The standard format for an apartment address

Write the apartment number on the same line as the street address, after the street number and name. The format is: [Street Number] [Street Name] [Apartment Number], [City], [State] [ZIP Code].

For example: 425 Oak Street Apt 12B, Denver, CO 80202. Some addresses use "Unit" instead of "Apt" — both are correct and widely recognized by mail carriers and delivery services. You can also write the apartment number on a second line if space is tight, though the single-line format is standard.

The apartment identifier comes before the city and state. Do not put it after the ZIP code, and do not put it on a line by itself. Mail sorting machines read addresses line by line from top to bottom, so placement matters for speed and accuracy.

Key Takeaways

  • Write the apartment number on the same line as the street address, using either "Apt" or "Unit" followed by the number.
  • The correct order is street address, then apartment number, then city, state, and ZIP code on the same or next line.
  • Use the apartment number exactly as it appears on your lease or building directory — some buildings use letters, numbers, or combinations.
  • If mail keeps arriving without the apartment number, contact your local post office to confirm the correct format for your building.

When to use "Apt" versus "Unit"

Both "Apt" and "Unit" are accepted by the United States Postal Service and all major delivery carriers. "Apt" is more common in older buildings and on the East Coast, while "Unit" appears more often in newer construction and on the West Coast, but this is not a hard rule.

Check your lease, building directory, or the mail you receive at your address — use whichever term your building uses. If your lease says "Apt 5C," write it that way. Consistency matters more than which abbreviation you choose, because mail carriers learn the format for each building.

How to format the apartment number itself

Write the apartment number exactly as it appears on your lease, mailbox, or building directory. Some buildings use only numbers (Apt 12), some use letters (Apt B), and some use combinations (Apt 12B or Apt 3-North).

If your building uses a hyphen, slash, or other character in the apartment designation, include it. Do not simplify or standardize it yourself — the exact format is what the post office has on file for your building. If you are unsure, look at mail you have received at that address or ask your landlord or building manager.

What to do if mail arrives without the apartment number

If you receive mail addressed only to your street address without the apartment number, the post office may not have your unit on file correctly. This happens most often in large buildings or when a building number was recently added or changed.

Visit your local post office with a piece of mail addressed to you and ask the clerk to confirm the correct format for your address. They can check what the building is registered as and whether your apartment number is in the system. If it is not, ask them to add it. This usually takes one visit and prevents future mail from going to the wrong unit.

Addressing mail to someone else in your building

When you address mail to another person in your apartment building, use the same format: street address, apartment number, city, state, ZIP code. You do not need to know the exact apartment number if you know the person's name — the post office can usually deliver it — but including the number ensures faster delivery.

If you are sending a package or important document, including the apartment number reduces the chance of it being delivered to the wrong unit. Many buildings have similar unit numbers on different floors, so the full designation matters.

International and non-standard apartment formats

If your building uses a non-standard format — such as a building name instead of a number, or a courtyard designation — write it as it appears on official documents for your building. Some older or historic buildings have names (The Riverside Apartments) instead of street numbers, or use terms like "Building A" or "Court 3."

Write the address as the post office knows it. If you are unsure whether your building uses a street number, building name, or both, check your lease or ask the building office. When in doubt, include both the street address and the building or courtyard name on separate lines.

Frequently Asked Questions

Can I write the apartment number on a separate line?

Yes, you can write it on a second line if space is limited. The format would be: 425 Oak Street / Apt 12B / Denver, CO 80202. However, the single-line format (425 Oak Street Apt 12B, Denver, CO 80202) is standard and processes faster through postal machines.

What if my apartment has both a number and a letter?

Write it exactly as it appears on your lease or mailbox — for example, Apt 12B or Unit 3-A. Do not rearrange or simplify it. The exact format is what the post office has registered for your building.

Does the post office care if I use "Apt" or "Unit"?

No. Both abbreviations are recognized and processed the same way. Use whichever your building uses, but either one will work. The apartment number itself is what matters most.

What should I do if my building does not have apartment numbers?

Some very small buildings or converted houses do not use apartment numbers. In that case, write the resident's name as the second line: 425 Oak Street / Sarah Chen / Denver, CO 80202. Contact your post office if you are unsure whether your building has numbers on file.