Put the recipient’s apartment, suite or unit details in Address Line 2 when the form labels that field for them. For example, enter APT 4B. If the address has no additional details and the field is optional, leave it blank. A form with a separate apartment or unit field needs a different split: follow that field’s instructions and check that the saved address contains the unit once.
“Optional” describes the field, not whether a particular recipient needs a unit number. For addresses containing secondary units, USPS requires their designators on the mailpiece. Removing the apartment information simply to get past a form can leave out part of the address. USPS Publication 28 §213
This guide concerns US addresses. The sample street 120 EXAMPLE AVE and unit APT 4B are an invented teaching combination, not a verified delivery address.
Match your information to the available fields
| What the form provides | How to enter the sample | What to check after saving |
|---|---|---|
| Street plus a dedicated Apt / Suite / Unit field | Street in the street field; unit details in the dedicated field, following its labels | APT 4B is retained once |
| Line 1 plus Line 2 labeled for apartment details | Line 1: 120 EXAMPLE AVE; Line 2: APT 4B | Both the street and unit survive |
| One street-address field with no separate unit field | If the site accepts a complete street address there, use 120 EXAMPLE AVE APT 4B | The end of the line is not cut off |
| An optional second line, but no additional address information | Leave it blank | No repeated street, city, state or ZIP |
Some interfaces ask you to select a type, such as Apartment, and then enter a number. If the number box asks only for that number, enter 4B there rather than repeating the type. Treat this as a way to read the current form, not a claim that every website stores addresses alike.
A combined confirmation line can be correct
The saved address might show APT 4B beside the street even though you entered it into a separate box. Check for missing information before treating a change in layout as an error.
The HTML standard defines address-line1, address-line2 and address-line3 as separate free-text street-address lines. Those field names do not establish how a particular website will print a mailing label. WHATWG autofill fields
For the US mailing layout, USPS prefers secondary unit information at the end of the Delivery Address Line. The delivery line in our teaching example is:
120 EXAMPLE AVE APT 4B
When all the delivery information will not fit on that line, USPS places the secondary information immediately above it:
APT 4B
120 EXAMPLE AVE
That is a mailpiece layout rule, not a test of a checkout system. The practical check is whether the recipient’s correct unit designator and number are still present. USPS §213.3
Keep the assigned unit type
Use the designation in the address the recipient gives you. Do not choose between Apartment and Suite by guessing what kind of building it looks like.
| Description | USPS abbreviation | Illustrative fragment |
|---|---|---|
| Apartment | APT | APT 4B |
| Suite | STE | STE 210 |
| Unit | UNIT | UNIT C |
| Room | RM | RM 12 |
| Floor | FL | FL 3 |
| Building | BLDG | BLDG 2 |
These are approved abbreviations in USPS Appendix C2. The fragments illustrate spelling, not whether a real address is complete. A floor number does not automatically replace an apartment number. If the recipient provides both a building and an apartment, preserve the necessary parts instead of selecting just one.
USPS says not to substitute # when the correct designator is known. If # is used, a space belongs between it and the number, as in # 4B. Confirm an unclear unit type with the recipient. Appendix C2 requires # on the mailpiece when the designator is blank or cannot be determined. That covers a known unit number with an undetermined type; it does not authorize inventing a number. Private mailbox addresses have their own conditions and should not be rewritten as apartments. See the PO Box and street address guide for that distinction.
Fix the saved result, not just the input box
The apartment appears to be missing. Expand the complete address or reopen the editor first. It may have been combined with the street. If it is missing from both the editor and the complete confirmation, restore it. If the site still drops it, report that specific problem to the site’s support team rather than removing the unit to make validation pass.
The unit appears twice. Compare the street field, Line 2 and any dedicated unit field. Find where the duplicate was entered, follow the field instructions, then check the saved address again.
An address suggestion removes the unit. Compare the suggestion with the recipient’s original address before accepting it. A suggested change that loses apartment information needs clarification with the recipient or the site.
Line 2 is mandatory but there is no unit. Read its help text: it may request another real part of the address. Do not assume that a made-up apartment, a repeated street or N/A is the right workaround. If no documented option fits, ask the site how it handles that address.
For length errors, inspect the field’s limit and use standard abbreviations that match the actual address. Check the final characters after saving. Cutting off a unit number is not a useful way to shorten the line.
Make one final comparison
Compare the complete saved address with the recipient’s version. Check the street, unit type, letters and digits, and any duplicated unit details. Inspect the mailing label too if it is available. A confirmation screen alone does not establish what was printed or whether mail can be delivered.
The USAddressGen US address tool can help with ordinary address-field practice. Its output, the examples here, and a form that accepts your input do not establish permission to use an address or verify a particular apartment’s deliverability.
Sources checked on October 21, 2026. This guide uses USPS and HTML documentation; it does not report tests of a particular merchant, live order or postal delivery.