How to Build Your Own Custom Mailbox in 7 Steps

A post-mounted mailbox has a vertical post that sticks out of the ground with a horizontal base attached for the mailbox to sit on.

First, determine how long you want your horizontal piece to be, then measure and cut your 10-foot four-by-four post accordingly. Your horizontal piece should be long enough for the length of the box, the width of your vertical post, and a length of wood to stick out of the back. For example, with a 20-inch mailbox and a four-inch post, you can cut a 30-inch horizontal piece that will stick 6 inches out of the back. 

Use your pencil, tape measure, and square to mark your desired length, then make the cut with your circular saw. Run your saw against your carpenter’s square for a straight cut. You may need to rotate the post to cut all the way through.

Decide where you want your two pieces to intersect, leaving enough room at the top for the height of your mailbox and a length of post that extends a bit higher. For example, if your mailbox is 10 inches tall, you can leave about 13 inches above where your pieces will intersect.

Line up your post pieces at your desired intersection, so they’re perpendicular and square. Use your pencil to mark lines on the vertical piece exactly the width of the horizontal piece. Set your circular saw to half the width of the post, and make cuts every eighth inch between, but not on, your pencil marks. Avoiding the pencil marks will help ensure a tight fit.

You’ll be left with thin pieces of wood between your marks that can be gently tapped out with a hammer. Run your circular saw sideways to smooth the surface between your marks. Ensure that your horizontal piece fits snugly in the notch you’ve created—if needed, widen the notch in tiny increments with your saw.

You’ll repeat this process with your horizontal piece so that when you join both pieces at the notch, they’ll fit together flush at a perfect 90-degree angle. Tap them together with a hammer, then use a few grooved nails to secure them in place.

Before attaching your mailbox to the post, you can touch it up with a custom paint job. If your existing mailbox is in good shape and you want to reuse it, you should be able to unscrew it from your old post easily. Otherwise, you can purchase a new plain mailbox for as little as $20 to $30 at a home improvement store.

Choose whatever color spray paint you want for the base. If you want to add an artistic touch, you can hand-paint an additional design using water-resistant acrylic paint. You can also purchase decals or house numbers to affix to the sides.

If you’ve chosen pressure-treated wood or hardwood such as cedar for your post, water damage likely won’t be an issue. However, you can add a coat of water-resistant paint for weatherproofing or decorative purposes.

Start by patching any cracks with wood filler, then sand the whole post down and apply a coat of primer. Add primer to the bottom of the post as well. Even though it won’t be seen, the primer helps with waterproofing. Sand between coats and add another coat of primer if recommended by the manufacturer’s guidelines.

Once your primer has dried, apply two coats of water-resistant paint in a color of your choosing. Allow it to dry fully before moving to the next step.

Using a shovel, clear out a roughly two-foot-wide hole for your mailbox. If you find any large rocks while digging, set those aside for later. The vertical length of your post will determine the depth you need.

According to USPS, the bottom of your mailbox should stand between 41 and 45 inches above the road. Place your post in the ground to ensure you’ve got the right amount of height, tamping down any loose soil at the bottom. If your soil is particularly soft or loose, you can also add a six-inch layer of all-purpose stone to create a solid surface for the post to stand on.

Place your level on the horizontal part of the post to ensure that it is standing straight vertically. Begin to fill the hole back up with soil, tamping it down firmly as you go. Any large rocks should be placed in front of the post to help brace it. When your hole is about halfway full, place a brick in front of the post horizontally for additional support.

Check again that your post is level both horizontally and vertically, as well as pointed toward the road in the exact direction you want. Make adjustments if needed before filling the rest of the hole, continuing to tamp the soil down firmly.

If you’d like, you also have the option to secure your mailbox post using concrete. To do this, mix two bags of concrete in your wheelbarrow according to the instructions on the package. Rather than fill your hole up with soil, fill it with your concrete. At the top, form your concrete into a rounded mound so that rainwater will slide away from your post. Cover the rest with topsoil.

Many mailboxes have a recess underneath, meaning they won’t sit flush with your mailbox post as-is. To solve this issue, use your circular saw to cut a mounting board out of a three-quarter-inch piece of dimensional lumber. Cut it to the length and width of the recessed area so it fits snugly into the recess, making it easier to attach the box to your new mailbox post.

Be sure to test your mailbox door with the mounting board in place. If the door doesn’t open properly, you may need to shorten the length of your mounting board to give it enough space.

Separated from your mailbox, center your mounting board on the horizontal part of your post where you’d like the mailbox to sit. Nail it down with a row of stainless steel nails. Your mailbox should be able to sit snugly on the mounting board. Using stainless steel screws and a power drill or screwdriver, attach the mailbox firmly to the mounting board.

With that, you should have a sturdy, beautiful new DIY mailbox that will last you many years. You can get even more creative with your project with other DIY mailbox ideas. For example, you can affix a simple hanging house number sign to the bottom of the horizontal post using screw-in hooks. You can also design a small garden around the base of your post.