letters and flats can carry extra services such as Certified Mail that generate a barcode of their own. So an envelope with no barcode has no number to look up, and no amount of searching will produce one.
| Carrier | Service | Price | Transit window returned |
|---|---|---|---|
| USPS | Ground Advantage | $9.91 | 8-9 business days |
| UPS | Ground | $10.31 | 2-3 business days |
| UPS | 3 Day Select | $15.13 | 5-6 business days |
| UPS | 2nd Day Air | $16.58 | 2-3 business days |
| FedEx | 2Day | $21.90 | 2-3 business days |
What the digits are actually doing
USPS publishes a barcode specification that spells this out field by field. It is the closest thing to a documented answer that exists.
| Field | Length in digits | What it does |
|---|---|---|
| Channel application identifier | 2 | Identifies the induction channel the piece came from. Valid values are 91, 92, 93, 94 and 95, with 92 and 93 for commercial mailers, 94 for PC Postage and online systems, and 95 for the retail counter. |
| Service type code | 3 | Identifies the mail class or product and the presence of any extra services. |
| Mailer identifier | 6 or 9 | The number USPS assigns to identify each mailer. |
| Serial number | 5, 7, 8, 10, 11 or 14 | Makes the piece unique. Nine-digit mailer identifiers use a 5, 7 or 11 digit serial; six-digit ones use 8, 10 or 14. |
| Mod 10 check digit | 1 | The final digit, calculated from the package identification code so a mistyped number fails validation. |
| Destination ZIP Code | 5 or 9 | Present in the barcode for routing and suppressed from the printed human-readable text. |
The specification shows worked examples with human-readable strings of 22, 26 and 30 digits depending on the construct in use. That range is why a flat digit count is the wrong question: the number of characters follows the barcode construct, not the service you bought.
Two practical consequences. A number that fails to load is often a transcription error caught by the check digit rather than a dead shipment. And USPS's own advice on locating the number is refreshingly plain: the tracking number is the series of numbers under the barcode.
How UPS and FedEx numbers differ
If you are trying to work out which carrier a number belongs to, the shape usually gives it away.
| Carrier | Published formats | Source |
|---|---|---|
| UPS | Tracking numbers appear as 1Z9999999999999999, 999999999999, T9999999999 and 999999999. A UPS delivery notice carries a reference number of 12 digits. UPS Mail Innovations pieces can carry a USPS delivery confirmation number described as a 22 to 34 numerical identifier, or an 18-digit internal sequence number. | What is a Tracking Number, ups.com |
| FedEx | A door tag number starts with DT followed by 12 numbers and can be entered into the tracking tool in place of the original number. | Door tag help, fedex.com |
| USPS | No per-service digit table is published on the customer help pages. The barcode specification describes 22, 26 and 30 digit human-readable constructs. | USPS2000508, postalpro.usps.com |
The leading 1Z is the single most reliable identifier in US parcel shipping, because no other carrier uses it. Beyond that, guessing from digit count alone is unreliable, which is why every carrier's tracking box accepts a paste and tells you immediately whether the number belongs to it.
How long the record lasts, and what to do about it
| Service | Record kept for |
|---|---|
| USPS Tracking | 120 days |
| Signature Confirmation | 1 year |
| Priority Mail Express | 2 years |
| Certified Mail | 2 years |
| Registered Mail | 2 years |
| Adult Signature Required and Adult Signature Restricted Delivery | 2 years |
| Numbered international mail services | 2 years |
- At purchase: save the number somewhere other than a browser tabUSPS states the Postal Service cannot track or locate an item without the tracking number, so the number is the shipment as far as any support conversation is concerned.
- Within the transit window: expect scan gaps, not a live feedUSPS says scanning information is updated through the day as scans become available, which is not the same as continuously.
- At the point of a dispute: screenshot the historyA standard USPS Tracking record is held 120 days. A marketplace case opened five months later will not have one to point at.
- Before day sixty, if something went wrong: fileUSPS puts the outer limit for damaged or missing contents at 60 days from the date of mailing.
The service type code is the interesting field
It encodes both the class and any extra services, which is how a scanner knows a piece needs a signature without anyone reading the label. It is also why two numbers for the same class often share a recognisable opening run.
A number that will not load is usually mistyped
The check digit exists precisely to catch that. Retype from the label rather than from a screenshot before assuming the shipment is missing.
What we do with the number
Labels bought through SMKlog keep their tracking number attached to the order, and our tracking page reads the carrier signal for it. The number itself is issued by the carrier, and it works everywhere the carrier works.
Tracking number questions
Does Ground Advantage include tracking?
Yes, at no extra charge. USPS states there is no additional cost for the tracking mechanism on products that automatically include USPS Tracking, and that you simply pay the product's postage. The exception it names is USPS Marketing Mail parcels, where adding tracking requires a fee.
What is a USPS tracking number made of?
The USPS barcode specification describes a package identification code as an application identifier, a service type code, a data string that varies by construct and a mod 10 check digit. The service type code is a three-digit field identifying the mail class or product and the presence of any extra services.
How many digits should a USPS tracking number have?
The customer-facing USPS help pages do not publish a digit count per service, and we will not invent one. The barcode specification shows human-readable strings of 22, 26 and 30 digits depending on which construct produced the label, with the destination ZIP portion suppressed from the printed text.
How long does tracking stay available?
USPS publishes the retention by service: 120 days for USPS Tracking, 1 year for Signature Confirmation, and 2 years each for Priority Mail Express, Certified Mail, Registered Mail and numbered international services.