How to Install a 4X4 Post Mailbox

Measure the post to determine how much of it needs to be in the ground. If the mailbox is mounted on top of the post, measure down from the top to between 42 and 48 inches. Make a mark, and then measure the rest of the post length from this mark. This is how deep the hole must be. If there is a horizontal arm that the box rests on, then measure the post from this point down 42 to 48 inches. Make a mark, and then measure the rest of the post length. The second measurement is the depth the hole needs to be. You will be adding 6 inches of gravel in the hole before you put the post in, so add 6 inches to the hole depth measurement that you have determined.

Dig a hole for the post to sit in. Use a posthole digger to dig out the hole and a measuring tape to check the depth as you go.

Fill the hole with 6 inches of gravel and tamp it down. Place the post on top of the gravel and fill in with layers of dirt and gravel until the hole is full. Tamp down the material as you fill to pack it in tight. Check the post to ensure it stays level as you fill in the hole.

Place the mailbox either on top of the 4-by-4 post or on the horizontal arm depending on the type you are using. Adjust it so that the holes in the bottom of the box are over wood, and so the door of the box is able to open completely. Mark the holes with a pencil so that you know where to drill.

Remove the mailbox and drill pilot holes in the wood. Use a bit that is smaller in diameter than the screws you are using.

Place the mailbox back in position and twist in 2-inch hex-head screws through the holes. Use a socket wrench to do this. A socket wrench will be able to get inside the mailbox, while a drill or screwdriver will not.