Put the apartment number on the same line as the street address

The standard format for an apartment address in the United States is: street number and name, then apartment number, all on one line. Write it like this: 123 Main Street, Apt. 4B or 123 Main Street #4B. Both formats work equally well with mail carriers and online forms.

If you are filling out a form with separate fields — one for "Street Address" and one for "Apartment/Unit" — put the street address in the first field and the apartment number in the second. The form will combine them correctly when it processes your information.

Never put the apartment number on a line by itself below the street address unless the form explicitly has a separate line labeled for it. Doing so can confuse mail sorting machines and delay delivery.

Key Takeaways

  • Write apartment numbers on the same line as the street address, separated by a comma or preceded by "Apt." or "#".
  • If a form has separate fields for street address and apartment number, fill each field in order — the form will handle the rest.
  • Never put the apartment number on its own line unless the form specifically asks for it in a separate field.
  • Use "Apt." for apartment, "Unit" for unit, or "#" for number — all three are recognized by mail carriers and delivery services.
  • Always include your apartment number, even if you think the mail carrier knows where you live, because mail sorting relies on the complete address.

How to fill out online forms and shipping addresses

Most online forms — whether for mail delivery, utility setup, or government documents — have a dedicated field for apartment or unit numbers. Look for a field labeled "Apartment," "Unit," "Suite," or "Apt. #." Type your number there without repeating the street address.

If the form has only one address field and no separate apartment field, type the full address on one line: 123 Main Street, Apt. 4B, City, State 12345. This works for shipping sites, online retailers, and most government portals.

Some forms will not let you proceed until you fill in every field, including an apartment field. If you do not have an apartment number — you live in a house or a building with no units — leave that field blank or type "N/A." Do not skip the field entirely, as this can cause the form to reject your submission.

Abbreviations and formats that work

The United States Postal Service (USPS) recognizes several ways to write an apartment number. The most common are:

  • Apt. 4B — the traditional abbreviation for apartment
  • #4B — the number sign, increasingly common in digital forms
  • Unit 4B — used for condos, townhouses, and some apartment buildings
  • Suite 4B — standard for office buildings and commercial addresses

Do not use periods or extra punctuation after the number itself. Write Apt. 4B, not Apt. 4B. or Apt #4B (mixing symbols). Stick to one format throughout a single address.

If your building uses a letter-only designation (like "Building A" or "Tower B"), include that before the unit number: 123 Main Street, Building A, Apt. 4B. This is especially important in large complexes where multiple buildings share the same street address.

What happens if you leave out the apartment number

Mail and packages sent to your street address without an apartment number will go to the building's main mailbox or the building manager's office, not to your door. If you live in a multi-unit building, this creates delays because the mail carrier cannot deliver to the correct unit.

For government documents, utility bills, and official mail, a missing apartment number can cause serious problems. Your mail may be returned to the sender, marked as "address incomplete." If you are waiting for a lease document, a court notice, or a housing-related form, a returned letter can delay your entire process.

When you move into an apartment, update your address with every organization that sends you mail: your employer, banks, insurance companies, the post office, and any government agencies. Include the apartment number every time.

Special cases: Buildings with unusual numbering

Some older buildings or complexes use non-standard numbering. You might see addresses like 123 Main Street, Rear (for a unit in the back), 123 Main Street, Lower (for a basement unit), or 123 Main Street, Upstairs. Write these exactly as they appear on your lease or mailbox.

If your building uses a letter instead of a number — 123 Main Street, Unit A — include the letter. If it uses both a letter and a number — 123 Main Street, 4A — write both. The more specific your address, the faster your mail arrives.

When you are unsure how your apartment number should be written, check your lease, your mailbox, or ask your building manager. They will tell you the official format for your address. Use that same format every time you fill out a form.

Addressing mail to someone in an apartment

If you are sending mail to someone who lives in an apartment, follow the same rules. Put their name on the first line, the full street address with apartment number on the second line, and the city, state, and ZIP code on the third line:

Jane Smith 123 Main Street, Apt. 4B Springfield, IL 62701

Do not put the apartment number on a separate line. The USPS sorting machines read addresses line by line, and if the apartment number is on its own line, the machine may not recognize it as part of the address.

Frequently Asked Questions

Should I use "Apt." or "#" for my apartment number?

Both work equally well with mail carriers and online forms. "Apt." is more formal and traditional; "#" is shorter and increasingly common in digital addresses. Choose one and use it consistently. The USPS recognizes both.

What if my apartment number has both a letter and a number, like 4B?

Write it exactly as it appears on your lease and mailbox: Apt. 4B or #4B. Do not separate the letter and number or rearrange them. The letter is part of your unit identifier.

Can I write my apartment number on a separate line if there is not enough space?

No. If space is tight, use the abbreviated format: 123 Main St #4B instead of 123 Main Street, Apartment 4B. Never move the apartment number to its own line, as mail sorting machines may not read it correctly.

What do I do if a form does not have a field for an apartment number?

Type the full address — street number, street name, and apartment number — all on one line in the address field. For example: 123 Main Street, Apt. 4B. The form will process it correctly.

Does the apartment number go before or after the city and state?

The apartment number goes after the street address but before the city and state. The correct order is: street address with apartment number, then city, then state, then ZIP code. Example: 123 Main Street, Apt. 4B, Springfield, IL 62701.