The Quick Answer

A USPS-approved curbside mailbox must hold at least 18.5 inches long by 5 inches wide by 6 inches high inside. The largest traditional curbside mailbox can measure up to about 22.8 inches long, 11 inches wide, and 15 inches tall. Commercial 4C mailboxes use a minimum compartment size of 12 inches wide by 15 inches deep by 3 inches high.

Whether you're replacing the mailbox at the end of your driveway or planning a mailroom for a new apartment building, the right size matters. A box that's too small leaves packages on the porch. A box that doesn't meet USPS specifications may not get delivery at all. This guide covers every major mailbox type, with the official numbers and what they mean for you.


Mailbox typeKey USPS dimensionWhere it comes from
Standard curbside (minimum)18.5" L x 5" W x 6" H interiorUSPS-STD-7B capacity test
Largest traditional curbsideUp to 22.81" L x 11" W interior, about 15" tallUSPS-STD-7B, Figure 1
Locking curbside mail slotAt least 1.75" H x 10" WUSPS-STD-7B
Curbside mounting height41"–45" from road to mailbox floorUSPS-STD-7B
Curbside setback6"–8" from curb to mailbox doorUSPS-STD-7B
4C tenant compartment (minimum)3" H x 12" W x 15" DUSPS-STD-4C
4C standard parcel locker15" H x 12" W x 15" DUSPS-STD-4C
4C large parcel locker18" H x 12" W x 15" DUSPS-STD-4C

Not sure which size fits your home or property? Call a US Post Company mailbox expert at (800) 977-2626 and we'll help you choose.


Curbside Mailbox Sizes and Dimensions

Curbside mailboxes are governed by the USPS standard called USPS-STD-7B. It covers any mailbox a carrier serves from a vehicle, on city, rural, and highway contract routes. USPS sorts these mailboxes into three classes:

  • Traditional (T): the classic tunnel-top or dome shape most people picture.
  • Contemporary (C): any other shape, as long as it meets the same size and function rules.
  • Locked (L): mailboxes that secure incoming mail behind a lock.


Minimum size: the 18.5" x 5" x 6" test

Every traditional and contemporary mailbox must pass a capacity test. USPS slides a test block measuring 18.50 inches long, 5.00 inches wide, and 6.00 inches high into the box. It must go in and come out easily, and the door must close fully with the block inside. Any mailbox that fails this test can't be approved.


Maximum size for traditional mailboxes

USPS also sets upper limits on traditional mailboxes. Inside, they can be up to 22.81 inches long and 11 inches wide, and the box can stand up to about 15 inches tall. Contemporary designs can take any shape but can't exceed these maximum dimensions either.


What about "No. 1" and "No. 2" sizes?

You'll often see mailboxes sold as "No. 1" (standard), "No. 2" (large), or T1/T2/T3. These labels come from older USPS drawings. The historical sizes were:


Old sizeLengthWidthHeight
T1 / C118.5"5"6"
T2 / C219.5"6"7"
T3 / C322.5"8"11.5"

When USPS adopted USPS-STD-7B, it dropped the requirement to build to those exact drawings and gave manufacturers more design freedom. Today, a mailbox is approved if it meets the minimum capacity, stays within the maximum size, and passes USPS testing. Look for "Approved by the Postmaster General" on the carrier door. USPS requires that marking on every approved curbside mailbox.

Shop USPS-approved curbside options: Post-Mounted Mailboxes | All Residential Mailboxes



Locking Mailbox Size Requirements

Locking mailboxes have more freedom in shape, but the mail slot has firm rules. Under USPS-STD-7B, the incoming mail slot must be at least 1.75 inches high by 10 inches wide. If the slot has a protective flap, it must swing inward so the carrier can slide mail in flat without extra effort. The slot must face the street and be easy for the carrier to see and reach.

Instead of the block test, USPS tests locking mailboxes with real mail. The slot must easily accept these standard items, and you must be able to retrieve them just as easily:

Test itemSize (L x H x thickness)
Priority Mail and Priority Mail Express envelope12.5" x 9.5" x 0.5"
Small Priority Mail box8.625" x 5.375" x 1.625"

One rule surprises many homeowners: mail carriers do not open locked mailboxes or carry your keys. USPS requires manufacturers to state this in their instructions. That's why every approved locking mailbox uses a slot or chute the carrier can drop mail into.

With mail theft on the rise, locking mailboxes have become one of the most popular upgrades we sell. Read our guide on how locking residential mailboxes work, or browse our USPS-approved locking mailboxes.



Wall-Mount Mailbox and Mail Slot Sizes

Wall-mount mailboxes don't carry the "Approved by the Postmaster General" seal. USPS-STD-7B applies only to curbside boxes, not boxes served at your door. USPS still recommends that door mailboxes follow the spirit of the curbside standard, and your local postmaster should be contacted before you install one.

In practice, that gives you more freedom in size and style. Choose a wall-mount mailbox that holds a normal day of mail, including letters, flats, and catalogs, and mount it near your main entrance where your carrier can see it. Many homeowners find about 55 to 65 inches off the ground comfortable for walk-up delivery.

Mail slots in a door or wall have their own minimums. The opening should be at least 1.5 inches by 7 inches, with the bottom of the slot at least 30 inches above the floor.

Important: If your home gets curbside delivery today, you can't simply move your mailbox to the house. You'll need your postmaster's permission first, because it changes how your route is served.

Ready to install? See our step-by-step wall mount mailbox installation guide, or shop wall-mounted mailboxes.


