A standard US curbside mailbox sits 41–45 inches above the road surface with its door 6–8 inches back from the curb, mounted on a breakaway post buried no more than 24 inches deep.
Two measurements decide whether your mailbox passes or fails, and both get taken to the wrong reference point more often than any other part of the job. The 41–45 inch figure runs from the road surface — not the dirt, not the sidewalk — to the bottom of the box or the point where mail enters a locked design. The setback runs 6–8 inches from the front face of the curb to the mailbox door. Miss either and a postal carrier can decline to deliver.
Everything below is the installation order that keeps those numbers right, from the phone call that comes before the shovel to the final flag test.
What Does USPS Require Before You Dig?
USPS requires you to contact the local postmaster before installing, moving, or replacing a curbside mailbox or its support. This one call settles the details no article can answer for your street.
Ask three questions: whether curbside delivery is authorized at your address, which side of the road the box belongs on, and the exact height and setback the postmaster wants. Local road conditions, state laws, and federal highway rules can add requirements on top of the USPS baseline.
The mailbox itself must be USPS-approved. Standard STD-7C governs curbside mailbox construction, and any custom-built or modified box needs the postmaster’s sign-off. New developments may also require centralized CBU pedestal equipment or STD-4C wall-mounted units instead of individual curbside boxes — worth confirming before you buy anything.
Which Post and Mounting Method Should You Use?
USPS points to Federal Highway Administration guidance recommending a 4-inch-by-4-inch wooden post or a 2-inch-diameter standard steel or aluminum pipe, set so it bends or falls away if a vehicle strikes it.
That breakaway behavior is the whole point. Concrete posts, heavy metal pipes, and improvised anchors like concrete-filled milk cans turn a minor bumper tap into a serious wreck, and USPS specifically discourages them. If a post can’t give, the vehicle and the people inside absorb the impact.
Bury the support no more than 24 inches deep. USPS does not publish a universal post-hole diameter, a concrete quantity, or a required post model — those depend on the post maker’s instructions, your soil, frost depth, and local rules. Follow the manufacturer’s spec sheet for hole size and whether concrete is appropriate at all.
| Specification | USPS Requirement | Measure To |
|---|---|---|
| Finished height | 41–45 inches | Road surface to mailbox bottom or mail-entry point |
| Setback | 6–8 inches | Curb face or road edge to mailbox door |
| Post type | 4×4 wood or 2-inch steel/aluminum pipe | Breakaway design, per FHWA guidance |
| Burial depth | No more than 24 inches | Ground level to post bottom |
| Box approval | USPS-approved, STD-7C | Postmaster confirmation for custom boxes |
| Prohibited supports | Concrete posts, heavy pipe, concrete-filled objects | Anything that won’t yield on impact |
| Mounting hardware | Must not project past the mailbox front | Corrosion-resistant fasteners |
How Do You Set the Post at the Right Height?
Measure the finished height before you dig, working from the road surface up through every component: buried depth, exposed post, bracket or platform, and the mailbox itself. The 41–45 inch target applies to the installed box, never to the bare post top.
Run the job in this order:
- Call 811 to have underground utilities marked, and keep the work area clear of traffic.
- Mark the setback so the mailbox door lands 6–8 inches behind the curb face.
- Dig the hole, stopping at 24 inches of burial depth for the post.
- Set the post plumb, facing the box toward approaching traffic as the postmaster approved, and brace it temporarily.
- Re-check height and setback against the road surface before the post is locked in place.
- Secure the support using the post maker’s specified method — no rigid concrete collar that defeats the breakaway design.
- Attach the mailbox with corrosion-resistant fasteners, keeping every mounting component behind the front face.
If you’d rather skip the guesswork on the support itself, this roundup of tested mailbox posts worth buying covers the breakaway designs that meet the USPS mounting requirements.
When it’s done, the carrier should be able to serve the box without leaving the vehicle, the door and flag should swing freely, and no snow, vegetation, or parked car should block access. Delivery can hinge on the postmaster’s final approval of placement and height, so confirm before the concrete or soil settles for good.
The full specification for curbside construction lives in the USPS STD-7C standard, which governs mailbox dimensions and approval.
Common Questions
Can I measure the 41–45 inches from the ground instead of the road?
No. USPS measures from the road surface, and a sloped or raised shoulder makes that distance very different from the soil line at the post base. Measure from the pavement or road edge the carrier’s vehicle actually drives on, then work backward to your burial depth.
Is a concrete post a good idea for stability?
USPS discourages it. Concrete posts, heavy metal pipes, and concrete-filled objects are unyielding, so a struck mailbox post transfers the full impact to the vehicle instead of bending away. A 4×4 wood post or 2-inch steel or aluminum pipe gives you the stability without the roadside hazard.
How deep should the post hole be?
USPS caps burial at 24 inches. Hole diameter and whether to use concrete depend on the post manufacturer’s instructions, your soil type, and local frost conditions — USPS sets no universal figure. Follow the manufacturer’s spec and check with the postmaster if your area has extra requirements.
Sources
- USPS. “U.S. Postal Service Standard Mailboxes, Curbside (STD-7C)” Governs curbside mailbox construction and approval.
- USPS. “Mailboxes” States the 41–45 inch height, 6–8 inch setback, post guidance, and postmaster contact requirement.
- USPS. “Publication 28: Postal Addressing Standards” Supports curbside delivery and mailbox placement standards.