The standard format for apartment addresses

Write the apartment number on the same line as the street address, after the street number and name. Use either "Apt." or "#" followed by the unit number. The format looks like this:

John Smith 123 Main Street Apt. 4B City, State 12345

Both "Apt. 4B" and "# 4B" work equally well with postal carriers. The key is putting it on the street address line, not on a separate line above or below. Mail sorting machines read addresses line by line from top to bottom, so the apartment number must be on the same line as the building address for the machine to process it correctly.

Key Takeaways

  • Write the apartment number on the same line as the street address, after the street number and name.
  • Use either "Apt." or "#" to mark the unit number — both formats are accepted by postal carriers.
  • Do not put the apartment number on a separate line or above the street address, as mail sorting machines may miss it.
  • Include the full apartment designation exactly as it appears on your lease or building directory.

When your apartment has both a number and a letter

Many apartments use a combination of numbers and letters — like 4B, 12-C, or 3A. Write the full designation exactly as it appears on your lease, your mailbox, or the building directory. Do not abbreviate or rearrange the letters and numbers.

If your unit is 4B, write "Apt. 4B" or "# 4B". If it is 12-C, write "Apt. 12-C" or "# 12-C". The hyphen, if present, stays in place. Postal carriers and mail sorting machines recognize these variations, but they work best when you match the exact format used by your building.

Spacing and punctuation on the address line

Leave one space between the street address and the apartment designation. The correct spacing is: "123 Main Street Apt. 4B" not "123 Main StreetApt.4B" or "123 Main Street Apt. 4B".

If you use the "#" symbol instead of "Apt.", the format is "123 Main Street # 4B" with a space before and after the symbol. Do not use both "Apt." and "#" together — choose one or the other. Avoid extra punctuation or abbreviations beyond what the postal system expects.

What to do if your building uses suite numbers instead

Some buildings, particularly older ones or those with commercial space, use "Suite" instead of "Apt." The format is the same: write it on the street address line. "456 Oak Avenue Suite 200" works just as well as "456 Oak Avenue Apt. 200".

If your building uses "Unit" as the designation, explore the same rule: "789 Elm Street Unit 5" goes on one line. The postal service recognizes all these terms — Apt., Suite, Unit, and # — so use whichever one your building uses on its official directory or your lease.

Addressing mail to an apartment building with no unit number

If you live in a small building where units are not numbered — for example, a two-family house where you are straightforward "the upstairs" — you can add a descriptive line. Write the street address on the first line, then add a second line with "Upstairs" or "Left Unit" or "Rear Apartment".

John Smith 123 Main Street Upstairs City, State 12345

This second line helps the mail carrier identify which door to use, even though it is not a formal apartment number. Postal carriers in small towns or rural areas often know buildings by these informal descriptions.

Common mistakes that slow down delivery

The most frequent error is putting the apartment number on a separate line below the street address. Mail sorting machines scan the street address line first and may not process the apartment number if it appears alone on the line below. Always keep it on the same line as the street address.

Another mistake is using unclear abbreviations or symbols. "Apt" without a period, "APT" in all capitals, or made-up symbols can confuse sorting machines. Stick to standard formats: "Apt." with a period, or the "#" symbol.

Do not write the apartment number in the return address area (top left of the envelope) without also including it in the main address. The return address is for mail that cannot be delivered, not for routing the current piece of mail.

Frequently Asked Questions

Should I write the apartment number before or after the street name?

Write it after the street name on the same line. The order is: street number, street name, then apartment number. "123 Main Street Apt. 4B" is correct. "Apt. 4B 123 Main Street" will confuse postal machines and may delay delivery.

Does it matter if I use "Apt." or "#"?

No. Both are standard and postal carriers recognize both. Use whichever you prefer or whichever matches your building's official format. Consistency helps if you receive mail regularly, but either one works for a single piece of mail.

What if I do not know the exact apartment number format?

Check your lease, your mailbox label, or the building directory. If none of those are available, ask your landlord or building manager. Using the exact format your building uses reduces the chance of mail being misdelivered to a neighboring unit.

Can I write the apartment number in parentheses instead?

Avoid it. Postal sorting machines are programmed to recognize "Apt." and "#" specifically. Parentheses, brackets, or other symbols may not be read correctly by automated systems, even though a human mail carrier might understand it.

Does the apartment number go on the same line if the street address is very long?

Yes. Even if the street name is long, keep the apartment number on the same line. Postal machines read line by line, and the apartment number must be on the street address line to be processed correctly. It is better to have a long line than to split the address across multiple lines.