How to Write an Apartment Address (USPS Format)

The short answer

To write an apartment address, put the apartment number at the end of the street line, after the street name and a designator such as APT: 4120 W OAK ST APT 3B, with the city, state, and ZIP Code on the line below. USPS Publication 28 calls that the preferred location; if the street line gets too long, the apartment goes on the line directly above the street line, not below it. Use the designator the building uses (APT, UNIT, STE) and use # only when you don’t know it, with a space after it. On IRIS, type the street in the Street address field and the unit in the optional Apt, suite, unit field, then review both before paying.

Mail a letter online How to address an envelope

What’s the proper way to write an address with an apartment number?

An apartment address has the same three lines as any US address: the name, the delivery address line, and the city, state, and ZIP Code. The apartment is part of the delivery address line, at the end. USPS Publication 28, section 213, says secondary unit designators such as APARTMENT or SUITE are required on the mailpiece wherever the address has one, and that the preferred location is at the end of the delivery address line.

Domestic Mail Manual 602.1.3 lists the required elements in order from the top and tells you to include the apartment number with the street and number. Write it in capitals if you can: Publication 28, section 212, prefers uppercase and a uniform left margin, though lowercase is acceptable. Punctuation may be left out (section 222), so APT 3B needs no period.

An apartment address on an envelope

MARIA LOPEZ
4120 W OAK ST APT 3B
DENVER CO 80204

Apartment address examples

Each example below is the delivery address line, the one that goes directly above the city, state, and ZIP Code. The examples are made up; the formats follow Publication 28.

How to write the street line for common apartment and unit types
Situation	Write it like this	Publication 28 basis
Numbered apartment	1802 MAPLE AVE APT 12	Sec. 213: designator at the end of the line
Apartment with a letter	1802 MAPLE AVE APT 4B	Sec. 213: designator, then the unit as the building writes it
Lettered apartment only	1802 MAPLE AVE APT C	Sec. 213
Condo or townhome unit	55 HARBOR WAY UNIT 210	App. C2: UNIT is approved
Office or business suite	900 MAIN ST STE 300	App. C2: STE for Suite
Floor of a building	77 RIVER RD FL 2	App. C2: FL for Floor
Basement apartment	312 ELM ST BSMT	App. C2: BSMT needs no number after it
Rear or front unit	312 ELM ST REAR	App. C2: REAR and FRNT need no number
Upper or lower unit of a duplex	48 PINE ST UPPR	App. C2: UPPR and LOWR need no number
Unit with an unknown designator	1802 MAPLE AVE # 12	Sec. 213.2: space after #
Fractional street number	123 1/2 MAIN ST APT 2	App. D4: fractional addresses
Hyphenated street number (New York City, Hawaii)	112-10 BRONX RD APT 5D	App. D1: keep the hyphen
Mobile home lot or trailer	400 SUNSET DR LOT 17	App. C2: LOT and TRLR
Street line too long for the unit	APT 1204 on the line above 1500 N LAKE SHORE BLVD	Sec. 213.3: line immediately above

USPS Publication 28, Appendix C2: Secondary unit designators

Where does the apartment number go in an address?

On the same line as the street, at the end. That’s the preferred location in Publication 28, section 213, and it’s what the USPS address-matching files expect: section 231 lists the parts of the delivery address line in order, ending with the secondary address identifier (APT) and the secondary address (3B).

If the street line is too long to hold the unit, section 213.3 says to put the secondary address information on the line immediately above the delivery address line. Don’t put it on its own line below the street. USPS reads the line just above the city, state, and ZIP Code as the delivery address, which is why Publication 28, section 215, puts the intended delivery address on that line when an envelope shows two addresses. An apartment line in that spot can be read as the whole address.

When the street line is too long, the unit goes above it

DANIEL OKAFOR
APT 1204
1500 N LAKE SHORE BLVD
CHICAGO IL 60610

Apt vs. unit vs. suite: which one should you use?

Use the designator the building actually uses. Publication 28 doesn’t assign APT to homes and STE to offices; it lists approved abbreviations and says not to fall back on # when the correct designator is known or shown in the USPS ZIP+4 file. Rental buildings commonly use APT, condo and townhome communities often use UNIT, and office buildings use STE, but your lease, the leasing office, or the label on your mailbox is the real answer.

There’s no difference in delivery between “an apartment” and “a unit” address. Both are secondary addresses: a specific place inside a building that shares one street number. The words matter only because the address should match the USPS record as closely as possible.

USPS-approved secondary unit designators (Publication 28, sec. 213.1 and Appendix C2)
Word	Abbreviation	Needs a number after it?
Apartment	APT	Yes
Unit	UNIT	Yes
Suite	STE	Yes
Building	BLDG	Yes
Floor	FL	Yes
Room	RM	Yes
Department	DEPT	Yes
Lot	LOT	Yes
Space	SPC	Yes
Trailer	TRLR	Yes
Basement	BSMT	No
Front	FRNT	No
Rear	REAR	No
Upper	UPPR	No
Lower	LOWR	No
Penthouse	PH	No
Office	OFC	No
Lobby	LBBY	No

USPS Publication 28, Appendix C2: Secondary unit designators

Do you need to put “Apt” before the apartment number?

Yes, when you know it. Publication 28, section 213, says the pound sign “should not be used” as a secondary unit designator if the correct designator, such as APT or STE, is known or shown in the ZIP+4 file. A bare number after the street, as in 1802 MAPLE AVE 12, is the weakest choice, because the 12 can be read as part of the street address.

If you don’t know the designator, use the pound sign. Appendix C2 lists “Blank, unable to determine” as a case that requires the # on the mailpiece. Section 213.2 adds that there must be a space between the # and the number, so write # 12, not #12. Don’t combine them, as in APT #12; use one or the other.

