Address Standardization API: What Changes, With Examples
The postal rules behind standardization, quoted from USPS Publication 28, and three live before-and-after examples from the sthan.io API.
An address standardization API rewrites a US address into postal format. It abbreviates street types, directions and unit designators (Avenue to AVE, Northwest to NW, Suite to STE) and uses the two-letter state code. Adding a missing ZIP+4 takes a lookup in postal data, so an API that checks postal data can also add the ZIP+4 when available. USPS Publication 28 defines a standardized address as one that includes all required elements and uses the Postal Service standard abbreviations.
The postal rules a standardization API applies
For US addresses, the rules come from USPS Publication 28, Postal Addressing Standards. These are the ones that change an address most often:
| Rule | What Publication 28 says | Section |
|---|---|---|
| What "standardized" means | "A standardized address is one that includes all required address elements and that uses the Postal Service standard abbreviations" | 211 |
| Street types | "The suffix of the address should conform to the standard suffix abbreviations listed in the ZIP+4 file". Examples from the table: AVENUE to AVE, BOULEVARD to BLVD, DRIVE to DR, STREET to ST | 234.1, Appendix C1 |
| Directions | The eight standard directionals are N, S, E, W, NE, NW, SE and SW, abbreviated to one or two characters | 233 |
| Unit designators | "The pound sign (#) should not be used as a secondary unit designator if the correct designation, such as APT or STE, is known". Approved abbreviations include Apartment APT, Building BLDG, Floor FL, Room RM, Suite STE | 213, Appendix C2 |
| Where the unit goes | "The preferred location is at the end of the Delivery Address Line." | 213 |
| Pound sign spacing | "If the pound sign (#) is used, there must be a space between the pound sign and the secondary number." | 213.2 |
| Casing | "Uppercase letters are preferred on all lines of the address block." Lowercase is acceptable if it meets OCR readability guidelines | 212 |
| Punctuation | "With the exception of the hyphen in the ZIP+4 Code, punctuation may be omitted in the delivery address block." | 222 |
These abbreviations apply to the part of the address the word plays. A directional word that belongs to the street name stays spelled out: Publication 28 section 233.3 says a directional between the street name and the suffix "is spelled out on the mailpiece."
Publication 28 also explains why this matters: "Complete addresses that have matched Postal Service files or directories have a better chance of being promptly and correctly delivered."
Before and after: live responses
We sent three US addresses, typed the way people type them, to the sthan.io verification endpoint on October 4, 2026. This is what came back:
| Typed as | Standardized result | What changed |
|---|---|---|
| 1600 pennsylvania avenue northwest washington district of columbia | 1600 Pennsylvania Ave NW, Washington, DC 20500-0005 | avenue to Ave, northwest to NW, district of columbia to DC, ZIP and ZIP+4 added |
| 233 South Wacker Drive Suite 3300, Chicago, Illinois 60606 | 233 S Wacker Dr STE 3300, Chicago, IL 60606-6387 | South to S, Drive to Dr, Suite to STE, Illinois to IL, ZIP+4 added for the suite |
| 400 broad street seattle wa | 400 Broad St, Seattle, WA 98109-4607 | street to St, casing fixed, ZIP and ZIP+4 added |
All three came back with dpvConfirmation Y and deliverableStatus Confirmed. Note the casing: sthan.io returns street names in mixed case ("Wacker Dr") with the unit designator in capitals ("STE"). Publication 28 prefers uppercase but accepts lowercase that machines can read. If you want the preferred uppercase format, convert the result in your own code.
Standardization comes with other postal fields when you use a verification API. For the Chicago address, the same response also returned:
| Field | Value |
|---|---|
| unitType / unitNumber | STE / 3300 |
| zip4 | 6387 |
| carrierRoute | C019 |
| deliveryPoint / checkDigit | 25 / 1 |
| recordType | H |
| dpvConfirmation | Y |
| deliverableStatus | Confirmed |
| matchTier / confidence | Exact / 1 |
The full list of verification fields is in what an address verification API returns.
Standardization alone vs standardization with verification
Standardizing changes the format. It does not, by itself, tell you whether the address exists. Publication 28 distinguishes complete, standardized and validated address output. It defines a standardized address as one with the required address elements and the Postal Service standard abbreviations, and a complete address as one with the elements needed for an exact match with the current ZIP+4 and City State files. In API terms, formatting alone does not prove that an address exists or can be delivered. A verification API does both in one call: it returns the standardized address and a deliverability status. More on the difference in cleansing vs verification vs standardization.
Other APIs that standardize US addresses
Read from each vendor's own page on October 4, 2026:
| API | What the vendor says | Free | Paid entry price |
|---|---|---|---|
| sthan.io US Address Verification | Returns the standardized address, ZIP+4 and a deliverability status (live responses above) | 100 a month, 10 a minute, no card | $12 a month for 10,000 |
| Smarty US Street Address API | "Standardizes and validates US addresses." | 42-day trial, 1,000 lookups, no card | $195 a year for 12,000 lookups, or $552 a year for 60,000 |
| PostGrid | "all addresses are formatted to local postal standards." | 500 free credits | Essential plan: $18 a month billed yearly, or $20 billed monthly, for 2,000 lookups |
| Geocodio (a geocoder) | "Address parsing and standardization is automatically included with all forward geocoding (i.e. address-based) lookups." | 2,500 lookups a day | $1 per 1,000 after the free tier |
Geocodio is a geocoding service: its main output is map coordinates. It belongs on this list only because it standardizes as a side effect of geocoding.
Where sthan.io fits. sthan.io standardizes US addresses as part of verification: each successful match returns the standardized address, the ZIP+4 and a deliverability status, where postal data has them. The free tier is 100 verifications a month (10 a minute) with no credit card. Paid plans start at $12 a month for 10,000. sthan.io is not CASS-certified.
Frequently Asked Questions
Sources
The example calls were made October 4, 2026. The sources below were read October 4 and rechecked October 7, 2026: USPS Publication 28 on pe.usps.com (sections 125, 211, 212, 213, 222, 233, 234 and Appendix C1 and C2); live sthan.io v2 address verification responses (3 calls shown); sthan.io/pricing/united-states; smarty.com/pricing; postgrid.com/pricing-address-verification; geocod.io/guides/address-parsing and geocod.io/pricing. Prices change; check the vendor page before you buy.
Standardize 100 US addresses a month free
No credit card. Standardized address and postal deliverability status, with ZIP+4 when available.