USPS Tracking Number Format: Every Prefix

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A USPS tracking number is a 20- to 34-digit numeric code. The most common format is 22 digits starting with 9400 (Priority Mail) or 9205 (First-Class). The last digit is a check digit calculated by the Luhn algorithm. USPS assigns different prefixes to each mail class.

If you are looking at a long string of numbers—usually 20 or 22 digits—and trying to figure out when your mail will arrive, you are in the right place. USPS tracking numbers follow specific formatting patterns. Once you understand the prefixes (like 9400 or 9205), you can easily identify which mail class is carrying your shipment.

Here is a quick look at the most common USPS tracking formats:

How to decode a USPS tracking number

Unlike some carriers that use arbitrary serial codes, the United States Postal Service uses structured sequences. The standard domestic tracking number is a 22-digit string known as the Intelligent Mail Package Barcode (IMpb), which was officially adopted in January 2013 to standardize package tracking across commercial networks.

A standard 22-digit domestic tracking number breaks down as follows:

9400 1 28206 22794350011 5

• 9400 (Service Type Code): Identifies the specific class of mail (e.g., First-Class, Priority).
• 1 (Source ID): Identifies the shipping system or label provider.
• 28206 (Mailer ID): A unique 5-to-9 digit ID assigned by USPS to the specific retailer or shipper.
• 22794350011 (Serial Number): A unique sequence assigned to your individual package.
• 5 (Checksum): A final calculated digit used by scanning systems to verify the number is authentic and free of typos. USPS domestic 22-digit tracking numbers use a weighted Modulo 10 algorithm to determine this check digit (using alternating multipliers of 3 and 1). By contrast, international 13-character numbers ending in "US" use a Modulo 11 formula.

Note: Shippers often format these numbers with spaces in emails (e.g., 9400 1282 0622 7943 5001 15) to make them easier to read. However, you should remove all spaces when pasting the code into a tracking search.

Guide to common USPS prefixes

The first few digits of the tracking number tell you the exact service level used to ship your package.

9400 (First-Class Package & Priority Mail Express)

First-Class Package is the most popular option for lightweight e-commerce shipments under one pound. It usually takes 2 to 5 business days. Priority Mail Express is an overnight service. Because they share the 9400 prefix, you will need to check the active status details to confirm which speed was purchased.

9205 & 9407 (Priority Mail)

Priority Mail is the standard 1-to-3 business day service used by most online retailers. USPS uses two different prefixes for this class due to historical system updates, but both offer the same transit priority and basic insurance.

9270 (Parcel Select)

Parcel Select is a ground shipping option designed for high-volume commercial shippers. It is the slowest domestic tracked option (taking 2 to 8 business days). It is often used by third-party logistics networks that carry the package long-distance and hand it to USPS for the "final mile" delivery.

EC, EA, & CP (International Mail)

International tracking numbers use a 13-character alphanumeric format matching universal postal standards. The two-letter prefix (like EA for Express Mail or CP for Registered Parcels) is followed by 9 digits and ends with the country code US, indicating the origin country.

Troubleshooting common tracking errors

If your tracking search returns an error or fails to update, check for these common issues:

Pre-shipment status

If the status shows "Pre-Shipment Info Sent to USPS" or "USPS Awaiting Item," the merchant has printed the label but has not handed the package over to the post office. It is normal for this status to remain unchanged for 24 to 48 hours.

Confusing "0" with "O"

For domestic shipments, USPS tracking numbers are entirely numeric. If you see a character that looks like the letter "O," it is always the number zero (0). Alphanumeric codes are only used for international shipments, and the letters will only appear at the very beginning or end of the string.

Missing transit scans

USPS handles millions of mail pieces daily. To keep mail moving, packages are sorted in batches. A package traveling cross-country might go 3 or 4 days without an active scan as it moves between large regional distribution hubs. The package is still in transit; it just has not passed a checkpoint scanner.

FAQs

Can I track a USPS package without a tracking number?

Yes. If you sign up for Informed Delivery (a free service on the USPS website), you will receive daily email previews showing images of incoming letter mail and active tracking links for any packages addressed to your residence, even if the sender did not provide a tracking number.

What should I do if my package is marked "Delivered" but is missing?

First, check your mailbox, front porch, and side doors. If you live in an apartment complex, check the shared mail room or look for a parcel locker key in your primary mailbox. Occasionally, carriers scan packages as "delivered" while they are still on the delivery vehicle. If it does not arrive by the next business day, contact your local post office immediately so they can look up the GPS coordinates of the delivery scan.

What does "In Transit, Arriving Late" mean?

This status indicates that the package has missed its expected delivery window due to transit delays, weather, or high volume. The package is still in the network and will be delivered, but USPS does not provide a new estimated date until the package is scanned at your local facility.

How long does tracking history remain active?

USPS keeps domestic tracking data active for 120 days after delivery. For certified or registered mail, the records are kept for up to two years.

Related Guides

• UPS Tracking Number Format
• FedEx Tracking Number Format
• DHL Tracking Number Format
• How to Track a Parcel

Source: Official Intelligent Mail Package Barcode specifications from usps.com.