How to Install a Curbside Mailbox

Installing a curbside mailbox takes an afternoon: call 811 to locate utilities, pick the spot on the carrier's side of the street (USPS puts boxes on the right-hand side of the road in the carrier's direction of travel), dig a hole no more than 24" deep, set a 4×4 wood post or a 2"-diameter steel or aluminum post so the mailbox bottom will land 41"–45" above the road surface with the door 6"–8" back from the curb face, plumb and backfill it, mount the box level, add house numbers at least 1" tall, and check that the door and flag operate freely. The only tools you need are a post-hole digger or shovel, a level, a tape measure, and a drill or wrench for the mounting hardware.

Key facts

• Height: 41"–45" from road surface to the bottom of the box. Setback: 6"–8" from curb face to the door.
• Post: 4"×4" wood or 2"-diameter steel/aluminum pipe; bury no more than 24" (USPS, citing the FHWA).
• Call 811 (free) at least two to three business days before digging.
• Don't set the post in a large concrete footing; it has to yield if hit.
• Put the house number on the box, on the side facing the carrier's approach.

Before you dig

1. Call 811. It's free and required in most states; buried gas and cable lines run along the road edge exactly where mailboxes go.
2. Confirm the location. Replace in the same spot as the old box unless the post office has told you otherwise. For a new address, ask the post office where they want it.
3. Check HOA rules. Some developments dictate post style or color.
4. Read the box's mounting instructions. Bolt patterns differ; a plate-mount box needs a post with a compatible plate, a bracket-mount box needs a 4×4.

Step 1: Choose the post

USPS defers to the Federal Highway Administration: a 4"×4" wooden post or a 2"-diameter standard steel or aluminum pipe, buried no more than 24" deep, so it breaks away on impact. Avoid heavy-wall pipe, concrete-filled posts, and masonry piers.

For a modern house, a slim powder-coated steel post — vertical or angled — is the usual choice. It matches the box and the FHWA guidance, and it doesn't rot.

Step 2: Dig and set the post

Dig a hole 20"–24" deep and about 8" in diameter. Set the post, check plumb with a level on two faces, and backfill in 4" lifts, tamping each. A few inches of gravel at the bottom improves drainage. If you want extra stability in loose soil, use a small collar of concrete near the surface or a bag of fast-setting mix rather than a large footing; the FHWA recommendation is against rigid, unyielding supports.

Steel posts with a plate base can also be surface-mounted to an existing concrete pad or footing with anchor bolts; keep the plate small so the post still yields.

Step 3: Set the height and setback

Measure from the road surface (not the top of the curb, not the lawn) to where the bottom of the box will be: 41"–45". Measure from the face of the curb to where the box door will be: 6"–8". Where there's no curb, measure from the edge of the pavement. The easiest method is to mount the box loosely, hold the assembly in position, mark the post at grade, then set the post to that mark.

Step 4: Mount the box

Set the box on the post plate or bracket, level it front-to-back and side-to-side, and tighten the fasteners. Use stainless or coated hardware. If the box has an internal mounting plate, be sure nothing protrudes into the mail area to snag envelopes. Open and close the door and raise and lower the flag a few times to confirm nothing binds.

Step 5: Numbers

Put the house number on the box in figures at least 1" tall, on the door or on the side the carrier approaches from. If the box is on a different street from the house, use the full street address. Match the number style to the box: for a modern box, a clean sans-serif in a contrasting color or in raw aluminum.

Step 6: Final checks

• Post plumb, box level.
• 41"–45" height, 6"–8" setback.
• Door swings fully, flag moves freely, nothing sharp inside.
• Numbers readable from a moving car.
• Old box and post removed and disposed of.

Common mistakes

• Wrong reference for height. Measuring to the top of the box, or from the lawn.
• Too close to the curb. Flush looks tidy and gets clipped by mirrors.
• Huge concrete footing. A hazard and, in some jurisdictions, prohibited on the right of way.
• Post too deep. More than 24" makes it rigid.
• Numbers on the wrong side. The carrier approaches from one direction.
• Digging without calling 811. Cable and gas lines are shallow at the road edge.

Replacing just the box

If the existing post is sound and at the right height, you don't need to redo it. See Can You Replace a Mailbox Without Replacing the Post?.

Where Modbox fits

Modbox's angled (65°) and vertical posts are 1¼" square steel tube with a 0.125" wall and a 4⅛" × 7⅞" plate, powder coated, 56.5" long, with installation instructions on each product page; measure to confirm the finished box lands in the USPS 41"–45" window. The post's bolt pattern aligns with the box so there are no exposed side holes. Installation PDFs are on each product page.

Frequently asked questions

How deep should a mailbox post be? No more than 24" (USPS/FHWA). 20"–24" is typical.

Do I need concrete? No. Tamped soil or gravel is enough for a 4×4 or a 2" post; a small surface collar of fast-set concrete is fine if the soil is loose. Avoid a large rigid footing.

Which side of the driveway does the mailbox go on? On the right-hand side of the road in the carrier's direction of travel, which is the USPS rule for new routes. For an existing address, keep it where the old box was; the post office assigns the location on new routes.

How high is a mailbox from the ground? 41"–45" from the road surface to the bottom of the box — the "ground" is the road, not the lawn.

Sources

• USPS, "How to Install a Mailbox" — https://www.usps.com/manage/mailboxes.htm
• Common Ground Alliance, Call 811 — https://call811.com/
• Modbox USA, "Modern Angled Curbside Mailbox Post" (post specifications) — https://modboxusa.com/products/modern-mailbox-post-modbox