How can I tell if an address is a PO Box?

Start with the address line. USPS Publication 28 writes a PO Box as “PO BOX” and the box number. If the address uses a street name instead, compare it with the local Post Office address. A Post Office street address followed by “#” or “UNIT” and a number points to Street Addressing, and DMM 508.4.1.2 treats that form as PO Box service.

Is an address with a # sign a PO Box?

Not always. USPS rules use the # sign in three kinds of address. The sign can mark a PO Box with Street Addressing, a private mailbox at a commercial mail receiving agency, or a suite or apartment. Check what the street address is before you decide.

Can a PO Box have a street address?

Yes, at some Post Offices. Street Addressing lets you use the Post Office’s street address with your box number after “#” or “UNIT”. The USPS PO Box page says the option is offered “if available.” DMM 508, section 4.1.2, still treats that form as PO Box service.

Can a ZIP Code tell me if an address is a PO Box?

Sometimes. USPS address data gives some ZIP Codes the classification code P, which means the ZIP Code has only Post Office Boxes. A PO Box in a ZIP Code that also has street delivery does not get that code. A ZIP Code lookup alone cannot rule out a PO Box.

Can I use a PO Box on my Employer Identification Number (EIN) application?

The instructions for Form SS-4, the EIN application, say not to enter a P.O. box number on lines 5a and 5b, the street address lines. The instructions for lines 4a and 4b, the mailing address, do not mention P.O. boxes. Lines 5a and 5b are needed only when the physical address is different from the mailing address.

Does the save office address checker detect PO Boxes?

No. The checker does not show a PO Box result. The tool is built to show whether USPS data records a street address as a commercial mail receiving agency. Under CASS Cycle O, USPS data also stopped identifying a PO Box with Street Addressing as a CMRA, so a result that is not a CMRA does not rule out a PO Box. A PO Box written with “PO BOX” is easy to recognize without a tool.

How to Tell if an Address Is a PO Box: Four Checks From USPS Rules

Short answer

You can usually tell from the address line, because USPS addressing standards write a PO Box as “PO BOX” and a number. The harder case is PO Box Street Addressing. There, the box uses the Post Office’s street address with “#” or “UNIT” and the box number.

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Key takeaways

USPS Publication 28, section 281, says a PO Box address is standardized as “PO BOX” and the box number. Section 283 names five other words that are changed to PO BOX: CALLER, FIRM CALLER, BIN, LOCKBOX and DRAWER.
Under Publication 28, section 285, an address at a private mailbox company may not combine the words PO BOX with the private mailbox number on the delivery address line. The same section says only the Postal Service is entitled to provide delivery to a PO Box.
The AIS Products Technical Guide defines PO Box markers in USPS address data files, including record type P and ZIP Classification Code P. Record type P marks a PO Box record in the ZIP + 4 file. ZIP Classification Code P marks a ZIP Code that has only Post Office Boxes.
PO Box Street Addressing is offered at some Post Offices. The box is written as the Post Office’s street address plus “#” or “UNIT” and the box number. Section 508.4.1.2 of the Domestic Mail Manual (DMM) says the # symbol in that form designates PO Box service.
USPS data has markers for Street Addressing boxes. The 2017 USPS guide to these addresses says carrier routes C770 through C779 indicate them. A later USPS summary of Coding Accuracy Support System (CASS) Cycle O adds a Delivery Point Validation (DPV) footnote, PB, for an identified PO Box Street Address.
The # sign alone does not prove a PO Box. Publication 28 also uses # for a suite or apartment number. The DMM lets a commercial mail receiving agency use # in place of PMB (private mailbox).
Some forms split the question by line. The Form SS-4 instructions, for an Employer Identification Number (EIN), say not to enter a P.O. box number on lines 5a and 5b, the street address lines.

How to tell if an address is a PO Box

Look at the address line first. An address written to USPS standards shows a PO Box as “PO BOX” and the box number. The address that needs a closer look is a PO Box with Street Addressing, which uses the Post Office’s street address and a “#” sign.

This guide gives four checks. Each check comes from a USPS document we read on October 14, 2026. The main documents are Publication 28, section 508 of the Domestic Mail Manual (DMM), and two USPS guides for address data.

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Four checks for a PO Box, and what each one misses

The table puts the four checks side by side. Read the “What it misses” column carefully, because each check has a gap.

