How many bags of concrete do i need to Set a mailbox post depends on diameter of post hole and bag size. Typically, you’ll need three 50-pound bags of fast-setting concrete to set a mailbox post in a hole that’s 12 inches in diameter and 18 inches deep.

As per the Federal Highway Administration, the standard size of a mailbox post should be a 4×4 inch wooden support or a 2″-diameter standard steel or aluminum pipe. The standard mailbox height above the ground should be 42 inches and below the ground is 18 inches for good stability.

A 50lb fast-setting concrete serves as a strong foundation for the mailbox post, making it is sturdy option for your mailbox. You have other options like Quikrete 60-pound or 40-pound bag of concrete, but however, to set a mailbox post a 50-pound fast-setting concrete is better option.

As a general rule, the depth of the concrete mailbox post hole should be 1/3 to 1/2 the height of the post and the diameter of the post hole should be 3 times the width of your post.

So, if your mailbox post is 3 feet high and 4 inches wide, then the hole should be 12 inches in diameter and 18 inches deep. A typical mailbox post should be set around 18 to 24 inches into the ground.

To calculate the number of bags of concrete needed for a mailbox post, you’ll first need to determine the volume of concrete required. Here’s a simple guide:

- Decide dimension of mailbox post: Lets it is about 12 inches (1 foot) in diameter and 18 inches (1.5 feet) deep
- Calculate the Volume of the hole: Use the formula for cylinder concrete, volume = (πD^2/4) ×h, such as V = (3.14× 1×1)/4 × 1.5 = 1.18 cubic feet
- Calculate volume of post: Lets a mailbox post measures 4 inches by 4 inches by 18 inches deep, so volume = (4x4x18)/ 1728 = 0.17 cubic feet (since there are 1728 cubic inches in a cubic foot).
- Subtracting the volume of the post from the volume of the hole gives you the amount of concrete required to fill the post hole of a mailbox, such as 1.18 _ 0.17 = approx 1 cubic foot
- Select bag size & find Bag Coverage: A 50lb bag of fast setting concrete yields volume about 0.375 cubic feet, while a 60lb bag yields 0.45 cubic feet volume
- Divide: Divide the total volume of concrete needed by the coverage per bag to get the number of bags required. Here’s the formula: Number of bags = Volume of concrete needed/Coverage per bag, so 1/0.375 = 2.66 bags (or rounded up to 3 bags).

**Conclusions**:-

Typically, you’ll need three 50-pound bags of fast-setting concrete to set a mailbox post in a hole that’s 3 times the size of the post (12 inches in diameter) and 1/2 to 1/3 to the height of the post (18 inches deep).