This Post Hole Concrete Calculator for Square Posts computes the volume and weight of Concrete required to fill any given size hole with a 4 x 4 post.  Just input hole diameter, hole depth and also the number of post holes. The output computes the total amount of Concrete needed and number of 40 pound, 60 pound, or 80 pound bags.  See below for more detailed information.


by Titus Nelson, PE

Inventor of the Holey-Moley Digger

Use the following 4 x 4 Fence Post Concrete Calculator to compute the volume of the space between a 4×4 post and any diameter hole larger than 5-inches.

4x4 Fence Post Concrete Calculator

The output is in (cubic inches) and (cubic feet).  In addition, the quick computation shows the amount of QUICKRETE in (pounds) to fill the void.  QUICKRETE weight is 133 lb/cuft for these calculations.  In addition, when you input the total number of fence posts, the total QUICKRETE requirement is output.  The total number of bags of QUICKRETE for your job is also shown for 30, 60, and 80 pound bags.

Another point to remember is that filling the hole a few inches below the ground level will require less concrete. Because the concrete is below the ground, less moisture will accumulate which reduces moisture rotting of fence posts.  Therefore,  reduce the depth of your hole by the distance you want the concrete underground.

Actual 4 x 4 Square Post Dimensions

In conclusion, it is important to note that a nominal 4 x 4 post is actually 3.5″ x 3.5″ and the diagonal is 4.95″.   The minimum hole diameter required for a 4 x 4 post is therefore 5 inches. This makes for a tight fit requiring almost no concrete.

The Holey-Moley Digger Head Sizes

The Holey-Moley Digger comes with standard digging heads of 4 inch, 5 inch and 6 inch.   As a result, the digger produces very straight and uniform holes so a calculation for the difference between the 4 x 4 post volume and the hole volume will be fairly accurate. Using these numbers will be quite accurate for 4×4 square posts.

In addition to calculators there are noteworthy blog article on this web page.  One describes Selecting The Best Hole Digging Shop Vacuum.   Likewise, another great article is Digging Deep, Straight, Uniform Holes with a Shop-Vac.  Please let us know how you like these calculators, we love feedback.  Finally, if you would like to be kept up-to-date on blog articles please sign up for more information here.

About the Square Post Concrete Calculator

This calculator outputs the hole volume in cubic inches. Likewise, the space between the post and hole wall is provided in cubic inches.   In addition, you will also find these values in cubic feet.  Since concrete comes in bags by weight, output is also shown in pounds.

Input (inches)

1. Select or Input the Hole “Diameter” in inches.

2. Select or Input the Hole “Depth” in inches

3. Select or Input the number of posts

Output (Volume & Weight)

1. Hole Volume – Cubic Inches

2. 4 x 4 Square Post Volume Underground – Cubic Inches

3. Volume Around the Post – Cubic Inches

4. Volume Around the Post – Cubic Feet

5. QUICKRETE Required Per Post – Pounds

6. QUICRETE Required For Job – Pounds

7. Number of 40 Pound Bags Required

8. Number of 60 Pound Bags Required

9. Number of 80 Pound Bags Required


In contrast to many calculators, this Square Post Concrete Calculator makes it easy to calculate the weight of concrete needed to fill a post hole of given diameter with a 4 x 4 post inside.  As a result the output conveniently provides the number of bags of various weights that you will need.

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