Letter Box Buying Guide: What to Look For | USPS Rules That Decide

A box at the wrong height gets skipped, and a box that’s too small turns every package into a trip to the post office. USPS sets the mounting rules and size classes; your job is matching one to how much mail shows up at your curb. This guide covers the four curbside sizes, the real install measurements, and where buyers go wrong.

What Size Mailbox Do You Actually Need?

USPS splits curbside residential boxes into four classes, each with a published footprint. Pick by household size and package frequency, not looks.

• T1 (small): about 6 inches wide, 19 inches deep, 8.5 inches high.
• T2 (medium): about 8 inches wide, 22.5 inches deep, 10.5 inches high.
• T3 (large): about 11 inches wide, 24 inches deep, 13.5 inches high.
• T4 (extra-large): about 12 inches wide, 26 inches deep, 15 inches high.

A single-person home with mostly letter mail is fine at T1 or T2.

A narrow decorative slot can fail that test and make the box undeliverable.

How High And How Far Back Should A Mailbox Sit?

That band lets the carrier reach your box from the vehicle without leaving the road. Miss the height and the carrier may skip the stop. Miss the setback and the box can catch a plow, mirror, or passing truck.

Two other requirements apply:

• Approval: curbside boxes must meet Postmaster General approval standards. A box carrying the PMG seal satisfies USPS size and construction rules. Custom-built boxes must meet the same standards.
• Mounting: the box must be positioned so the carrier can serve it from the road — true for post-mounted curbside boxes and decorative pillars alike, as long as measurements land in range.

Comparing real products against these numbers beats guessing — the tested letter box roundup with USPS sizing lines up models against these exact classes.

Curbside Or Wall Mount — Which Applies To You?

USPS dimensions and approval rules cover curbside boxes. Wall-mounted mailboxes aren’t standardized the same way, and USPS publishes no wall-mount dimensions — those setups can also be shaped by local codes.

So the logic splits: if your carrier delivers from the road, you’re shopping against the T1–T4 classes and the 41–45 inch height rule. If mail arrives at a door slot or wall box, USPS sizing doesn’t govern it; local building rules do. Check those before ordering, because a wall box that violates a municipal or HOA requirement is a return either way.

Local restrictions apply on top of USPS standards, not instead of them. HOAs and municipalities can dictate style, color, placement, and mounting even when the box is PMG-approved. USPS standards decide whether the box is deliverable; local rules decide whether it’s allowed.

Full current requirements live in USPS mailbox guidance and the agency’s engineering standards for curbside boxes.

FAQs

Can I buy any decorative mailbox and use it curbside?

Only if it meets Postmaster General approval standards. A decorative box that never went through approval — or a custom build ignoring the size and construction rules — can be refused service. Look for the PMG seal or confirm the box matches a published USPS size class before mounting it.

Does a bigger mailbox always mean better?

No. Oversized boxes still have to sit 6 to 8 inches back at 41 to 45 inches high, and a bulky box on a short post can end up out of position. Match the class to your actual volume: T1 or T2 for single occupants and couples, T3 or T4 once packages and multiple people are in the mix.

What happens if my mailbox height is slightly off?

USPS sets a 41 to 45 inch range so carriers can reach the box from the vehicle without stepping out. A box below or above that band can be skipped or left unserved. Measure from the road surface to the bottom of the box, or to the mail entry point, before you set the post.