Check	What to look for	What it misses	USPS source
Read the address line	“PO BOX” and a number. Publication 28 also changes CALLER, FIRM CALLER, BIN, LOCKBOX and DRAWER to PO BOX	A box written with Street Addressing has no “PO BOX” in it	Publication 28, sections 281 and 283
Look up the ZIP Code’s classification	ZIP Classification Code P, which means the ZIP Code has only Post Office Boxes. The code is a USPS data field and is not printed on mail	A PO Box in a ZIP Code that also has street delivery does not get code P	AIS Products Technical Guide, City State product
Check the ZIP + 4 record type	Record type P for a standard PO Box. The record type is a USPS data field and is not printed on mail	You need address software or a service that returns this field. Our free address checker does not show it. A Street Addressing box uses different markers, covered in Check 4	AIS Products Technical Guide, ZIP + 4 product
Look for Street Addressing	The Post Office’s own street address, followed by “#” or “UNIT” and a number. In USPS data, a carrier route from C770 to C779 (2017 PBSA guide) or the Delivery Point Validation (DPV) footnote PB	Nothing printed in the address says Street Addressing. You have to look up the Post Office or use address software that returns these fields	DMM 508, sections 4.1.2 and 4.5.4. Publication 28, section 284. Guide to Post Office Box Street Addresses (2017). CASS Cycle O summary

From USPS Publication 28, DMM 508, the AIS Products Technical Guide (April 2022), the Guide to Post Office Box Street Addresses (January 2017) and the CASS Cycle O summary, read on October 14, 2026.

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Check 1: How USPS writes a PO Box in the address line

Publication 28 is the USPS book of addressing standards. Section 281 says a PO Box address “must be standardized as follows: PO BOX ##.” The ## stands for the box number, so a standard address line reads PO BOX 1234.

Section 283 names five other words that sometimes appear in place of PO Box. The words are CALLER, FIRM CALLER, BIN, LOCKBOX and DRAWER. The standard says to change those words to PO BOX on a mailpiece.

Section 285 adds a rule for addresses at private mailbox companies. That section says: “The words POST OFFICE BOX or PO BOX and the private mailbox number cannot be used on the Delivery Address Line.” The next sentence reads: “Only the Postal Service is entitled to provide delivery to a PO Box.”

Section 285 says the words PO BOX and the private mailbox number cannot be used together on the delivery address line. The same section says only the Postal Service is entitled to provide delivery to a PO Box, so the words PO BOX point to a box at a Post Office.

Checks 2 and 3: What USPS address data records about PO Boxes

USPS offers address data files, called Address Information System (AIS) products, for mailers and software makers. The fields in those files are defined in the AIS Products Technical Guide. The edition linked from PostalPro when we checked is dated April 2022.

We read the guide’s definition of the Record Type Code and counted the codes. The guide lists seven: G for general delivery, H for high-rise, F for firm, S for street, P for PO Box, R for rural route or contract, and M for multi-carrier. Six of the seven are used in the ZIP + 4 product. M is used only in the Carrier Route product.

A PO Box record is simple to spot in the ZIP + 4 file. The guide says: “All Post Office Box records are indicated as such by the words ‘PO BOX’ in the Street Name field in the ZIP + 4 product.”

The City State product describes whole ZIP Codes. Its ZIP Classification Code has four values. M marks a military ZIP Code. P marks a “ZIP Code having only Post Office Boxes.” U marks a ZIP Code assigned to a single organization, and a blank marks a standard ZIP Code with many addresses.

The same product also has a P.O. Box Only file. The guide says those records help mailers find communities “where the customer's only acceptable form of postal delivery is through a P.O Box address.” The guide also says that flag is different from ZIP Classification Code P.

A PO Box written with Street Addressing uses different markers in USPS data. Check 4 covers them.

Check 4: The PO Box that uses a street address

Some Post Offices offer an option called Street Addressing. The USPS PO Box page describes it as using “the street address of your Post Office location, combined with your PO Box number, as your mailing address.” The same page says the option is offered “if available.” Its purpose is to let private carriers such as UPS and FedEx deliver packages to the box.

DMM section 508.4.5.4 lists Street Addressing as an option of competitive PO Box service. The section gives this example: John Smith, 123 Main Street #4567, Any Town, NY 10001. Publication 28, section 284, says the box number follows a “#” sign or a “UNIT” designation.

An address in that form looks like a suite in an office building. The DMM still treats it as a PO Box. Section 508.4.1.2 says: “Post Office Box, PO Box, or the # symbol when a Post Office street address is used as allowed under 4.5.4, designates this service in an address.”

