Can I use a # symbol instead of APT?

Use APT when APT is the correct designator for the address. USPS says the # symbol should not be used when the correct designator is known. If # must be used, leave a space between the symbol and the unit number, as in # 4.

Apt vs Unit vs Suite: USPS Secondary Address Format Examples

Q: What is the correct abbreviation for apartment?

APT is the USPS standard abbreviation for apartment. Use the specific secondary designator assigned to the address, such as APT, STE, UNIT, FL, or BLDG.

Q: Is a private mailbox the same as a PO Box?

No. A private mailbox at a commercial mail receiving agency is identified with PMB or # and may not be represented as a USPS PO Box.

Getting apartment, suite, floor, or building information in the right place on an address can be a pain. This guide sorts through where each part belongs and how to format it.

Early in our journey with The Letter Pilot, address formatting was one of our biggest pain points. We built functionality to format addresses automatically and validate them against USPS standards, but in some cases it dropped apartment numbers, suite numbers, building numbers, and other important details. We had unhappy customers, and honestly, that was our fault. We did not understand secondary address information well enough at the time. We know much more about the process now, although it can still be confusing. I still find myself doing a double take when I see something like a floor number instead of an apartment, or a PMB instead of a PO Box. Hopefully, this guide can pass along what we have learned about how these address details work.

Address Formatting Basics

Here are the basics: start with the street address, then put the apartment, suite, unit, or building information after it on the same line. A complete address with secondary information usually follows this format:

Standard Format

RECIPIENT NAME

STREET NUMBER STREET NAME APT/STE/UNIT/BLDG NUMBER

CITY STATE ZIP

What USPS prefers

Place the apartment, suite, unit, floor, or building information at the end of the street-address line.
Use an approved designator such as APT, STE, UNIT, FL, or BLDG.
Use uppercase letters when practical. USPS prefers uppercase, although readable lowercase text is acceptable.
Punctuation can be omitted. USPS shows unpunctuated addresses as preferred, but punctuated addresses can still be acceptable.

What to avoid

Do not put secondary information below the city, state, and ZIP Code line.
Do not leave out the apartment or unit number when the address requires one.
Do not use # as a substitute when the correct designator, such as APT or STE, is known.
If you use #, include a space before the unit number, as in # 4B.

Sources: USPS Publication 28, Section 213 for placement and secondary designators; Section 212 for uppercase formatting; and Section 222 for punctuation.

Standard USPS Address Designators

Most people are familiar with common designators such as APT, STE, and UNIT. PMB is where things can get confusing. The first time I saw it, I assumed it was another way to write PO Box. It is actually short for Private Mailbox, which is a mailbox at a commercial mail receiving agency. A PO Box is different because it is provided by USPS. USPS specifically says a private mailbox number must use PMB or # and cannot be written as PO BOX. I still see PMB addresses every once in a while, so use this quick chart for the common abbreviations you may encounter when mailing.

USPS secondary address abbreviations
Full Name	Abbreviation	What it identifies
Apartment	APT	Apartment number
Suite	STE	Suite number
Unit	UNIT	Unit number
Floor	FL	Floor number
Building	BLDG	Building identifier
Room	RM	Room number
Department	DEPT	Department identifier
Space	SPC	Space number
Lot	LOT	Lot number
# (Number)	#	Fallback when the correct designator cannot be determined
Private Mailbox	PMB	Private mailbox number at a commercial mail receiving agency

Sources: USPS Publication 28, Appendix C2 for APT, STE, UNIT, FL, BLDG, RM, DEPT, SPC, and LOT; and Section 285 for PMB formatting.

Which Address Designator Should You Use?

Use the designator that belongs to the actual address. If the apartment complex identifies the address as APT 5, write APT 5. If the building uses UNIT 5, write UNIT 5. The same idea applies to STE, FL, BLDG, RM, and the other approved designators. You are not choosing the label based on what sounds best. You are preserving the address information assigned to that delivery point.

USPS says the pound sign should not replace a known designator such as APT or STE. Use # only when the proper designator cannot be determined, and include a space before the number, as in # 5. Publication 28, Section 213 explains both rules.

PMB is the important exception. A private mailbox at a commercial mail receiving agency must use PMB or # followed by the mailbox number. It cannot be relabeled as APT, STE, or PO BOX. USPS lays out those requirements separately in Publication 28, Section 285.

Where to Put the Apartment Number

Here are some common placement mistakes I see when people mail through our service. Our system now catches these kinds of errors and generally formats the address correctly. If it cannot confidently fix an address, we reach out to the customer and apply a manual correction on their behalf. These are simply a few things to keep in mind when sending your letters.

✅ Preferred: Same Line as Street Address

JOHN SMITH

123 MAIN STREET APT 4B

NEW YORK NY 10001

This is the USPS-preferred format.

Handwritten envelope showing APT 4 on the same line as the street address — A simple handwritten example with APT 4 included directly after the street address.

✅ Acceptable: Line ABOVE Street Address

JOHN SMITH

APT 4B

123 MAIN STREET

NEW YORK NY 10001

Use this only if the address is too long for one line.

Not preferred: Line Below Street Address

JOHN SMITH

123 MAIN STREET

APT 4B ← Wrong position!

NEW YORK NY 10001

This is not the USPS-preferred placement. If the apartment information will not fit on the street-address line, place it immediately above the street address instead.

Source: USPS Publication 28, Section 213 specifies the preferred position and the alternate position used when the complete delivery address will not fit on one line.

