The correct format includes the apartment number on the same line as the street address or on the line below it

Mail carriers need three pieces of information to deliver a letter to an apartment: the recipient's name, the street address, and the apartment number. The United States Postal Service (USPS) accepts two standard formats. The first puts the apartment number on the same line as the street address, separated by a comma or the abbreviation "Apt." The second puts the apartment number on a separate line between the street address and the city/state/ZIP code.

Both formats work equally well. The key is making sure the apartment number is clearly visible and not squeezed into a corner or written in tiny letters. Mail sorting machines read addresses line by line, so if your handwriting is hard to read or the number is in an unusual spot, the letter may be delayed or sent to the wrong unit.

The USPS prefers uppercase letters and no punctuation, though lowercase and punctuation will not prevent delivery. What matters most is legibility — a mail carrier should be able to read your address in one glance without having to puzzle out abbreviations or guess which number belongs where.

Key Takeaways

  • Write the apartment number either on the same line as the street address (after "Apt.") or on its own line between the address and city/state/ZIP.
  • Use clear, legible handwriting or print the address; mail sorting machines and carriers need to read it quickly and accurately.
  • The USPS prefers "Apt." as the abbreviation, though "Unit," "#," or "Apartment" also work.
  • If you are unsure of the correct format for a specific building, check the lease or ask the landlord or property manager for the official mailing address.

Standard USPS format for apartment addresses

The USPS publishes official addressing guidelines, and they show two acceptable layouts. The first is called the "on-line" format:

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

The second is the "on-separate-line" format:

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

Either one will reach the recipient. The on-line format saves space and is common on envelopes and packages. The on-separate-line format is easier to read when the address is long or the apartment number is complex (like "Apt. 12-B" or "Unit 304"). Choose whichever is clearer for your handwriting or printing.

Do not put the apartment number on the same line as the city and state. Mail sorting machines expect the street address and apartment number to be together, and the city/state/ZIP to be on the last line. Putting them in the wrong order can slow delivery or cause misrouting.

What to do if you do not know the apartment number format

Some buildings use numbers only (Apt. 4), some use letters (Apt. B), and some use both (Apt. 4B). A few use unit names or descriptors instead (like "The Garden Unit" or "East Wing"). If you are unsure which format applies to a specific address, the fastest way to find out is to ask the person who lives there or check their lease or mail.

If you are sending mail to a rental property and do not have contact with the tenant, call the landlord or property management office. They can tell you the exact format the USPS uses for that building. Some buildings have their own internal numbering that differs from what the post office recognizes, so the official mailing address matters.

You can also look up an address on the USPS website or Google Maps. Both often show the standardized mailing format for apartment buildings. If the address appears in multiple formats online, the one on the USPS Postal Service website or the building's official website is the most reliable.

Common mistakes that slow delivery

The most common error is writing the apartment number in a place the mail carrier does not expect — for example, in the top right corner, in parentheses after the ZIP code, or squeezed into the margin. Mail sorting machines scan addresses in a specific order, and if the apartment number is not where the machine looks for it, the letter may be flagged for manual sorting, which adds days to delivery.

Another frequent mistake is using inconsistent abbreviations. If you write "Apt." on one line and "#" on another, or spell out "Apartment" instead of abbreviating it, the address may not match what the post office has on file. Stick to one format throughout the address.

Illegible handwriting is also a problem. If the apartment number looks like it could be a "5" or an "S," or if the street number is unclear, a mail carrier may deliver to the wrong unit or building. If you are addressing many letters, printing is safer than handwriting.

How apartment addresses work in different states and cities

The USPS rules are the same nationwide, but some cities and states have additional local conventions. For example, some older buildings in New York City use "Apt." while newer ones use "Unit." Some buildings in other states use "Suite" instead of "Apartment." These variations do not affect delivery — the post office recognizes all of them — but using the format the building itself uses reduces the chance of confusion.

If you are sending mail to a building in a state or city you are unfamiliar with, ask the recipient or the building's management office what format they use. This is especially important for commercial buildings or mixed-use properties, where "Suite" or "Office" may be more common than "Apartment."

Rural addresses and properties with PO boxes sometimes have different rules. If the address includes a PO box, the apartment or unit number usually goes on the line above the PO box. Check with the post office or the recipient if you are unsure.

Addressing mail to apartment buildings with no unit numbers

Some small apartment buildings do not use unit numbers — instead, mail is addressed to the recipient's name and the building address only. This is common in older buildings with just a few units, or in buildings where tenants are known by name rather than number. If you are unsure whether a building uses unit numbers, ask the recipient or check their mail.

If a building has no unit number but you know which floor or side of the building the person lives in, you can add that information in parentheses on the address line (for example, "123 Main Street (2nd Floor, Left)" or "123 Main Street (Rear Unit)"). This is not required by the USPS, but it can help a mail carrier who is unfamiliar with the building.

Frequently Asked Questions

Should I use "Apt." or "Unit" or "Apartment"?

All three are recognized by the USPS. "Apt." is the most common abbreviation and is preferred by the post office. Use whichever format the building itself uses — check the lease, a piece of mail already sent to that address, or ask the landlord.

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

Write it exactly as it appears: "Apt. 4B" or "Unit 4B." Do not separate the number and letter or rearrange them. If you are unsure of the exact format, ask the tenant or property manager.

Can I write the apartment number in a different location if the envelope is small?

It is best to keep the apartment number on the address block itself, either on the same line as the street address or on the line when ready below it. If space is very tight, the on-line format ("123 Main Street Apt. 4B") takes up less room than the separate-line format.

Does the post office deliver to apartment buildings without unit numbers?

Yes. If the building has no unit numbers, address the letter to the recipient's name and the street address only. The mail carrier will deliver it to the building, and the recipient or building staff will sort it to the correct unit.

What happens if I address a letter to an apartment but forget the unit number?

The letter may be delayed while the post office tries to match the name and address to the correct unit. If the building is large or the name is common, the letter might be returned to sender. Always include the apartment number if you know it.