USPS address verification checks an address against the Postal Service’s Address Management System (AMS) and returns a USPS standardized address: the correct street abbreviations, the ZIP+4 code, and a Delivery Point Validation (DPV) result that says whether mail is actually delivered there. For a single address, the free USPS ZIP Code Lookup tool does this in seconds; for whole mailing lists, mailers use CASS-certified software.
This guide covers the USPS side specifically: what a standardized address looks like, how the AMS database and DPV codes work, what CASS certification requires, and what changed when USPS retired its Web Tools APIs in January 2026. If you ship with several carriers and want the general checklist, our guide on how to verify an address before you ship walks through the step-by-step process.
What Is USPS Address Verification?
USPS address verification is the process of matching an address to the official USPS address database and correcting it to postal standards. The Postal Service serves about 170.4 million delivery points nationwide (FY2025), and every one of them has a single standardized form in the AMS file. Verification answers three questions: does this address exist, is it complete, and does USPS deliver mail to it?
When you submit an address, the matching engine runs it through several layers:
- Address matching: the street, city and state are matched to a known address range in the AMS file.
- ZIP+4 assignment: the four-digit add-on that identifies a specific delivery segment, such as one side of a block or one floor of a building, is appended.
- Delivery Point Validation: DPV confirms that the exact house or unit number is a real delivery point, not just a number that falls inside a valid range.
- Standardization: the address is rewritten in the format of USPS Publication 28, with uppercase letters, standard suffix abbreviations (ST, AVE, BLVD) and secondary designators (APT, STE).
What Does a USPS Standardized Address Look Like?
A USPS standardized address follows Publication 28, the Postal Service’s addressing standard. It uses capital letters, no punctuation except the hyphen in the ZIP+4 code, standard abbreviations, and the secondary unit on the same line as the street address. The last line holds the city, the two-letter state code and the ZIP+4 code.
| As typed by a customer | USPS standardized address |
|---|---|
| 123 north main street, apartment 4b | 123 N MAIN ST APT 4B |
| Springfield, Illinois 62701 | SPRINGFIELD IL 62701-1234 |
| 500 Oak Avenue Suite #210 | 500 OAK AVE STE 210 |
| P.O. Box 77 | PO BOX 77 |
The ZIP+4 values above are illustrative. The point is the pattern: standardized addresses read well on automated sorting equipment, so they move through the network with fewer manual touches. Putting the unit number on the street line, rather than on a separate line above it, is the single most common fix verification makes.
How Do You Verify an Address With USPS for Free?
USPS offers address verification free of charge through three channels, each suited to a different volume.
ZIP Code Lookup for single addresses
The USPS ZIP Code Lookup tool at tools.usps.com is the quickest option. Choose “Find by Address,” enter the street, city and state, and the tool returns the standardized address with its ZIP+4 code. If the address can’t be matched, the tool says so, which is your cue to confirm the details with the recipient.
USPS APIs for developers and online stores
USPS retired its legacy Web Tools APIs on January 25, 2026. The replacements are listed in the API catalog on the USPS APIs developer portal, which includes an Addresses API for standardizing addresses and looking up ZIP codes. The new APIs use OAuth 2.0 tokens and JSON requests instead of the old XML calls, so any store or script still pointing at Web Tools endpoints needs to be updated.
CASS-certified software for mailing lists
Businesses that process thousands of addresses at once, especially for bulk mail, run their lists through CASS-certified address-matching software. CASS output is what USPS accepts as proof of address quality for automation and presorted mail.
What Is CASS Certification?
CASS, the Coding Accuracy Support System, is the USPS program that tests and certifies address-matching software. Developers process a test file of about 150,000 addresses, and the CASS department grades the results against strict accuracy minimums. Only software that passes may produce the PS Form 3553 summary that mailers submit with automation-rate mailings.
Testing has two stages. Stage I is an optional self-test that comes with answers so developers can debug their software. Stage II is the required test, with no answers supplied, and it decides certification. Certification runs by cycle; the current cycle, Cycle O, expires July 31, 2028.
| CASS component | What it does | Minimum Stage II score |
|---|---|---|
| Five-digit ZIP, ZIP+4 and carrier route coding | Assign the correct ZIP Code, add-on and delivery route | 98.5% |
| LACSLink | Converts old rural-route or renamed-street addresses to the current city-style address | 98.5% |
| Delivery point coding | Builds the 11-digit delivery point barcode value | 100% |
| DPV or DSF2 | Confirms the exact address is a deliverable point | 100% |
| eLOT | Assigns the carrier’s line-of-travel sequence | 100% |
| RDI | Flags whether the address is residential or business | 100% |
You don’t need CASS software to check a handful of addresses. It matters when you mail at volume and want your list to qualify for automation and presort prices, or when your shipping software runs every order through a certified engine before printing labels.
What Is the USPS Address Management System?
The AMS database is the master file behind every USPS address check, whether it’s the ZIP Code Lookup page, the Addresses API or a CASS engine. It holds residential, business, PO Box and military (APO/FPO/DPO) delivery points, and it changes constantly as new homes, businesses and units are built: USPS added about 1.8 million delivery points in FY2025 alone.
