The standard format for apartment addresses
An apartment address needs three parts: the street address, the apartment number, and the city, state, and ZIP code. The apartment number goes on the same line as the street address, after the street number and name. Write it as "Apt." or "#" followed by the unit number.
The correct format looks like this:
123 Main Street, Apt. 4B Springfield, IL 62701
Some addresses use "Unit" instead of "Apt." — both are correct. The key is putting the apartment identifier on the first line, not on a separate line by itself. Mail carriers and delivery services use the apartment number to route your mail to the right unit, so placement matters.
Key Takeaways
- Write the apartment number on the same line as the street address, after the street name, using "Apt." or "#" as the identifier.
- The second line should contain only the city, state, and ZIP code — never put the apartment number on its own line.
- Use the exact apartment designation from your lease or building directory, whether that is a letter, number, or combination.
- When filling out online forms, use the street address line for "Apt. 4B" and the city line for the city only, unless the form has a separate apartment field.
What to do when forms have a separate apartment field
Many online forms and applications split the address into multiple fields: street address, apartment/unit, city, state, and ZIP. When you see a dedicated apartment field, enter only the apartment number or unit designation there — not the word "Apt." or the street address again.
If the form shows "Street Address" and "Apartment/Unit" as separate boxes, fill them like this:
| Field | What to enter |
|---|---|
| Street Address | 123 Main Street |
| Apartment/Unit | 4B |
| City | Springfield |
| State | IL |
| ZIP Code | 62701 |
This separation helps government agencies and service providers organize records correctly. Do not repeat information across fields — if you put "123 Main Street, Apt. 4B" in the street address field and then put "4B" again in the apartment field, the system may reject the entry or create duplicate records.
Handling unusual apartment designations
Not all apartment numbers follow the standard format. Some buildings use letters only (Apt. A, Apt. B), some use numbers with letters (4B, 2C), some use suite numbers (Suite 200), and some use building names or descriptors. Write whatever designation appears on your lease or mailbox.
If your building uses a non-standard system, check your lease or ask the landlord or property manager what the official unit designation is. Write it exactly as it appears in the building's records. For example, if your lease says "Unit 4-B" with a hyphen, use the hyphen. If it says "Apt 4B" without a period, write it without a period.
When mailing something to yourself or having a package delivered, consistency matters more than style. Use the same format every time so mail carriers and delivery services recognize the address as the same place.
When you do not have an apartment number
Some smaller buildings or converted houses do not have formal apartment numbers. In that case, use the street address alone on the first line, and add a descriptor on the second line if needed — for example, "Rear Unit" or "Upstairs" — before the city and state.
123 Main Street, Rear Unit Springfield, IL 62701
If there is no descriptor and no number, just use the street address. The postal service will deliver to the building, and your landlord or neighbors can help sort it from there. This is less common now, but it happens in rural areas or very small properties.
Addressing mail to an apartment building
When you are sending mail or a package to someone else's apartment, use the same format: street address and apartment number on the first line, city and state on the second. Include the recipient's name on the line above the address.
Jane Smith 456 Oak Avenue, Apt. 12 Portland, OR 97201
If you do not know the apartment number, ask the recipient before sending anything. Mail without an apartment number may end up at the building's main office or get returned. For packages, the carrier may leave it at the front desk, but letters often get lost without the unit designation.
Common mistakes to avoid
The most common error is putting the apartment number on a separate line by itself. This confuses mail sorting systems and can delay delivery. Write "123 Main Street, Apt. 4B" on one line, not "123 Main Street" on one line and "Apt. 4B" on the next.
Another mistake is using inconsistent formatting. If you write "Apt. 4B" on one form and "Apartment 4B" on another, or "4B" without any prefix on a third, the addresses may not match in records. Stick with one format and use it every time.
Do not add extra information like your landlord's name, the building name, or a suite number that is not part of your official address. Extra details can confuse automated sorting systems. Stick to the street address, apartment number, city, state, and ZIP code.
Frequently Asked Questions
Should I write "Apt." or "Apartment" or just the number?
"Apt." is the standard abbreviation and works everywhere. "Apartment" is also correct but takes more space. Just the number (like "4B") works on forms with a separate apartment field, but on a single-line address, use "Apt." or "#" so it is clear what the number means.
What if my lease says "Unit" instead of "Apt."?
Use "Unit" if that is what your lease and building use. Both "Apt." and "Unit" are correct and recognized by mail carriers. Consistency with your building's official records matters more than which word you choose.
Can I write the apartment number in parentheses instead?
Parentheses like "123 Main Street (4B)" are not standard and may confuse automated mail sorting. Stick with "123 Main Street, Apt. 4B" or "123 Main Street #4B" — both are recognized by postal services and delivery carriers.
Do I need to include the apartment number when writing my address on a check?
Yes. Write it the same way you would on any other mail: street address and apartment number on the first line, city and state on the second. Banks and check processors use the full address to route payments correctly.
What if I moved and my mail is still going to my old apartment?
File a change of address with the U.S. Postal Service, not just with individual companies. Go to usps.com or visit your local post office to submit a forwarding request. Include both your old address (with the old apartment number) and your new address (with the new apartment number) so the system knows which mail to forward.