Mailbox Post Dimensions and Placement

USPS does not approve mailbox posts. Posts are often governed by local rules, state laws, and highway safety guidelines, so check with your town before you dig. That said, a few dimensions are standard across most of the country:


MeasurementStandard
Mailbox height41"–45" from the road surface to the inside floor of the box (or the mail slot on locking boxes)
Setback6"–8" from the front of the curb or road edge to the mailbox door
Post sizeA 4x4 wood post or a 2" diameter steel or aluminum pipe is typical
Burial depthNo more than 24", so the post can break away if a vehicle hits it
House numbersAt least 1" tall, on the box or post

The height and setback numbers matter most. They put your box right where a carrier's arm lands from a mail truck window. For the full breakdown, read our Ultimate Guide to USPS Mailbox Height Regulations.

Skip the post-hole digger. The Spin-Digger™ no-dig mailbox post screws into the ground and sets a mailbox in under 10 minutes. See how it works. Need matching numbers? Browse our mailbox number tags.


Commercial Mailbox Dimensions: 4C, Parcel Lockers, and Cluster Box Units

Apartments, condos, offices, and new subdivisions use centralized mailboxes, and they follow a different set of standards.


STD-4C horizontal mailboxes

USPS-STD-4C is required for new construction and major renovations. Its size rules are:


CompartmentMinimum interior size (H x W x D)
Tenant (customer) compartment3" x 12" x 15"
Standard parcel locker15" x 12" x 15"
Large parcel locker18" x 12" x 15"

Taller tenant compartments are allowed, and one unit can mix sizes. The standard also sets installation heights, measured from the finished floor:

  • No tenant lock higher than 67 inches
  • The USPS Arrow lock between 36 and 48 inches
  • At least one tenant compartment below 48 inches for accessibility
  • No tenant compartment floor lower than 28 inches
  • No parcel locker floor lower than 15 inches

The published STD-4C standard calls for at least one standard parcel locker for every ten tenant compartments. USPS guidance has pushed many projects toward more lockers since then, so confirm the ratio with your local postmaster. Our post on USPS parcel locker requirements explains the change.

For the full rulebook, see What Are the USPS Requirements for 4C Mailboxes? and ADA Accessibility Guidelines for Commercial Mailboxes. Shop: STD-4C Mailboxes


Cluster box units (CBUs)

CBUs are freestanding outdoor units on a pedestal, used for subdivisions and HOAs. They come in standard configurations by door count, such as 16, 13, 12, 8, and 4 doors, with some models adding built-in parcel lockers. Because the pedestal is part of the design, a properly installed CBU meets USPS height requirements without extra planning. Standalone outdoor parcel lockers can be added beside them for more package capacity.

Not sure whether a CBU or 4C system fits your project? Read CBU vs 4C Mailboxes. Shop: Cluster Mailboxes


4B+ vertical and horizontal mailboxes

Older buildings often have 4B+ mailboxes, including vertical units with narrow compartments about 5 inches wide, 6 inches deep, and 15 inches tall. These no longer meet the size rules for new construction. They're allowed only as direct replacements where the wall opening isn't changing. If your renovation alters the opening, you'll need STD-4C. Compare the two in 4C vs 4B+ Mailboxes. Shop replacements: Vertical (1250) Mailboxes | 4B+ / 1400 Series

Planning a multi-unit project? Our team has installed commercial mailboxes to local postmaster standards since 1998. Request a free quote and we'll help you size the system, parcel lockers included.


Which Mailbox Size Is Right for You

The USPS minimum is only a starting point. The best size depends on how much mail and how many packages you get.


If you...ConsiderWhy
Mostly get letters and a few magazinesA standard (No. 1) curbside mailboxMeets USPS minimums, lowest cost
Get catalogs, flats, and small packages weeklyA large (No. 2) curbside mailboxFits more without folding or overflow
Order online often or worry about theftA locking parcel mailboxKeeps packages secure until you retrieve them
Live in the country with a long drive to the roadA large or rural-style mailboxFewer trips, more room between pickups
Manage an apartment, condo, or officeSTD-4C mailboxes with parcel lockersRequired for new construction and major renovations
Manage an HOA or new subdivisionCluster box units (CBUs)Freestanding, USPS-approved, built-in height compliance

A good rule of thumb: if packages already end up on your porch, size up. Rural homeowners can find more ideas in our guide to the best rural mailboxes, and our residential mailbox post guide helps you pick a post strong enough for a larger box.


Frequently Asked Questions


What is the standard size of a mailbox?

A USPS-approved curbside mailbox must fit a test block of 18.5 inches long, 5 inches wide, and 6 inches high. Most standard (No. 1) mailboxes are built slightly larger than this minimum.


What is the largest mailbox USPS allows at the curb?

A traditional curbside mailbox can be up to 22.81 inches long and 11 inches wide inside, and about 15 inches tall. Contemporary designs must stay within those same limits.


Does my mailbox have to be USPS-approved?

Curbside mailboxes should be approved and marked "Approved by the Postmaster General." Wall-mount mailboxes and door slots aren't approved the same way, but your local postmaster should sign off before you install one.


How big does a locking mailbox slot need to be?

At least 1.75 inches high by 10 inches wide. It must accept a flat Priority Mail envelope and a small Priority Mail box.


What size is a 4C mailbox compartment?

The minimum tenant compartment is 3 inches high, 12 inches wide, and 15 inches deep. Standard parcel lockers are at least 15 by 12 by 15 inches.


Can I install a larger mailbox than USPS requires?

Yes, as long as it stays within the maximum curbside dimensions and is installed at the correct height and setback. Larger mailboxes are one of the easiest ways to protect packages from weather and theft.


Get the Right Size the First Time

Choosing the right mailbox size saves you from porch pirates, soggy packages, and a note from your postmaster. At US Post Company, every mailbox we sell is built to meet or exceed USPS standards. We've been helping homeowners, builders, and property managers get it right since 1998.


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