Put the apartment number at the end of the delivery address line, using the correct designator — APT, STE, UNIT — rather than the # sign. Here are the exact USPS rules, the approved abbreviations, the awkward cases, and why the format matters for mail forwarding, utilities, deliveries, and your movers.
Put the apartment number at the end of the delivery address line, using the correct designator — APT, STE, UNIT — rather than the # sign.
That is it. Everything below is the detail for the cases that do not fit the simple version.
Format rules from USPS Publication 28, Section 213: Secondary Address Unit Designators.
| Rule | What USPS says |
|---|---|
| Where the unit goes | "The preferred location is at the end of the Delivery Address Line." |
| When it does not fit | Place the secondary address information on the line immediately above the delivery address line. |
| Using the # sign | "The pound sign (#) should not be used as a secondary unit designator if the correct designation, such as APT or STE, is known." |
| If you do use # | "There must be a space between the pound sign and the secondary number." |
| Designator | Abbreviation | Example |
|---|---|---|
| Apartment | APT | 1010 HYDE ST APT 4B |
| Suite | STE | 500 MARKET ST STE 1200 |
| Unit | UNIT | 24 OCEAN AVE UNIT C |
| Building | BLDG | 800 BRANNAN ST BLDG 2 |
| Floor | FL | 1 POST ST FL 9 |
| Room | RM | 100 LARKIN ST RM 210 |
| Department | DEPT | 350 MISSION ST DEPT 4 |
The full approved list is in Appendix C2 of Publication 28. Use the designator the building itself uses — if the letterboxes say "Unit", write UNIT.
USPS prefers uppercase, no punctuation, and left alignment for machine reading. Handwritten mail does not have to follow that to be delivered, but a package label should.
Long street names. If the whole thing will not fit on one line, put the unit on the line above the street address, not below it.
Two buildings on one lot. BLDG comes before the unit: 800 BRANNAN ST BLDG 2 APT 5.
Basement or garden flats. Use what the building uses. If it is simply "Lower", write it as the unit: 1420 PAGE ST UNIT LOWER.
Letters and numbers. Keep them together as the building lists them: APT 4B, not APT 4 B.
Rural routes and PO boxes. A PO box is not a secondary designator; it replaces the street address entirely.
Deliveries that need a buzzer code. Do not put it in the address. Put it in the delivery instructions field, where the carrier or driver actually reads it.
A wrongly formatted unit number is one of the quieter ways a move goes wrong. It affects more than mail.
On a move, the unit number belongs in the written scope, not just in a phone conversation. It determines which door the truck goes to, whether an elevator reservation is needed, and how long the carry is.
When you get a quote from us, the sales specialist confirms the full address including the unit, the floor, and the elevator situation — that is what a dispatcher plans around. See apartment moving for how building access is handled, and pricing for how it affects the estimate.
Related reading: setting up utilities, how to pack for a move, and moving to California if you are arriving from another state.
At the end of the delivery address line: 1010 HYDE ST APT 4B. USPS states this is the preferred location.
USPS says not to when the correct designator is known. If you do use it, leave a space: # 4B.
Put the unit information on the line immediately above the delivery address line — above, never below.
USPS prefers uppercase with no punctuation for automated processing. Either will be delivered by hand; use uppercase on labels.
Not required, but it improves sorting accuracy. USPS has a lookup tool for it.
In the delivery instructions, not the address. The address line is read by machines.
Formatting rules from USPS Publication 28; reviewed September 2026.
Tell us the full address, unit number included, and we will come back with a starting quote.