A USPS PO Box is a shelf inside a Post Office, rented in five sizes, that gives an address other than the home for mail and — with Street Addressing — for packages from UPS and FedEx as well. USPS lists the sizes and the ID requirement on its PO Boxes page, read 2026-08-11; the reasons somebody actually signs the lease come down to six recurring situations that keep bringing people to a Post Office window.
Some mail belongs on a metal door at a Post Office lobby, not a slot on a front door.
Every use case reduces to a mismatch between where the person lives and where their mail should arrive. The mismatches are boring and universal.
Registering an LLC, a sole proprietorship, or a domain does not require a home address on public records if the operator does not want it there. USPS itself markets the PO Box for “a side hustle or a storefront”. Business correspondence, invoices, tax notices and vendor checks all sit in the box; suppliers who want a physical street format go through Street Addressing.
Apartments with a single lobby mail slot, rooming houses, and shared driveways lose mail routinely. Sensitive envelopes — tax documents, credit-card offers, replacement cards — land safely in a box with a lock and a key, and the household knows what came without asking.
Porch theft is the plain reason. Even where insurance covers a loss, replacing an item and refiling a claim is a day nobody wants. Packages arrive at the Post Office and wait behind the counter or in the box until the owner walks in.
An address that does not change when the lease does. Anyone moving between apartments in the same town, traveling for months, or between houses can hold the same PO Box across the interval; the driver's license and the bank keep working.
A hold on residential mail lasts 30 days at most. A PO Box holds everything indefinitely while the traveler is away; the box does not care whether it is filled today or in six weeks, and the neighborhood mail person is not accumulating a suspicious pile at the front door.
The Street Addressing service lets a PO Box holder use the Post Office's street address followed by their PMB (Private Mailbox Number) style suffix, and UPS, FedEx and Amazon can then deliver packages to the Post Office where staff place them in the box. Not every Post Office offers it — call before signing the lease.
USPS publishes the five inside dimensions and a rough capacity for each on the PO Boxes page. What follows are the dimensions USPS lists; pricing is set per Post Office and shown on the reservation page for that location.
| Size | Inside dimensions | Typical capacity |
|---|---|---|
| XS (Size 1) | 3 in x 5.5 in | 10 to 15 letters and 3 rolled magazines |
| S (Size 2) | 5 in x 5.5 in | 15+ letters, 5 rolled magazines, or 1 Small Priority Mail Flat Rate Box |
| M (Size 3) | 5.5 in x 11 in | Large envelopes, magazines, and 2 Small Priority Mail Flat Rate Boxes |
| L (Size 4) | 11 in x 11 in | Small and Medium Priority Mail Flat Rate Boxes plus 10 to 15 letters |
| XL (Size 5) | 12 in x 22.5 in | Multiple packages |
The right size for a small business is usually M or L. A pure mail-only use case is fine with XS or S; a store that receives supply samples or returns walks into XL more often than expected. Where a box overflows, the Post Office holds the excess at the counter for pickup and slips a notice into the box.
Reserve online at usps.com/manage/po-boxes.htm for the target Post Office, choose the size and rental term, and take the confirmation to the Post Office window with a completed PS Form 1093 and two forms of acceptable U.S. identification. USPS's own instructions, read 2026-08-11, name that ID requirement in the “pick up your keys” step. The clerk hands over two keys, or a combination for a keyless lock, and the box is live once opened.
Where Street Addressing is offered, ask at the same visit to enroll. The address format switches from “PO Box 1234” to “123 Main Street # 1234” using the Post Office's street address, which is what UPS and FedEx labels need to actually deliver.
Six recurring reasons: a business address that is not the owner's home, mail privacy on a rented apartment or shared house, package security away from a porch, a fixed address while moving or traveling, mail forwarding when temporarily away, and a physical street-formatted address for accepting non-USPS packages via Street Addressing.
Five sizes, from XS through XL. USPS lists the inside dimensions and typical capacity on the usps.com PO Boxes page, read 2026-08-11; sizes range from a 3 by 5.5 in envelope box up to a 12 by 22.5 in package box.
For most business mail, yes. USPS markets the address for “side hustles and storefronts” on its PO Box page. Some registrations that need a physical street address (business licenses, some bank accounts, some state filings) will not accept a PO Box; the Street Addressing service supplies a street-format address for those cases where it is offered at the Post Office.
Not to the box number directly. UPS and FedEx do deliver to the Post Office's street address when the PO Box holder enrolls in Street Addressing, and the mail is placed in the box on arrival. Not every Post Office offers Street Addressing; ask before signing the lease.
A completed PS Form 1093 and two forms of acceptable U.S. identification when picking up keys or the combination at the Post Office, per the usps.com instructions read 2026-08-11.
Local availability, Street Addressing rollout and per-location fees change; treat the details above as what USPS's own page said on the date shown and check the reservation page for the target Post Office.