Removing the Old Post from Concrete
The fastest way to remove a mailbox post from concrete is to cut it as close to the ground as possible, then dig out the concrete stub. Use a reciprocating saw or angle grinder with a metal-cutting blade to cut the post 2 to 3 inches below ground level—low enough that you won't trip over it, but high enough to give you something to grab. Wear safety glasses and work gloves; metal shavings and concrete dust will fly.
Once the post is cut, dig around the concrete base with a shovel. You don't need to remove all the concrete—just enough to loosen it. Dig down 6 to 8 inches on all sides of the stub. If the concrete is old and crumbling, it will come apart as you dig. If it's solid, use a pry bar to rock the stub back and forth; this breaks the bond between the concrete and the ground. A sledgehammer works too, but it's harder to control and may damage surrounding pavement.
Once the stub moves freely, pull it out with a pair of locking pliers or a come-along if it's stuck tight. If concrete still clings to the base, chip it away with a cold chisel and hammer. You want the hole as clean as possible before you set the new post.
Key Takeaways
- Cut the old post 2 to 3 inches below ground with a reciprocating saw or angle grinder, then dig around the concrete base to loosen and remove it.
- A new 4x4 post set in concrete will last 10 to 15 years; pressure-treated wood resists rot longer than untreated wood.
- Mix concrete to a thick peanut butter consistency, pour it around the post in the hole, and let it cure for at least 24 hours before hanging the mailbox.
- Check that the post is plumb (perfectly vertical) in both directions before the concrete sets, using a level on two adjacent sides.
- Concrete that is too wet will not support the post; concrete that is too dry will not bind—follow the bag instructions for water ratio.
Choosing the Right Replacement Post
A standard mailbox post is a 4x4 piece of wood, usually 3 to 4 feet tall above ground. Pressure-treated wood is the best choice because it resists rot and insect damage. Untreated wood will rot within 5 to 7 years in most climates; pressure-treated wood typically lasts 10 to 15 years. The cost difference is small—usually $15 to $25 more per post.
If you want the post to last even longer, consider a composite or vinyl post, which will not rot at all. These cost $40 to $80 but require no maintenance and will outlast the mailbox itself. Wood is easier to work with and easier to cut if you need to adjust the height, so most homeowners stick with pressure-treated 4x4.
Buy a post that is at least 4 feet long. You will set 18 to 24 inches in concrete and leave 36 to 42 inches above ground. If the post is too short, you won't have enough concrete depth to hold it steady in wind.
Preparing the Hole and Setting the Post
Dig a hole 10 to 12 inches wide and 24 inches deep. The hole should be roughly centered where the old post was, but if the old concrete is still in the way, move it 6 to 12 inches to one side. A wider hole gives the concrete more surface area to grip the post and the surrounding soil.
Place the new post in the hole and check it with a level. The post must be plumb, meaning perfectly vertical. Check it on two sides—the front and one side—because a post can be plumb on one side and tilted on the other. Adjust the post by adding or removing soil under the base until the bubble sits in the center of the level vial on both sides. Have someone hold the post steady while you work, or brace it with scrap wood wedged against the sides.
Once the post is plumb, you are ready to pour concrete. Do not skip the plumb check—a post that leans even slightly will look wrong and may not hold the mailbox level.
Mixing and Pouring Concrete
Use a concrete mix designed for fence posts or mailbox posts. Quikrete and Sakrete both make 50-pound bags that are formulated to set fast and hold posts steady. One bag will fill a hole about 10 inches wide and 24 inches deep. Buy two bags to be safe; leftover concrete can be used for other projects.
Mix the concrete in a wheelbarrow or large bucket. Add water slowly while stirring with a shovel or mixing paddle. The concrete should look like thick peanut butter—wet enough to pour, but stiff enough to hold the post upright without slumping. If it is too wet, it will not support the post; if it is too dry, it will not flow around the post and bind properly. Follow the bag instructions for the exact water ratio, as different brands vary slightly.
Pour the concrete into the hole around the post. Work it into the corners with a stick or shovel handle to remove air pockets. Fill the hole until the concrete is level with the ground or slightly above it. Smooth the top with a trowel so water will run off instead of pooling around the post.
Curing Time and When to Hang the Mailbox
Concrete reaches about 50 percent strength in 24 hours and full strength in 7 days. You can hang a light mailbox after 24 hours, but it is safer to wait 48 hours. If you hang the mailbox too soon and someone bumps it hard, the post may shift slightly before the concrete is fully set.
During the first 24 hours, keep the concrete damp by misting it with a garden hose if the weather is hot or windy. Concrete that dries too fast can crack. If rain is forecast, cover the hole loosely with plastic to keep it from getting waterlogged, but do not seal it completely—concrete needs some air circulation as it cures.
After 48 hours, the post will be solid enough for normal use. Attach the mailbox bracket to the post at the height you want—usually 42 to 48 inches from the ground to the bottom of the mailbox, which puts it at a comfortable height for mail carriers and homeowners.
Common Mistakes to Avoid
The most common error is not checking that the post is plumb before the concrete sets. A post that is even 1 inch out of plumb over 3 feet of height will look noticeably tilted and may cause the mailbox door to swing shut on its own. Always use a level on two perpendicular sides before pouring concrete.
The second mistake is pouring concrete that is too wet. Wet concrete will not hold the post upright; the post will sink or tilt as the concrete settles. If you mix it too wet, you cannot fix it—you have to wait for it to cure, then dig it out and start over. Err on the side of stiff; you can always add a little water, but you cannot take it out.
A third mistake is not digging the hole deep enough. A 24-inch depth is the minimum for a 4x4 post in most soils. If your soil is sandy or loose, go 26 to 28 inches deep. Shallow concrete will not anchor the post against wind and frost heave, which can lift the post up over winter.
Finally, do not hang a heavy mailbox or install a post-mounted sign on the same post. A mailbox post is designed to hold a mailbox only. Adding extra weight will stress the concrete and may cause it to crack or the post to lean.
Frequently Asked Questions
Can I reuse the old concrete hole?
You can if the old concrete is completely removed and the hole is clean. If chunks of old concrete remain, they will create weak spots in the new concrete and the post may shift. Dig out all the old concrete, even if it takes an extra 30 minutes. A clean hole is worth the time.
How deep should the post go in the concrete?
Bury 18 to 24 inches of the post in concrete. The deeper you go, the more stable the post will be in wind and frost. For a standard mailbox post, 24 inches is ideal. If you live in a climate with deep frost, ask neighbors how deep they set their posts—frost heave can lift a shallow post.
What if the post starts to lean after the concrete sets?
If the post leans slightly and the concrete is still soft (within 24 hours), you can tap it back into plumb with a rubber mallet and a block of wood. If the concrete is hard and the post leans, you will need to remove it and start over. This is why checking plumb before the concrete sets is so important.
Do I need to treat the part of the post that goes in the ground?
If you bought pressure-treated wood, no additional treatment is needed. The pressure treatment goes all the way through the wood. If you are using untreated wood, you can brush wood preservative on the part that will be buried, but pressure-treated wood is a better choice and saves you the step.
Can I use a metal post instead of wood?
Yes. Metal posts are more durable and will not rot, but they conduct cold and can be uncomfortable to touch in winter. They are also harder to cut if you need to adjust the height. Wood is easier to work with and looks more traditional, but metal will last longer with zero maintenance.