Key Takeaways
- Write the apartment number on the same line as the street address, after the street name, using "Apt" or "Unit" as the label.
- The order is always: street number, street name, apartment number, city, state, ZIP code — never rearrange these.
- Use the abbreviations your lease or building uses (Apt, Unit, #, or Apartment) to match what the post office has on file for your building.
- If you are unsure which format your building uses, check a recent piece of mail you received or ask your landlord or property manager.
The Standard Single-Line Format
The United States Postal Service (USPS) prefers apartment addresses written on one line. This format moves through sorting machines faster and reduces the chance a letter goes to the wrong unit. Write it as: [Street Number] [Street Name] [Apt/Unit Number], [City], [State] [ZIP Code].
The apartment label can be "Apt", "Unit", "#", or "Apartment" — all are correct. Use whichever one appears on your lease or building directory. If your building uses "Unit 3B", write "Unit 3B". If it uses "Apt 3B", write "Apt 3B". Consistency with what the post office already has on file matters more than which label you choose.
The comma always comes after the apartment number and before the city. There is no comma between the street name and the apartment number.
When to Use a Second Line for the Apartment Number
Some addresses work better on two lines, especially when the apartment designation is long or complex. The format is then:
[Street Number] [Street Name] [Apt/Unit Number] [City], [State] [ZIP Code]
This two-line approach is acceptable and still reaches you, but USPS sorting machines process single-line addresses faster. Use the two-line format only if your building's address is unusually long or if your lease specifically shows the address this way. For most apartments, the single line is the better choice.
What Happens If You Get the Format Wrong
Mail written with the apartment number in the wrong place usually still reaches you, especially if the ZIP code is correct. Postal carriers know their routes and recognize buildings even when the format is not perfect. However, mail can be delayed or sent to the building's main office instead of your unit, which means you might not receive it for several extra days.
Packages are more at risk than letters. A delivery driver with fifty packages and an incorrectly formatted address may leave it at the building entrance or with a neighbor rather than climb stairs to find the right unit. This is why getting the format right matters most for packages and time-sensitive mail.
How to Find Your Building's Correct Address Format
The easiest way to confirm the format is to look at mail you have already received — a utility bill, bank statement, or package label. The address printed on incoming mail is exactly how USPS has your unit recorded. Copy that format for any mail you send out or give to others.
If you have not received mail yet, ask your landlord or property manager for the official building address. They can tell you whether the building uses "Apt", "Unit", or "#", and whether the apartment number should be written as "3B" or "03B". Some older buildings use leading zeros; some do not. Match what your building uses.
You can also check your lease — the address listed there is usually the format the post office recognizes. If your lease shows "425 Oak Street Unit 3B", use that exact format.
Special Cases: Suites, Buildings, and Numbered Complexes
Some addresses include more than just an apartment number. A building might have a suite number, a building letter, or a complex name. Write these the same way: on the same line as the street address, in the order they appear on your lease.
Example: 425 Oak Street Building C Unit 3B, Denver, CO 80202. Or: 425 Oak Street Suite 200, Denver, CO 80202. If your lease shows the address with these extra designations, include them in the same order when you write it out.
Addressing Mail to Someone Else at Your Apartment
When you address mail to a friend or family member at your apartment, use the same format as your own address. Put their name on the first line, then the full address on the lines below:
Sarah Chen 425 Oak Street Apt 3B Denver, CO 80202
Do not put the apartment number on the same line as the name. The postal service reads the name first, then the address, and mixing them can slow sorting. If the address is very long, you can move the apartment number to its own line, but keep the name separate at the top.
Frequently Asked Questions
Should I use "Apt" or "Unit" or "#"?
Use whichever one your building uses on its directory, lease, or previous mail you have received. All three are correct and will reach you. Matching what the post office already has on file is more important than which abbreviation you choose. If you are unsure, "Apt" is the most common.
Does the order of the address matter if I include the ZIP code?
The ZIP code helps, but the order still matters. A ZIP code covers many buildings and streets. Postal machines read the full address in order — street number, street name, apartment number — to sort mail to the right unit. A wrong order can delay delivery even with the correct ZIP code.
What if my apartment number has letters and numbers, like 3B?
Write it exactly as it appears on your lease or building directory. If it is "3B", write "3B". If it is "03B" or "Unit 3B", write that instead. Consistency with your building's official format prevents mail from going to the wrong unit.
Can I write the address all in capital letters?
Yes. USPS machines read both uppercase and lowercase. Many people write addresses in all capitals, and it does not slow delivery. Use whichever is easier for you to write clearly.
What if I do not know my apartment number yet?
Contact your landlord or property manager before you move in. They can give you the official address format and apartment designation. You will need this for mail forwarding, utility setup, and other services before you arrive.