Where to Put an Apartment or Unit Number in a Shipping Address
A practical labels & addressing guide to where to put an apartment or unit number in a shipping address, with a repeatable method, worked example, checklist, limitations, and official source links.
Quick answer
Put the apartment, suite, or unit designator and number in the delivery-address portion immediately associated with the street address—not in the recipient-name or city-state-ZIP line. The USPS standardized delivery address line includes the primary address and secondary unit elements. A common verified output looks like “123 MAIN ST APT 4B.” Use the exact result returned by USPS ZIP Code Lookup or supplied by the building; do not invent an abbreviation or omit the unit.
Why placement matters
A street address can identify a building while the secondary unit identifies the delivery point within it. Missing, ambiguous, or misplaced unit information can delay routing or prevent an exact match. In Publication 28, section 23, USPS describes the delivery address line as components that include the primary number, street elements, secondary address identifier, and secondary address.
The DMM 602 complete-address definition also lists a secondary address unit designator and number, such as an apartment or suite, as an address element. Current official records and the recipient’s verified address control over a generic example.
Three-line structure
| Block position | Correct kind of information | Do not put here |
|---|---|---|
| Recipient line | Person, business, or department name | A floating apartment number without context |
| Delivery address line | Street address plus verified APT, STE, UNIT, or other secondary element | City, state, personal directions |
| Last line | USPS-recognized city, state abbreviation, ZIP | Gate code, building nickname, phone number |
Use the right designator
Publication 28’s current Secondary Unit Designators appendix lists USPS-approved abbreviations such as APT, STE, UNIT, BLDG, FL, and RM. They are not interchangeable. Use what the official address output or recipient’s building uses. A private mailbox may require PMB or a number form under separate DMM rules; do not label it PO BOX unless it is actually a USPS Post Office Box.
The pound sign by itself can be ambiguous and may not match the official record. Never infer “APT” from a number written after the street. Confirm whether that number is a unit, building, floor, route element, or part of the primary address.
Verification workflow
- Ask the recipient for the complete mailing address exactly as used for mail.
- Enter the street, unit, city, and state into USPS ZIP Code Lookup.
- Compare the returned delivery line with the recipient’s information. Resolve discrepancies rather than selecting whichever looks shorter.
- Copy the complete output to the shipping label without dropping the secondary number.
- Check that label software did not move “APT 4B” into an optional note field that will not print.
- Print a draft and compare every address component before purchasing postage.
When the line seems too long
Do not delete the unit or shrink the label until it becomes unreadable. Use the current USPS standardization and the label provider’s supported address fields. Publication 28 contains compression and abbreviation guidance for mailers; carrier support or the recipient’s mailroom can clarify complex business addresses. Handwritten packages still need a complete, legible block.
Apartment-label checklist
- Recipient name matches the mailbox or receiving record where applicable.
- Street number, street name, suffix, and directional are verified.
- Secondary unit designator and unit number are both present.
- City, state, and ZIP match the current USPS lookup result.
- Unit is printed in the delivery block, not hidden in order notes.
- Interior address card repeats the verified address when appropriate.
Worked example
A marketplace order shows “Taylor, 123 Main, #4B,” while the recipient messages “Building West, apartment 4B.” Do not choose a format by intuition. Ask for the official mailing address, run the full location through USPS ZIP Code Lookup, and use the standardized delivery line it returns. Put building or department information only in the supported position confirmed by the recipient or official result.
Limitations
This article gives general placement, not a deliverability decision. Apartment complexes, campuses, military housing, Puerto Rico, rural routes, commercial receiving agencies, and business mailrooms can require additional elements. USPS databases and standards change, and third-party checkout fields may truncate data. Verify the live USPS result and recipient information before mailing. Never publish an unredacted apartment label or access code.