Build a Mailbox Post

A mailbox post takes a beating. Every delivery driver who overshoots your driveway, every snow plow that cuts the corner, every freeze-thaw cycle that heaves the ground — your post absorbs it all. The difference between a post that lasts three years and one that lasts fifteen comes down to how deep you set it and how you protect the wood from ground contact. Most mailbox posts fail at the soil line where moisture and freeze cycles gang up on exposed wood. A properly built post goes deep, uses pressure-treated lumber rated for ground contact, and gets a full concrete collar that sheds water. This is a Saturday morning project that pays dividends for years. The postal service has specific requirements about mailbox height and setback from the road. Your carrier needs to reach the box without leaving their vehicle, which means 41-45 inches from the road surface to the bottom of the mailbox, and the box face should be 6-8 inches back from the curb. Get these wrong and you'll be rebuilding. Get them right and you'll have a post that stands straight through a decade of deliveries.

1. Call Before You Dig and Mark the Location. Dial 811 at least two business days before you dig. Utilities will mark underground lines for free. While you wait for clearance, measure 6-8 inches back from the curb edge and drive a stake. Check with neighbors about mailbox alignment — a post that's noticeably forward or back from the row looks off and might irritate your carrier.
2. Dig the Post Hole. Dig a hole 10-12 inches in diameter and 24-30 inches deep. In frost-prone regions, go to 30 inches or below your local frost line. A clamshell digger works better than a shovel for pulling dirt from a narrow hole. Keep the sides relatively straight — a bottle-shaped hole where the bottom flares wider actually reduces holding strength.
3. Cut and Prep the Post. Cut your 4x4 to 6 feet total length. This gives you 30 inches below grade and 42 inches above — right in the sweet spot for mailbox mounting. Sand any rough edges that might catch clothing or hands. Do not paint or seal the below-grade portion — pressure treatment needs to breathe, and sealing can trap moisture.
4. Set the Post and Add Gravel Base. Pour 4 inches of gravel into the hole bottom for drainage. Set the post in the hole and have a helper hold it plumb while you check with a level on two adjacent sides. Add another 2 inches of gravel around the post base and tamp it firm. This gravel layer prevents the post from sitting in standing water.
5. Mix and Pour Concrete. Mix an 80-pound bag of concrete following package directions for a firm, workable consistency. Pour it around the post, leaving 2-3 inches at the top for soil and sod. Poke a rod or stick down into the concrete repeatedly to eliminate air pockets. Recheck plumb in both directions and make final adjustments before the concrete starts to set.
6. Brace and Cure. Drive two stakes into the ground on opposite sides of the post, about 2 feet away. Screw temporary diagonal braces from the stakes to the post to hold it perfectly plumb. Leave these in place for 48 hours while the concrete cures. Mist the concrete lightly once or twice during hot weather to prevent cracking.
7. Mount the Mailbox. After the 48-hour cure, remove braces and measure 41-45 inches from ground level to where the mailbox bottom will sit. Most mailboxes mount with a separate bracket that screws to the post. Use 3-inch stainless steel or galvanized screws — standard steel rusts through in two years. Attach the bracket, then bolt the mailbox to the bracket according to manufacturer instructions.
8. Backfill and Finish. Fill the remaining 2-3 inches above the concrete with topsoil. Replace the sod patch if you saved it, or seed the disturbed area. Add reflective numbers to the post or mailbox if your address isn't clearly visible from the street. Apply a coat of exterior primer and paint to the above-ground portion of the post if desired.