USPS published a separate guide on these addresses, the Guide to Post Office Box Street Addresses (PBSA), version 1.1 from January 2017. The guide says its purpose is to explain “how to determine if a Street-Style address actually represents a Post Office Box address.” In 2017 the guide said about 6,400 Post Offices offered the option.

The PBSA guide also lists limits. Street Addressing is not available to caller service customers or to alphanumeric box numbers. Customers with a free Group E box cannot use it either.

1Compare the street address with Post Office addresses. USPS lists them in its Locate a Post Office tool. Two things together point to Street Addressing. First, the street address belongs to a Post Office. Second, a “#” or “UNIT” number follows it.
2Look at the carrier route in USPS data. The 2017 PBSA guide says: “Carrier route numbers C770 through C779 indicate PBSA addresses.” The same guide says address software certified under the USPS Coding Accuracy Support System (CASS) can return that carrier route.
3Look for the DPV footnote PB. A USPS summary of CASS Cycle O added a Delivery Point Validation (DPV) footnote, PB, for an “Identified PO Box Street Address.” The same summary says “PBSA records will no longer be identified as a ‘CMRA’.”

The PBSA guide also explains a change in format. When CASS software processes one of these addresses, “the ‘#’ value is standardized to the term ‘Unit’.” In the guide’s example, 131 S Center St # 3094 becomes 131 S Center St Unit 3094.

The 2017 guide says all PBSA addresses were included in the DPV table for commercial mail receiving agencies. Under the later Cycle O summary, PBSA records are no longer identified as a CMRA. The PBSA guide does not give a record type for these addresses, so we do not say they carry record type P.

You will not see any of these fields on an envelope. Our free address checker does not show the record type, the carrier route or the PB footnote. USPS data under Cycle O also stopped identifying these boxes as CMRAs. For that reason, a result that is not a CMRA does not rule out a PO Box. The checker is not a PO Box test.

Our guide to the USPS virtual address question covers what Street Addressing does for package delivery.

Three kinds of address that use the # sign

The # sign is the main reason Street Addressing is easy to miss. USPS rules use the same sign in three different kinds of address.

Address type	Who receives the mail	How the number is written	USPS rule
PO Box with Street Addressing	The Post Office, which puts it in your PO Box	Post Office street address, then “#” or “UNIT” and the box number	Publication 28, section 284. DMM 508.4.5.4
Private mailbox at a commercial mail receiving agency (CMRA)	A private business that receives mail for its customers	“PMB” or “#” and the private mailbox number	DMM 508.1.8.3. Publication 28, section 285
Suite or apartment	The person or business at that unit	A designator such as STE or APT. Publication 28 says # should not be used when the correct designator is known	Publication 28, section 213

From USPS Publication 28 and DMM 508, read on October 14, 2026.

A private mailbox at a CMRA looks like a street address and works like a mailbox. Under DMM 508.1.8.2, each CMRA must register with the Post Office responsible for delivery. Under DMM 508.1.8.3, mail delivery to a CMRA requires PS Form 1583, the Application for Delivery of Mail Through Agent.

The CMRA case is the one our free address checker is built for. The checker shows whether USPS data records a street address as a CMRA. Our guide on how to check if an address is a CMRA explains how to read that result.

Why business forms ask whether an address is a PO Box

Some forms care about which line an address goes on. The Form SS-4 instructions are one example. Lines 4a and 4b ask for the mailing address. Lines 5a and 5b ask for the street address, and the instructions say: “Don’t enter a P.O. box number here.”

The same instructions say to provide the physical address on lines 5a and 5b “only if different from its mailing address shown on lines 4a–4b.” The instructions for lines 4a and 4b do not mention P.O. boxes.

Some banks write similar rules. Mercury is one example. Its company address page says: “We can not accept a P.O. box as your legal address.” For the physical address, where the team works day to day, Mercury does not accept “a P.O. box, virtual address, commercial mail receiving agency, mail center, or registered agent address.”

As we read it, Street Addressing does not change the answer on those lines. The reason is DMM 508.4.1.2, which says the # form with a Post Office street address still designates PO Box service. Our guide to business bank account address requirements covers the address rules of more banks.

What to use when a line does not accept a PO Box

Match the address to the line. A PO Box can work on a mailing line that accepts one. A physical address line needs the place where the business actually operates. Mercury’s rule shows that at least one bank also excludes virtual addresses on that line.

You may want a street address for your business mail instead of a PO Box. A save office address is a commercial street address with mail handling. Use it on lines that accept a mail service address, such as a mailing address line.

Our comparison of a virtual office and a PO Box looks at the two services in more detail.