ZIP+4 Lookup API: Get the 4-Digit Add-On by API
What the four extra digits mean, in USPS's own words. Three live sthan.io responses that show where the ZIP+4 comes back. And what seven APIs call the field and charge for it, read from each vendor's own page on the same day.
A ZIP+4 lookup API takes a street address and returns its ZIP+4 code: the 5-digit ZIP Code plus a 4-digit add-on that points to a smaller delivery area, such as one side of a street or one building. In practice you get it from an address verification API, which standardizes the address and returns the add-on as its own field. The old free USPS Web Tools route was retired on January 25, 2026.
What ZIP+4 means, according to USPS
The USPS mailing standards (Domestic Mail Manual, section 602.1.8.2) define it like this:
"The most complete ZIP Code is a nine-digit number consisting of five digits, a hyphen, and four digits, which the USPS describes by its trademark ZIP+4."
The same section explains the four extra digits. The first two name a sector and the last two a segment, a smaller area inside it. Together they can point to a side of a street between intersections, a building, a floor or group of floors, a firm within a building, a span of boxes on a rural route, or a group of PO Boxes that one carrier serves.
Two rules in that manual matter when you look up a ZIP+4:
- Send the whole address. USPS says a complete address has all the elements needed "to obtain the finest level of ZIP+4 code for the delivery address." The unit number is one of those elements.
- Firms can have their own code. "If a firm name is assigned a unique ZIP+4 code in the USPS ZIP+4 Product, the unique ZIP+4 code must be used in the delivery address."
How to get a ZIP+4 by API
You send one address and read one field back. With sthan.io it is a single GET request:
curl "https://api.sthan.io/v2/address-verification/usa/1025%20Briar%20Ave%2C%20Washington%20Court%20House%2C%20OH%2043160" \
-H "Authorization: Bearer YOUR_API_KEY"The fields sit inside the response's Result object. For a matched address it holds the standardized address with the full ZIP+4 in Result.fullAddress, the four digits on their own in Result.zip4, and a Result.matchCode object. Result.matchCode.zip4 reads Inferred when the field was missing from your input and the API filled it in.
One tip: do not expect a precise ZIP+4 from a five-digit ZIP Code alone. Five digits cover many sectors and segments, so the lookup normally needs the street address, and a unit or firm name may be needed to get the finest-level ZIP+4.
Live sthan.io responses: three address types
We sent these three addresses to the sthan.io verification API on October 4, 2026. The table copies fields from the raw responses.
| Type | Input | fullAddress returned | zip4 | recordType | dpvConfirmation | deliverableStatus |
|---|---|---|---|---|---|---|
| House on a street | 1025 Briar Ave, Washington Court House, OH 43160 | 1025 Briar Ave, Washington Court House, OH 43160-1007 | 1007 | S | Y | Confirmed |
| Apartment with unit | 1255 University Ave Apt 142, Sacramento, CA 95825 | 1255 University Ave APT 142, Sacramento, CA 95825-6836 | 6836 | H | Y | Confirmed |
| Government office | 1600 Pennsylvania Ave NW, Washington, DC 20500 | 1600 Pennsylvania Ave NW, Washington, DC 20500-0005 | 0005 | S | Y | Confirmed |
In all three, matchCode.zip4 was Inferred: none of the inputs had a ZIP+4, and each response added one. The apartment also came back with unitType "APT" and unitNumber "142" as separate fields. Each response also carries deliveryPoint (two digits) and checkDigit (one digit). The ZIP+4 plus the two delivery point digits make the delivery point routing code; the response provides the check digit separately, and the numeric delivery point barcode adds it as well, which is why the postal rules describe that barcode as the ZIP+4 followed by three digits.
