Writing an apartment address seems like it should be simple, and yet it is one of those everyday tasks that most people are quietly unsure about. Does the apartment number go on the same line as the street, or its own line? Should you use commas? All caps? These small questions matter more than they appear, because a poorly formatted apartment address is one of the most common reasons mail and packages get delayed, misdelivered, or returned. The good news is that the United States Postal Service has clear standards, and once you understand them, addressing an apartment correctly becomes straightforward. This guide walks through the correct format, the standard abbreviations, and the special notations that ensure your mail reaches the right unit every time.

The Basic Three-Line Format

The USPS recommends writing an apartment address on three lines, the same structure used for most standard addresses. The first line contains the recipient’s full legal name, the second line contains the delivery address including the apartment number, and the third line contains the city, state abbreviation, and ZIP code. This clean, three-line layout reduces confusion and helps postal workers and sorting machines read the address at a glance.

A correctly formatted apartment address looks like this:

JANE SMITH
123 MAIN ST APT 2B
NEW YORK NY 10001

The recipient’s address is placed in the center of the envelope or package, since this is where mail sorters and carriers automatically look first, while the return address goes in the top left corner using the same format. Keeping the address to three lines whenever possible is the ideal, as this concise structure processes most reliably.

Where the Apartment Number Goes

The single most common point of confusion is where to place the apartment number, and the preferred answer is that it belongs on the same line as the street address, following the street name. Many people mistakenly put the apartment number on a separate line, but USPS prefers it on the delivery address line together with the street information. The formula to follow is street number, then street name, then the unit designator abbreviation, then the unit number, all on the second line: 123 MAIN ST APT 2B.

There is a practical fallback for cases where everything will not fit comfortably on the second line. If the line becomes too crowded, you can move the apartment number to its own third line and shift the city, state, and ZIP code down to a fourth line. This looks like:

JANE SMITH
123 MAIN ST
APT 2B
NEW YORK NY 10001

Either arrangement works, but the single-line version is preferred whenever the information fits, since fewer lines generally process more cleanly through automated sorting.

Standard Abbreviations to Use

Abbreviations make it easier to fit all the necessary information onto the delivery line, and postal services actually prefer them because they save space and improve processing speed. The standard abbreviation for apartment is APT, which should appear before the unit number. While you can write out the word apartment in full, the abbreviation is preferred.

Different types of units use different designators. For an apartment, use APT. For a suite, which some modern buildings use instead of apartment, use STE, as in 123 MAIN ST STE 4B. For a unit, use UNIT. The number sign, written as #, is also acceptable as a general designator before the unit number. The key is to use the term that matches your actual building, since using the correct designator ensures mail and packages are routed to the right door, and to be consistent in how you write it.

If you need to specify which building within a larger complex the unit belongs to, you can add the building number or letter to the delivery line as well, as in 123 MAIN ST BLDG A APT 2B. When this makes the line too long, moving the unit information to its own line using the fallback format above keeps everything readable.

Capitalization and Punctuation Rules

Two formatting details significantly affect how reliably your address is read, and both may surprise people accustomed to standard writing conventions.

The first is capitalization. USPS recommends writing the entire address in all capital letters. Capital letters give postal sorting machines a near guarantee of recognizing an address, which is especially true for printed addresses but applies to handwritten ones as well. If you cannot write neatly in all capital letters, it is worth considering a label maker or printed labels, since legibility directly affects whether your mail arrives.

The second is punctuation. Contrary to a lifetime of adding commas and periods to addresses, USPS advises minimizing punctuation, because it does not help the post office’s sorting machines and can occasionally hinder them. This means dropping the commas and periods that might feel natural, writing 123 MAIN ST APT 2B rather than 123 Main St., Apt. 2B. A clean address with minimal punctuation and full capitalization gives automated systems the best chance of reading it correctly.

It is worth noting that some sources, including several major platforms, do include a comma after the street address when adding apartment information, writing 123 MAIN ST, APT 2B. Both approaches are widely used and generally deliver successfully, but the strict USPS machine-readability guidance favors minimal punctuation.