How to write an address with a building and apartment number

Many complexes have several buildings that share one street name. If each building has its own street number, the building number is already in the address, and you write only the apartment: 210 CEDAR CT APT 4 for building 210. If the unit number already includes the building, such as 3204 for building 3, apartment 204, write APT 3204 and nothing more.

If a complex uses one street number for every building and repeats unit numbers, ask the leasing office or your carrier how its mail is addressed, and check the address in the USPS ZIP Code Lookup, which has its own Apt/Suite/Other field. Publication 28 lists BLDG as an approved designator, but section 231 describes the delivery address line with a single secondary designator and number, and Publication 28 gives no example combining BLDG and APT. When the property tells you to use both, put them together at the end of the street line, building first, as in 900 PARK LN BLDG 3 APT 204.

USPS ZIP Code Lookup

What goes in address line 2?

On online forms, address line 1 is the street address: house number, street name, and any direction or suffix, such as 4120 W OAK ST. Address line 2 is for the apartment, suite, unit, building, floor, or room: APT 3B. Splitting them is a form-design choice that makes data easier to check; when the address is printed, the unit belongs at the end of the street line or on the line above it, as above. USPS’s own ZIP Code Lookup splits it the same way, with a Street Address field and an Apt/Suite/Other field.

Can you put the apartment number in address line 1? Yes. 4120 W OAK ST APT 3B in line 1 matches the Publication 28 format exactly. Leave line 2 empty in that case so the unit isn’t entered twice, and check that line 1 doesn’t hit the form’s character limit.

What doesn’t go in line 2: a second street address, “c/o” or attention lines (those go above the street line), or a PO Box when you’ve already entered a street address.

How to write an apartment address on a form

Paper forms often give you one line for the street. Put the apartment on that same line, at the end, exactly as on an envelope. IRS Form W-9, for example, labels line 5 “Address (number, street, and apt. or suite no.)” and its instructions say to enter your address with the “apartment or suite number” on that line.

When a form has a separate “Apt. no.” box, put only the unit there, such as 3B, without APT or #, unless the form asks for more. When it has no room at all, write the unit on the line above the street, never squeezed in after the city.

IRS Form W-9 (PDF)

Apartment mailbox numbers, college mailrooms, and private mailboxes

In many apartment buildings, each mailbox is labeled with its apartment number, so the apartment is all you need. If your building assigns mailbox numbers that differ from unit numbers, ask the manager which one belongs in your address, and use the apartment number unless told otherwise.

College mailrooms are different. The Domestic Mail Manual, 602.1.4.2, notes that “PO Box” is used incorrectly when it comes before a private box number such as a college mailroom box, so follow the format your school publishes instead of writing PO BOX.

A private mailbox at a commercial mail receiving agency, such as a shipping store, isn’t an apartment. Publication 28, section 285, requires PMB or # followed by the box number, and DMM 507 lists “Undeliverable as Addressed, Missing PMB or # Sign” as a reason mail can be returned.

What happens if you leave the apartment number off?

It depends on the building and the carrier. A carrier who knows the name on the envelope may still put it in the right box. In a large building, the carrier may have no way to tell which box is right, and the letter can come back to the sender. DMM 507, Exhibit 1.4.1, lists the endorsements USPS uses for mail it can’t deliver as addressed, including “Insufficient Address” and “Attempted—Not Known.”

If a letter comes back, fix the address and mail it again in a new envelope with new postage; our returned-to-sender guide explains each endorsement.

What’s my address if I live in an apartment?

Your address is your name, your building’s street address followed by your apartment, and the building’s city, state, and ZIP Code. The building’s street number is not your apartment number. Use the same format for your return address. If you leave your name off a return address, DMM 602.1.5.2 says another clear designation, such as an apartment, suite, or room number, is required so USPS can return the piece.

A return address from an apartment

J PATEL
88 LINCOLN ST UNIT 6
PORTLAND ME 04101

Apartment addresses vs. PO Boxes and c/o addresses

A PO Box is not a secondary unit. It replaces the street line, as PO BOX 458, and you shouldn’t add an apartment to it. If an envelope shows both a street address with an apartment and a PO Box, USPS delivers to the one on the line directly above the city, state, and ZIP Code.

If you’re writing to someone staying in another person’s apartment, keep the apartment on the street line and add a c/o line with the resident’s name under the recipient’s name.

How to enter an apartment address on IRIS

IRIS’s letter form has a Name or company field, a Street address field, an optional Apt, suite, unit field, City, a two-letter State, and ZIP. Type the street in Street address (4120 W Oak St) and the unit in Apt, suite, unit (Apt 3B). The unit field holds up to 64 characters.

Before you can pay, the form checks that each address has a name, a street address, a valid two-letter state, territory, or military code, and a five-digit ZIP Code or ZIP+4. Street suggestions appear as you type; picking one fills the street, city, state, and ZIP but leaves the Apt, suite, unit field for you. Those checks catch format mistakes, not a missing or wrong apartment, so compare the unit with what the recipient gave you. You review the document, both addresses, and the total before paying.

A one-page black-and-white letter costs $3.20 with First-Class postage included. IRIS mails one letter to one US address per order.

Sources & further reading

Product details checked against the IRIS API contract when this article was published. Platform references can change; consult the linked documentation when implementing.

Published by IRIS Postal Forms, operated by Great Lakes Computing Office LLC. Send a correction.

Put it into practice

Mail a letter to an apartment without the stamp run

Upload a US Letter-size PDF, enter the recipient’s street and apartment in separate fields, and IRIS prints and mails it by USPS First-Class Mail to a US address. A one-page black-and-white letter is $3.20 with postage included, and no account or subscription is needed.