The basic format for an apartment address

An apartment address follows the same structure as any street address, with one addition: the apartment number. Write it as: your name on the first line, the street address with apartment number on the second line, then the city, state, and ZIP code on the third line.

The apartment number can go two ways. The most common format in the United States places it after the street address on the same line, separated by a comma or the abbreviation "Apt." — for example, "123 Main Street, Apt. 4B" or "123 Main Street, #4B". Some buildings use "Unit" instead of "Apt." The postal service accepts all three formats, though "Apt." is the most widely recognized.

If you are filling out a form with separate fields for street address and apartment number, put the street address in the first field and the apartment number in the second. Never leave the apartment number off when mailing something to an apartment building — it will either be returned or delivered to the wrong unit.

Key Takeaways

  • Write the apartment number on the same line as the street address, using "Apt.", "Unit", or "#" followed by the number.
  • The complete address includes your name, street address with apartment number, city, state, and ZIP code in that order.
  • If a form has separate fields for street and apartment, use the street field for the building address and the apartment field for your unit number.
  • The postal service recognizes multiple formats, but "Apt." is the most standard and least likely to cause confusion.
  • Leaving off the apartment number can result in mail being returned or delivered to the wrong unit in the building.

When to use "Apt." versus "Unit" or "#"

"Apt." is the abbreviation for apartment and is the most widely used format across the United States. It is recognized by the U.S. Postal Service and understood by mail carriers, delivery services, and online forms. If you are unsure which format to use, "Apt." is the safest choice.

"Unit" is also acceptable and is sometimes preferred by newer buildings or those with non-traditional layouts. Some buildings use "Unit" because not all spaces are technically apartments — a building might have offices, studios, or mixed-use units. The postal service treats "Unit" the same way it treats "Apt."

The "#" symbol is less formal and is sometimes used in casual writing or on packages, but it is not the standard format for official mail. If you are writing a formal letter, filling out government forms, or sending something through the postal service, use "Apt." or "Unit" instead.

How apartment numbers are assigned

Most apartment buildings number units in a logical pattern, though the system varies by building. In many buildings, the first digit indicates the floor — so 201 is the first unit on the second floor, 305 is the first unit on the third floor. The remaining digits go up as you move down the hallway.

Some buildings use letters instead of numbers for individual units on each floor — for example, 2A, 2B, 2C on the second floor. Others use a completely different system based on when the building was constructed or how the units are arranged. If you do not know your apartment number, check your lease, your mailbox, or the building directory.

Knowing how your building numbers its units can help you find your way around and ensures you give the correct address to delivery services and guests. If you move to a new apartment, always verify the exact unit number before you give out your address.

Writing your address on envelopes and packages

When addressing an envelope by hand, write your 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. Leave a space between the street address and apartment number, or use a comma. Write clearly in blue or black ink so the postal service can read it easily.

For packages, follow the same format. If you are using a shipping label from a carrier like UPS or FedEx, fill in the street address and apartment number in the appropriate fields. Do not write the apartment number in a separate location on the package — it should be part of the delivery address field.

If your building has a main lobby or mailroom, some carriers may ask you to note that on the label so they know where to leave the package. Check your building's delivery policy or ask your landlord whether carriers should leave packages at your door, in the mailroom, or with a doorman.

Filling out online forms with an apartment address

Most online forms have separate fields for street address and apartment number. Put the building's street address in the "Street Address" or "Address Line 1" field. Put your apartment number in the "Apartment", "Unit", or "Address Line 2" field. Some forms label this field differently — look for anything that says "Apt.", "Unit", "Suite", or "Additional Address Information".

If a form has only one address field and no separate apartment field, write the full address on one line: "123 Main Street, Apt. 4B". The form will accept it. Never split the apartment number onto a separate line in a single-field form — the system may not process it correctly.

When ordering something online for delivery to your apartment, double-check that the apartment number appears in the delivery address before you submit the order. Mistakes here are common and can result in your package being delivered to the wrong unit or left in a common area.

Special situations: PO boxes, mailboxes, and alternate addresses

If your building uses a cluster mailbox system (the kind with multiple locked boxes in a common area), your address still includes your apartment number. The postal service uses the apartment number to sort mail into the correct box. Do not use a PO box number in place of your apartment number.

If you rent a private mailbox at a commercial mailbox service, the address format is different. You would write "PMB" (Private Mailbox) or "Box" followed by the number, then the street address of the mailbox location. This is not the same as an apartment address and should only be used if you actually rent a mailbox at that location.

If you are having mail sent to a temporary address or a family member's house while you are moving, use their full address with their apartment number (if applicable). Once you move into your new apartment, update your address with the sender so future mail goes to the correct location.

Common mistakes to avoid

The most common mistake is leaving the apartment number off entirely. This happens when people write only the street address and city, state, ZIP. Mail carriers will attempt delivery, but it may end up at the building's main office, with a neighbor, or returned to the sender. Always include your apartment number.

Another mistake is writing the apartment number in the wrong place. Do not put it on a separate line below the street address when mailing a physical letter — it should be on the same line as the street address. Do not write it in the city field or anywhere else on the envelope.

Some people abbreviate the street name (like "St." for Street or "Ave." for Avenue) inconsistently or use non-standard abbreviations. While the postal service can usually figure it out, stick to standard abbreviations or spell out the street name completely to avoid delays. The same applies to city names — always spell them correctly.

Frequently Asked Questions

Should I write "Apartment" or just "Apt."?

"Apt." is the standard abbreviation and is preferred by the postal service. Writing out "Apartment" is also correct, but "Apt." is shorter and universally understood. Either way works, but "Apt." is more common on official documents and mail.

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

Write it exactly as it appears on your lease or mailbox: "Apt. 4B" or "Unit 4B". The number and letter together identify your specific unit. Do not separate them or rearrange them — use the format your building uses.

Can I use a hashtag symbol instead of "Apt."?

The "#" symbol is less formal and not the standard format for official mail. Use "Apt." or "Unit" on formal letters, government documents, and anything going through the postal service. The "#" symbol is fine for casual notes or informal communication, but it is not recognized as clearly by automated mail sorting systems.

What happens if I give the wrong apartment number?

Mail or packages will likely be delivered to the wrong unit or returned to the sender. If this happens, contact the sender and provide the correct address. For packages, reach out to the carrier as soon as possible — they may be able to intercept it before it is delivered to the wrong apartment.

Do I need to include my apartment number when writing a return address?

Yes. Your return address should be complete and include your apartment number so that if the mail cannot be delivered, it can be returned to you. Write it in the same format as you would write it on the front of an envelope: your name, street address with apartment number, city, state, and ZIP code.