Using ATTN and C/O Notations

Two special notations help direct mail to the right person in shared or professional situations, and knowing when to use each prevents confusion.

The notation ATTN, meaning attention, is useful when mailing to a specific person within a larger organization or a shared apartment, ensuring the mail goes directly to that individual. You place ATTN at the start of the address with the person’s name, as in ATTN JANE SMITH.

The notation C/O, meaning care of, is used when sending mail to someone at an address that is not their own, such as a letter going to a friend’s place or to an organization on someone’s behalf. You write the apartment address as usual but add C/O before the name of the person receiving the mail on the intended recipient’s behalf. These notations make mail more efficient and help it reach the correct person without misplacement.

When a company is based in an apartment, room, or suite, you add the company name on its own second line and shift the remaining lines down, keeping the recipient name on the first line and the address information below.

Writing the Address Online

When entering an apartment address into online forms for shopping, shipping, or services, follow the same logic as a written address. Most online forms provide a dedicated field labeled Address Line 2, which is commonly used to specify the apartment or unit number, and can also hold additional details like a building name, floor number, or suite designation. Input the street address in the first field, the apartment or unit number in the second field or Address Line 2, and then the city, state abbreviation, and ZIP code in their designated fields. Using this structure online mirrors the correct written format and ensures accurate delivery.

Getting an apartment address right comes down to a few consistent habits: use the three-line format, keep the apartment number on the delivery line when it fits, write in all capital letters, minimize punctuation, use the correct unit designator, and add ATTN or C/O when needed. Applying the ZIP+4 code when you know it, which adds four digits after the standard five-digit ZIP for more precise routing, further improves delivery accuracy. Each detail matters, from the street address to the ZIP code, and following these standards ensures your letters and packages arrive at the right unit without the frustrating delays that come from a poorly formatted address.

FAQ

Where does the apartment number go in an address?

The apartment number should go on the same line as the street address, following the street name, which is the format USPS prefers. The correct order on that line is street number, street name, unit designator abbreviation, and unit number, as in 123 MAIN ST APT 2B. If the information will not fit comfortably on one line, you can place the apartment number on its own separate line and move the city, state, and ZIP code down to the next line, though the single-line version is preferred whenever it fits.

Should an apartment address be written in all capital letters?

Yes, USPS recommends writing the entire address in all capital letters. Capital letters give postal sorting machines a near guarantee of correctly recognizing an address, which applies to both printed and handwritten mail. If writing legibly in all caps is difficult, using a label maker or printed labels is a worthwhile investment to ensure your mail is read and delivered correctly.

What is the correct abbreviation for apartment?

The standard USPS abbreviation for apartment is APT, placed before the unit number, as in APT 2B. While you can write out apartment in full, the abbreviation is preferred because it saves space and improves processing speed. Other designators follow the same logic: STE for suite, UNIT for a unit, and the number sign # as a general option. Use the designator that matches your actual building type.

Do you use commas when writing an apartment address?

USPS advises minimizing punctuation, including commas and periods, because it does not help automated sorting machines and can sometimes hinder them. The machine-readable format writes 123 MAIN ST APT 2B without punctuation. That said, some major platforms do include a comma after the street address, as in 123 MAIN ST, APT 2B, and both approaches generally deliver successfully. For the most reliable machine reading, the minimal-punctuation approach is best.

What do ATTN and C/O mean in an address?

ATTN stands for attention and is used to direct mail to a specific person within a larger organization or shared apartment, placed at the start of the address with the person’s name. C/O stands for care of and is used when sending mail to someone at an address that is not their own, such as a friend’s home, written before the name of the person receiving the mail on the intended recipient’s behalf. Both notations help ensure mail reaches the correct person without confusion.

How do you write an apartment address online?

When entering an apartment address into an online form, use the same format as a written address. Put the street address in the first field, the apartment or unit number in the second field, often labeled Address Line 2, and then the city, state abbreviation, and ZIP code in their designated fields. Address Line 2 can also hold additional details like a building name or floor number. Following this structure online mirrors the correct written format and helps ensure accurate delivery.