To find the last four digits, use USPS ZIP Code Lookup and choose By Address. Supply the complete street address, including the actual apartment or suite when applicable. A city, state or nearby address is not enough to choose the extension for your address.
ZIP+4 is a five-digit ZIP Code followed by a hyphen and four additional digits. It helps identify an address more precisely for postal processing. It is not an identity check: a result does not confirm that a person or company is at the address, or give you permission to use it.
Choose the address search, not the city list
USPS offers three searches. By Address works with a specific street address. By City and State lists ZIP Codes for a city, while Cities by ZIP Code lists the cities associated with a ZIP. Those broader lists do not supply the extension for a particular street address and unit.
Gather the house number, street name, any directional part of the address and the actual unit designation. The address search accepts a street address with city and state, or a street address with a known ZIP Code. Enter apartment or suite information in Apt/Suite/Other when applicable; do not invent a unit to obtain a result.
If the unit information is unclear, review Address Line 2 and apartment or suite numbers before searching. Ask the address provider to clarify missing details.
Read the returned address before copying the code
USPS says the lookup displays a standardized address, so abbreviations or other formatting may differ from your input. Compare the street number, street, directional information, unit, city, state and ZIP. A formatting difference does not automatically mean the result is wrong; it also does not justify accepting a different address.
If several addresses match, USPS recommends narrowing the search with a street address and, when applicable, a unit number. Do not select the closest-looking result or copy the extension from another unit in the building.
What to do when no result appears
| Situation | Useful next step |
|---|---|
| A street or unit detail may be incorrect | Compare the entry with the original address and correct it. Keep necessary address details. |
| Searching with a company name gives unexpected results | USPS suggests trying without the company name or using another version of it. The street and unit must still be correct. |
| The address is new or has recently changed | Confirm it with the address provider. USPS lists new construction as one possible reason for a missing match; it is not a diagnosis of your search. |
| The mailing address is confirmed but still absent | Ask the local Post Office for assistance, as USPS recommends. If a website insists on nine digits, ask that website how to proceed. |
A failed lookup does not establish that a person or business does not exist. A successful lookup does not establish that they occupy the address either. Keep postal matching separate from identity or occupancy checks.
A mailing format is not every form’s input format
The USPS ZIP Code basics specify five digits, a hyphen and four digits when using ZIP+4 on a mailpiece. A website may provide one field or separate ZIP and +4 fields. Follow the field’s explicit instructions and inspect the saved result.
Do not force nine digits into a five-digit field. If your service needs the full ZIP+4 but offers no place for it, ask the service to clarify. Conversely, a required nine-digit field is not permission to append arbitrary zeros. Look up or confirm the extension first. This guide does not claim that every mailing requires ZIP+4.
Leading zeros also matter. If a spreadsheet or CSV import has removed one, use the ZIP Code leading-zero guide to inspect the original value and data type. Restoring a known field width does not verify its relationship to an address.
Check the saved address, not just the success message
Before using the record, compare the submitted and saved address. Keep the correct unit, the first five digits and the extension; check for missing zeros and the punctuation expected for that use. A form accepting a value is not postal verification.
The US address tool can support address-field learning and testing. Generated data is not a substitute for looking up your actual mailing address and cannot establish an assigned ZIP+4. Keep test fixtures separate from shipping records.
Sources were checked on October 30, 2026: USPS Lookup, ZIP Code Basics and Publication 28 §211. We read the public guidance but did not submit a real address search, test a third-party form or perform a delivery test. The checking workflow is editorial guidance.