Write your apartment number after the street address on the same line, or on a separate line below it

The standard format is: street number and name, then apartment number. You can write it as "123 Main Street, Apt 4B" all on one line, or split it across two lines as "123 Main Street / Apt 4B". Both formats work equally well with the postal service and on official documents. The key is making sure the apartment number is clearly visible and not buried in the middle of other information.

If your building uses "Unit" instead of "Apt" or "#", that works too — the postal service recognizes all three. What matters is that mail carriers and delivery services can tell which unit is yours without guessing. A missing or unclear apartment number is the most common reason mail gets delivered to the wrong unit or held at the building office.

Key Takeaways

  • Write the apartment number on the same line as the street address or on the line directly below it, never buried in other text.
  • Use "Apt," "Unit," or "#" before the number — all three formats are recognized by mail carriers and government agencies.
  • Always include the city, state, and ZIP code below the street and apartment information.
  • If your building has a secondary address (like a suite or building letter), place it between the street address and apartment number.

The correct order for a full mailing address

A complete apartment address follows this order from top to bottom: your name, street address with apartment number, city, state, and ZIP code. Each line should contain only one piece of information. This layout is what the postal service expects and what sorting machines can read accurately.

Here is what it looks like in practice:

Sarah Chen 123 Main Street, Apt 4B Portland, OR 97201

Or with the apartment number on its own line:

Sarah Chen 123 Main Street Apt 4B Portland, OR 97201

Both versions are correct. The postal service reads the ZIP code first to route the mail to the right facility, then the street address and apartment number to get it to your door. Skipping the apartment number or putting it in the wrong spot can send mail to the building's main office or to a neighbor's unit.

When your building has a secondary address or suite number

Some apartment buildings have a secondary address — a building letter, suite number, or building name — that comes between the street address and your apartment number. This is common in large complexes or converted commercial buildings. Write it in this order: street address, then secondary address, then apartment number.

Example:

James Rodriguez 456 Commerce Avenue, Building C, Apt 210 Denver, CO 80202

If the secondary address is long, you can split it across two lines:

James Rodriguez 456 Commerce Avenue, Building C Apt 210 Denver, CO 80202

Check your lease or the building directory to see exactly how the address is listed. Use that format when filling out official documents, bank forms, or government paperwork — consistency matters when records need to match across different agencies or services.

How to format your address on online forms and applications

Online forms usually have separate fields for street address, apartment number, city, state, and ZIP code. Put the street number and name in the "Street Address" field, and the apartment number in the "Apartment," "Unit," or "Suite" field. Do not combine them into one field unless the form explicitly says to do so.

If a form has only one address field and no separate apartment box, write it as "123 Main Street, Apt 4B" on a single line. Some forms will not accept a second line, so combining them prevents the address from being cut off or rejected. When in doubt, include the apartment number on the same line as the street address rather than leaving it out.

For government forms — mortgage applications, tax documents, voter registration — use the exact address on your lease or deed. Mismatches between your address on file and your actual address can delay processing or cause mail to be returned. If you have recently moved, update your address with the post office, your bank, and any government agencies before submitting new forms.

Common mistakes that delay mail delivery

The most frequent error is leaving off the apartment number entirely or writing it in a way mail carriers cannot read — for example, "123 Main St Apt4B" with no space, or "123 Main St, 4B" without "Apt" or "Unit." Mail carriers need to see the apartment identifier clearly to know which unit gets the mail.

Another common problem is writing the apartment number in the city or state line by mistake, like "123 Main Street, Portland Apt 4B, OR 97201." This confuses sorting machines and may cause the mail to be returned or held. Keep each piece of information in its proper place: street address and apartment together, then city, then state and ZIP on the final line.

Abbreviating the state incorrectly or using an old ZIP code also causes delays. Use the two-letter state abbreviation (OR for Oregon, CA for California) and the full five-digit ZIP code. If you are unsure of your ZIP code, the USPS ZIP Code Lookup tool on USPS.gov will find it when you enter your street address and apartment number.

What to do if your apartment number is missing or unclear

If you notice your apartment number is missing from an address you have already submitted — on a job process, loan form, or government document — contact the organization and provide the correction in writing. Include your full name, the incorrect address, and the correct address with the apartment number. Ask them to update their records before processing your process or sending mail.

If mail is being delivered to the wrong unit or the building office, talk to your mail carrier or building management. They can add a note to your address in the postal system. You can also file a mail forwarding request with the post office if you need to redirect mail temporarily, though this does not solve the underlying address problem.

For ongoing issues, consider using a mail label or mailbox marker with your apartment number in large, clear numbers. This helps delivery drivers and mail carriers confirm they have the right unit, especially in buildings where unit numbers are not clearly visible from the street.

Frequently Asked Questions

Should I write "Apartment" or "Apt" or just use the # symbol?

All three are recognized by the postal service: "Apartment 4B," "Apt 4B," and "#4B" all work. Use whichever fits the space available or matches the format requested on the form. "Apt" is the most common abbreviation and saves space on envelopes and labels.

What if I do not know my apartment number?

Check your lease, utility bills, or the building directory posted near the entrance. If those do not show it, ask your landlord or building management. Your apartment number should be on any official document related to your lease or tenancy.

Can I use a PO box instead of my apartment address?

A PO box is a separate mailing address, not a replacement for your street address. You can use a PO box if you want mail to go there instead, but government agencies, employers, and financial institutions usually require your physical street address for official records. You may need both — your apartment address for official documents and a PO box for general mail.

Does the postal service accept "Apt" written without a period?

Yes. "Apt 4B" and "Apt. 4B" are both accepted. The period is optional and does not affect delivery. Use whichever format matches the form or envelope you are filling out.

What if my building does not have apartment numbers, only names?

Write the unit name or identifier in place of the number. For example, "123 Main Street, The Garden Unit" or "123 Main Street, East Wing." Make sure it matches exactly how the building identifies units in its official records and on the lease.