How other APIs return ZIP+4, and what they charge
Every product below except Geocodio is an address verification API. Geocodio is a geocoder that sells ZIP+4 as an add-on field. Prices were read on each vendor's own page on October 4, 2026. The 10,000 column is the cost of 10,000 lookups in one month at list price.
| API | ZIP+4 field | 10,000 lookups a month | Free |
|---|---|---|---|
| sthan.io | Result.zip4 | $12 (Basic plan, 10,000 a month) | 100 a month, no card |
| PostGrid | data.zipPlus4 | $45 (Business plan billed monthly, 10,000 lookups) | 500 free credits listed on each plan |
| Google Address Validation | result.uspsData.standardizedAddress.zipCodeExtension (US and PR only; Google says it is not always filled) | $85 (5,000 free, then 5,000 at $17.00 per 1,000; our math) | 5,000 Pro events a month, billing account required |
| Smarty | components.plus4_code | $88 (monthly 10,000 plan) | 42-day trial, 1,000 lookups |
| Lob | components.zip_code_plus_4 | Published pricing differs: Lob's address verification help center lists a Business plan at $120 a month for 10,000; its main pricing page shows verification rates inside its mail plans ($25 a month with 1,000 included, then $0.025 each, about $250 for 10,000; our math). Confirm the price with Lob | Help center: 300 a month on the $0 Developer plan; main pricing page: none shown ($0.05 per verification) |
| Melissa | Not checked: this row compares its US Address Verification credits and its free Global Address Verification plan, and we did not confirm either response's ZIP+4 field | Unclear: Melissa's pricing page gives two credit rates for a US address, 10 on its pay-as-you-go card and 3 in its FAQ. Its 100,000-credit pack ($350) would cover 10,000 or about 33,000 addresses (our math). Confirm the rate with Melissa | Free Global Address Verification Starter plan: 250 records a month |
| Geocodio (geocoder) | zip4 append: plus4 and zip9 arrays | $0 if limited to 1,250 ZIP+4 lookups a day; $17.50 if all 10,000 are processed in one day. Each ZIP+4 lookup uses 2 credits (1 lookup + 1 append), $1 per 1,000 credits after the free tier (our math) | 2,500 credits a day, so 1,250 ZIP+4 lookups a day |
Three details from the vendors' own docs are worth knowing before you pick:
- Google:
uspsDatais "Only provided in region US and PR" and "is not guaranteed to be fully populated for every US or PR address." - Geocodio: businesses with a range of ZIP+4 codes get an array with two items, and you can pass the firm name as
addresseeto get "a more precise USPS ZIP+4 code for businesses." - Melissa: its pricing page shows two rates. The US Address Verification pay-as-you-go card says "10 Credits Per Address"; the FAQ says "each US address equals 3 credits." Because those two published rates conflict, confirm the applicable rate with Melissa.
What happened to the free USPS lookup
The old free route was the USPS Web Tools address API. USPS says "The USPS Web Tools API platform was retired on January 25, 2026." We cover the dates, the replacement and what its terms allow in USPS Web Tools retirement.
Where sthan.io fits. For a matched address, sthan.io verification returns the standardized address with its ZIP+4 when postal data has one, the four digits on their own in zip4, and a deliverability status. 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
Read October 4, 2026, unless noted: USPS Domestic Mail Manual section 602 (pe.usps.com/text/dmm300/602.htm), sections 1.4 and 1.8; usps.com/business/web-tools-apis; sthan.io/pricing/united-states; live responses from api.sthan.io/v2/address-verification/usa; postgrid.com/pricing-address-verification; developers.google.com/maps billing-and-pricing pricing list and SKU details, and the Address Validation validateAddress reference; smarty.com/pricing and the US Street API reference; lob.com/pricing and docs.lob.com; melissa.com/pricing; geocod.io/pricing and geocod.io/docs. Rechecked October 6, 2026: help.lob.com address verification pricing; avdocs.postgrid.com (zipPlus4 field); geocod.io pricing and docs (credits per append); melissa.com/pricing (credit rates and free plan); the Google validateAddress reference; docs.lob.com and the Smarty US Street API reference (field paths). Prices change; check the vendor page before you buy.
Get ZIP+4 on 100 free verifications a month
No credit card, no expiry. Standardized address, ZIP+4 when available and deliverability status with each successful match.