The correct format puts the apartment number on the same line as the street address
Write the apartment number directly after the street address on the same line, separated by a comma or the abbreviation "Apt." followed by the number. The most common format in the United States is: 123 Main Street, Apt. 4B, City, State 12345. Some postal services also accept 123 Main Street Apt 4B without the comma after the street address, and both formats will reach you.
The key is that the apartment number must appear before the city and state line. Mail carriers use the street address and apartment number together to sort mail to the correct unit, so leaving it off or putting it in the wrong place can delay delivery or send your mail to the building's main office instead of your door.
If your building uses a unit number instead of an apartment number, or uses letters and numbers together (like Unit 12-A or #204), write it exactly as it appears on your lease or building directory. Consistency matters more than following a single rule—mail carriers learn the exact format your building uses.
Key Takeaways
- Write the apartment number on the same line as the street address, after the street number and name, separated by a comma or "Apt."
- The format 123 Main Street, Apt. 4B, City, State 12345 is the standard that postal services recognize.
- Use the exact unit designation from your lease or building directory—whether that is "Apt.", "Unit", "#", or a letter-number combination.
- Leaving off the apartment number or placing it on a separate line can cause mail to be delivered to the building lobby instead of your unit.
When to use "Apt." versus "Unit" or "#"
"Apt." (short for apartment) is the most widely recognized abbreviation and works in any building. "Unit" is equally correct and is often used in condominiums, townhouses, or mixed-use buildings. The "#" symbol is less formal but still accepted by mail carriers. Choose whichever matches what appears on your lease, your building directory, or your mailbox label—that consistency is what matters to postal delivery.
If you are unsure which term your building uses, check your lease agreement or look at the label on your mailbox. Your landlord or property manager can also tell you the official designation. When you give your address to banks, insurance companies, or government agencies, use the same format every time so records stay consistent.
The order of address lines for mail and forms
When filling out a form with separate lines for address information, the standard order is:
- Street address with apartment number: 123 Main Street, Apt. 4B
- City, State, ZIP code: City, ST 12345
Some forms have a separate field labeled "Apartment/Unit Number" or "Suite." If that field exists, you can write just the number there (4B or Unit 12) and put only the street address in the main address field. Either approach works, but filling in the apartment number in the first address line is safer if you are unsure whether the form will route correctly without it.
For international mail or if you are sending something to a building with a secondary address (like a mail center or management office), ask your landlord for the exact mailing address they use. Some large complexes have a separate mailing address from the physical street address.
Common mistakes that delay mail delivery
The most frequent error is writing the apartment number on a second line by itself, like this:
123 Main Street Apt. 4B City, State 12345
Mail sorting machines read the top line as the complete address and may not process the apartment number correctly. Postal workers can usually catch this, but it slows delivery. Always keep the street address and apartment number together on one line.
Another common mistake is abbreviating the apartment number inconsistently. If your lease says "Apartment 4B," do not write "Apt 4" or "A4B" on different forms. Use the exact designation every time. Inconsistency can cause mail to be flagged for manual review or sent to the wrong unit if the building has similar numbers.
Do not leave the apartment number off entirely when giving your address to anyone, even if you think "everyone knows" which unit you live in. Banks, insurance companies, and government agencies need the complete address on file. If your address record is incomplete, refunds, tax documents, or legal notices may not reach you.
What to do if your building does not have standard apartment numbers
Some older buildings or converted houses use non-standard designations like "Front," "Back," "Upper," "Lower," or only a suite number. Write these exactly as they appear on your lease and mailbox. For example: 123 Main Street, Back Unit, City, State 12345 or 456 Oak Avenue, Suite 200, City, State 12345.
If your building has no official apartment or unit designation at all, ask your landlord or property manager to assign one or to provide the official mailing address. This is especially important if you live in a multi-unit building where mail carriers need to distinguish between units. Without a clear designation, your mail may be delivered to a shared mailbox or the building office, and you could miss important documents.
Frequently Asked Questions
Should I use a comma after the street address before the apartment number?
A comma is standard and helps mail carriers read the address clearly, but it is not required. Both "123 Main Street, Apt. 4B" and "123 Main Street Apt. 4B" are accepted by postal services. Using a comma is slightly safer because it is the format most postal machines are programmed to recognize.
What if my lease says "Apartment 4B" but my mailbox says "Unit 4B"?
Use whichever appears on your mailbox, since that is what mail carriers see every day. If there is a discrepancy, contact your landlord or property manager and ask them to clarify the official designation. Consistency between your mailbox label and the address you give to others prevents delivery delays.
Can I write my apartment number in parentheses instead of after a comma?
Parentheses are not standard for postal addresses and may confuse mail sorting machines. Stick with a comma or "Apt." followed by the number. If a form requires a specific format, follow the form's instructions, but for general mailing addresses, use the comma or "Apt." format.
Do I need to include the apartment number when writing my address by hand on an envelope?
Yes. Always include the apartment number on envelopes, even if you think the recipient knows where you live. Mail carriers rely on the complete address to sort mail correctly, and handwritten addresses are read by both machines and people. Leaving it off risks your mail being delivered to the building office or a neighbor's unit.