Correct Examples by Type

Here are correctly formatted examples for different situations:

Apartment

JANE DOE

456 OAK AVENUE APT 12

CHICAGO IL 60601

Suite

ACME CORPORATION

ATTN JOHN SMITH

100 CORPORATE BLVD STE 500

DALLAS TX 75201

Unit

MIKE JOHNSON

789 MAPLE COURT UNIT 3

SEATTLE WA 98101

Building + Apartment

SARAH WILLIAMS

200 PARK PLACE BLDG C APT 301

BOSTON MA 02101

Floor Only

ABC COMPANY

555 BUSINESS CENTER FL 12

LOS ANGELES CA 90001

Mobile Home

BOB MILLER

123 TRAILER PARK RD LOT 45

PHOENIX AZ 85001

Common Mistakes to Avoid

Compare these commonly written addresses with the USPS-standardized format:

Common apartment address mistakes
Less standardized	USPS-standardized format	Why
123 Main St. Apt. #4	123 MAIN ST APT 4	Removes punctuation and the unnecessary #
123 Main Street Apartment Four	123 MAIN ST APT 4	Use abbreviations and numbers
123 Main St New York NY 10001 Apt 4	123 MAIN ST APT 4 NEW YORK NY 10001	Apt goes BEFORE city line
123 Main St, Suite 5	123 MAIN ST STE 5	Uses the USPS abbreviation and omits unnecessary punctuation

Sources: USPS Publication 28, Section 213 for secondary-unit placement and use of #; Section 212 for uppercase formatting; and Section 222 for punctuation.

Don't Forget Your Apartment Number

This is the single biggest mistake I see. The street address itself may be valid, so an address-validation system can recognize the building without realizing that an apartment number is missing. The mail then reaches a building with multiple doors or mailboxes, and the carrier has no reliable way to know which one belongs to the recipient. They should not have to guess.

USPS says secondary information such as an apartment or suite designator is required when the delivery location contains one. Without it, the mail may be delayed or returned because the address is not specific enough for delivery. See USPS Publication 28, Section 213.

Special Cases

I have also run into confusing secondary addresses in my own life. It can be incredibly frustrating when an address looks correct but mail still does not arrive. I once lived in a condo complex at an address we will call 2441 W 3RD ST. The complex had two separate buildings, and each building had its own UNIT 4. My address was in Building B, so the complete secondary address information was BLDG B UNIT 4, not UNIT 4B. Leaving out the building identifier could send someone to the wrong side of the complex. Situations like this can get confusing quickly, so let's cover a few of the less obvious scenarios below.

Multi-story apartment building with many separate residential units — In a large apartment property, the building and unit identifiers can both be necessary to locate the correct door.

Letter + Number Combinations (APT 4B)

Preserve the complete identifier assigned to the address. If it is APT 4B, do not shorten it to APT 4 or add a space between the 4 and B. The same principle applies to identifiers such as STE 100A or UNIT 12C.

Multiple Designators (Building + Apt)

An address may contain more than one secondary element, such as BLDG C and APT 301. Preserve the complete address supplied by the recipient or a validated address record rather than inventing an order. USPS Appendix C2 lists the approved designators.

Apartment building entrance marked C512 — What does C512 mean here? The sign alone does not tell us. It may be one combined building identifier, or it may be the property's shorthand for Building C and a numbered location. Do not automatically rewrite it as BLDG C UNIT 512. Use the address supplied by the resident or confirmed by the property.

Very Long Addresses

USPS prefers the secondary unit at the end of the delivery-address line. If the complete address will not fit there, place the secondary information on the line immediately above the street address. See USPS Publication 28, Section 213.

PO Box + Street Address

Avoid listing both when one delivery address will do. If both a street address and PO Box must appear, put the address where the recipient wants delivery immediately above the city, state, and ZIP Code, and use the ZIP Code that corresponds to that address. USPS normally uses the PO Box as the intended delivery address in a dual-address format. See Publication 28, Section 215 and our PO Box addressing guide.

Private Mailbox (PMB)

A private mailbox at a commercial mail receiving agency must use PMB or #, as in 123 MAIN ST PMB 456 or 123 MAIN ST # 456. USPS prohibits using another identifier, such as STE, in place of PMB, and a private mailbox cannot be represented as a USPS PO Box. If the agency's street address already contains a secondary element, USPS requires a separate PMB line. See Publication 28, Section 285.

Frequently Asked Questions

Where does the apartment number go in an address?

USPS prefers the apartment number at the end of the street-address line, as in 123 MAIN ST APT 4B. If the complete address will not fit on one line, place the apartment information on the line immediately above the street address. USPS Publication 28, Section 213 covers both formats.

What is the correct abbreviation for apartment?

APT is the USPS standard abbreviation for apartment. USPS lists other specific designators, including STE, UNIT, FL, and BLDG. Use the one that belongs to the actual address rather than swapping one term for another. See the USPS secondary address designator table.

Can I use # instead of APT?

Use APT when APT is the correct designator. USPS says the # symbol should not replace a known designator. When # must be used, leave a space between it and the unit number, as in # 4. Private mailboxes are a special case: USPS permits either PMB 456 or # 456. See Publication 28, Section 213 and the separate PMB rules in Section 285.

Is a private mailbox the same as a PO Box?

No. A private mailbox is provided by a commercial mail receiving agency and is written with PMB or #. USPS specifically says it may not be represented as a PO Box. A USPS PO Box is addressed with PO BOX followed by its box number. See Publication 28, Section 285 for private mailboxes and Section 281 for PO Box formatting.

Not sure if your address is valid?

Use our free USPS Address Validator to check it before you mail. Get the standardized format with ZIP+4 codes.

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