Because of that churn, USPS issues updated AMS data to licensed software providers on a regular schedule, and CASS-certified products must use a current file. A list verified a year ago can go stale, so high-volume mailers re-run their lists before each large mailing rather than relying on an old pass.
What Do DPV Codes Mean?
The DPV confirmation code tells you how much of the address USPS could confirm. It’s the most useful field for deciding whether to print a label or contact the customer first.
| DPV code | Meaning | What to do |
|---|---|---|
| Y | Primary address and any secondary unit confirmed | Ship as entered |
| D | Building confirmed, but the apartment or suite number is missing | Ask the recipient for the unit number |
| S | Building confirmed, but the unit number given could not be confirmed | Check the unit number; it may be new or mistyped |
| N | Address not confirmed as a delivery point | Hold the order and contact the recipient |
A “D” result is the classic multi-unit problem: the package reaches the right building but has no apartment to go to. Catching it before label creation saves a return and a reship. If a label has already been printed with a bad ZIP, our guide to a wrong ZIP code on a USPS package covers the fixes.
How Does Address Verification Affect USPS Mailing Requirements?
For automation and presorted mail, address quality is a requirement, not a nice-to-have. Mailers submit a PS Form 3553 from CASS-certified software to show the addresses were coded by an approved engine. Mail claiming these prices must also meet the USPS Move Update standard, which means updating the list for customer moves (typically through NCOALink) within 95 days before the mailing.
Individual package shippers don’t file any of these forms. For them, verification is about avoiding undeliverable packages, address-correction returns and unhappy customers.
What Are the Limits of USPS Address Verification?
USPS verification handles most domestic addresses correctly, but a few categories need a manual check:
- New construction: brand-new homes and units may not appear in the AMS data until the local post office adds them.
- Rural and non-standard addresses: properties on unnamed roads or without house numbers may not match cleanly.
- Military and diplomatic mail: APO, FPO and DPO addresses follow their own format rules, with the unit and box on the street line.
- Addresses USPS doesn’t deliver to: some locations receive mail only at a PO Box, so a street address can be real yet return DPV “N” for mail delivery.
An unverifiable address is not always a wrong one. When the tool can’t confirm it, ask the recipient or the local post office before you ship. Our USPS locations guide helps you find the post office that serves an area.
Does USPS Address Verification Work for UPS Shipments?
Yes, a USPS standardized address is accepted by every U.S. carrier, because Publication 28 formatting is the common baseline for domestic addresses. UPS also checks addresses in its own systems; if you’re comparing options, see our page on UPS address validation and changing a UPS delivery address.
Once your addresses are clean, the next decision is the carrier. You can compare live UPS and USPS prices for the same package in the ParcelPath rate calculator; ParcelPath is free to use and offers up to 89% off USPS and up to 85% off UPS. Packing the box correctly matters too, and our USPS packaging guidelines explain the rules.
Frequently Asked Questions
Is USPS address verification free?
Yes. The USPS ZIP Code Lookup tool at tools.usps.com verifies and standardizes single addresses at no charge, and the Addresses API on the USPS developer portal is free to register for. Paid options exist only on the software side: CASS-certified address-matching products are sold by private vendors, and they are mainly needed by bulk mailers processing large lists.
What is a USPS standardized address?
A USPS standardized address is an address rewritten to Publication 28 rules: all capital letters, standard abbreviations such as ST, AVE and APT, the unit number on the street line, a two-letter state code, and the full ZIP+4 code. Standardized addresses read reliably on USPS sorting equipment, so they move through the network with fewer manual corrections.
What does DPV code D mean in USPS address verification?
A DPV code of D means USPS confirmed the building or primary address but the secondary information, such as an apartment or suite number, is missing. Mail to that address may not reach the right unit. The fix is to contact the recipient for the unit number and add it to the street line before you print the shipping label.
Do I need CASS-certified software to verify addresses?
No, not for everyday shipping. CASS-certified software is needed when you mail at automation or presort prices and must submit a PS Form 3553, or when you clean large lists at once. To check a few addresses, the free USPS ZIP Code Lookup tool uses the same USPS address data and returns a standardized result.
What replaced the USPS Web Tools address API?
USPS retired the Web Tools APIs on January 25, 2026. Their replacements are the USPS APIs listed in the catalog at developer.usps.com, including an Addresses API for standardization and ZIP Code lookups. The new APIs use OAuth 2.0 authentication and JSON, so older integrations built on Web Tools XML requests must be migrated to keep verifying addresses.
Sources
- USPS PostalPro: Coding Accuracy Support System (CASS)
- USPS Publication 28: Postal Addressing Standards
- USPS ZIP Code Lookup
- USPS: Web Tools APIs Retired January 25, 2026
- USPS APIs Developer Portal
- USPS Postal Facts: Size and Scope
For more ways to run shipping smoothly, browse our roundup of the best shipping services and solutions.
Part of our USPS Locations guide. Related: USPS Shipping: Rates, Tracking and Services, USPS Collection Box.
