How Much Concrete for a Mailbox Post? Bags and Depth

How much concrete for a mailbox post is usually 2 to 3 bags of 80-pound concrete. A typical hole is 10 to 12 inches wide and about 24 inches deep, which holds roughly 1 to 1.6 cubic feet of concrete once the post takes up its share. Many people use fast-setting post mix instead, which needs a few more bags because each bag yields less. Before you set a rigid post in concrete next to the road, check local rules, because many road agencies and postal guidelines prefer posts that bend or break away when hit.

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Hole size and concrete volume

Hole	Depth	Concrete (4x4 post)	80-lb bags	50-lb fast-set bags
8 in round	24 in	0.7 cu ft	1 to 2	2
10 in round	24 in	0.9 cu ft	2	3
12 in round	24 in	1.4 cu ft	3	4
12 in round	30 in	1.8 cu ft	3 to 4	5

Volume of a round hole is π × radius² × depth. A 12-inch hole 24 inches deep holds about 1.57 cubic feet; a 4x4 post removes about 0.17 cubic feet of that. An 80-pound bag yields about 0.6 cubic feet, and a 50-pound fast-setting bag yields roughly 0.375 cubic feet.

How deep to set a mailbox post

Mailbox posts are commonly set about 18 to 24 inches deep. Deeper holes help in loose soil and in areas with deep frost, where a shallow post can heave. Most mailboxes need the bottom of the box roughly 41 to 45 inches above the road surface, set back 6 to 8 inches from the curb or road edge. Your local post office can confirm the height and setback for your route.

The roadside safety question

A mailbox post sits where a car may leave the road. A solid post in a big block of concrete can act like a barrier in a crash. For that reason, many state and local road agencies recommend or require posts such as a 4x4 wood post or a light steel pipe that will bend or break away, and some limit how much concrete can be used. Ask your highway department or town public works office before you pour. If concrete is allowed, many people set the post in tamped gravel or a small collar instead of a large block.

Steps for setting the post

Call 811 before digging. Utility lines often run along the road.
Dig the hole 3 times the post width, with a slightly wider bottom.
Add 3 to 4 inches of gravel in the bottom for drainage.
Set and brace the post plumb at the right height.
Fill with concrete to just below grade, then crown the top so water runs away from the post.
Wait about 20 to 40 minutes for fast-setting mix before attaching the box, or a day for standard mix.

Asphalt or concrete driveways

If the post sits in a paved area, you will cut and patch the surface after setting the post. See mailbox post set in asphalt for how to patch around it.

Bottom line

Most mailbox posts take 2 to 3 bags of 80-pound concrete, or 3 to 5 bags of fast-setting mix, in a 10 to 12 inch hole about 24 inches deep. Call 811 first, set the box at the height your post office wants, and check local road rules, since many areas prefer breakaway posts and little or no concrete.

Questions people ask

How many bags of concrete do I need for a mailbox post?

Usually 2 to 3 bags of 80-pound concrete for a 10 to 12 inch hole about 24 inches deep, or 3 to 5 bags of fast-setting mix.

How deep should a mailbox post be?

About 18 to 24 inches is common, deeper in loose soil or frost-prone areas.

Should a mailbox post be set in concrete?

Check local rules. Many road agencies prefer posts that bend or break away when struck, and some limit concrete.

How high should a mailbox be from the road?

The bottom of the box is commonly about 41 to 45 inches above the road surface. Confirm with your post office.

How long before I can attach the mailbox?

About 20 to 40 minutes with fast-setting post mix, or a day with standard concrete.

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