Setting Posts in Concrete
Whether you are building a backyard fence, a retaining wall, a pergola, or sinking a new mailbox, setting your posts in concrete is the best way to ensure they remain perfectly plumb and rot-free for decades.
When you dig a post hole, you are essentially creating a rough cylinder in the earth. Filling that void with concrete anchors the wooden or metal post securely into the subsoil, preventing it from leaning under wind loads or lateral pressure.
Because filling post holes rarely requires ordering a massive ready-mix concrete truck, it is crucial to know exactly how many cubic feet (and thus, how many bags) of concrete you need to purchase from the hardware store.
How to Calculate Volume for a Post Hole
A post hole is a cylinder. However, unlike calculating a pure concrete column, you are usually putting a wooden post inside the hole. That wooden post displaces some of the concrete!
For the absolute highest level of precision, you would calculate the volume of the entire hole, and then subtract the volume of the buried portion of the wooden post. However, because holes dug by hand (or even with a motorized auger) are never perfectly straight or smooth, professional fence builders simply calculate the total volume of the hole and use the displaced volume of the wood as their built-in waste factor.
The Formula
To find the total volume of the hole in cubic yards:
- Divide the Hole Diameter (in inches) by 2 to find the Radius.
- Convert the Radius to feet (divide by 12).
- Square the radius (multiply it by itself).
- Multiply the squared radius by Pi (π ≈ 3.14159) to get the area of the hole in square feet.
- Multiply that area by the Hole Depth (ft) to get the total Cubic Feet.
- Divide the total cubic feet by 27 to convert to Cubic Yards.
Volume (yd³) = (π × ( (Diameter ÷ 2) ÷ 12 )² × (Depth ÷ 12)) ÷ 27
Example Calculation
Imagine you are setting a 4x4 fence post into a hole that is 10 inches in diameter and 36 inches deep.
- Find Radius in feet:
10 ÷ 2 = 5 inches. Then5 ÷ 12 = 0.416 ft. - Square the Radius:
0.416 × 0.416 = 0.173. - Find Area:
3.14159 × 0.173 ≈ 0.545 square feet. - Find Depth in feet:
36 ÷ 12 = 3 ft. - Find Cubic Feet:
0.545 sq ft × 3 ft = 1.635 cubic feet.
How many bags is that?
If you are using standard 80 lb bags of concrete (which yield roughly 0.60 cubic feet each), you would need 1.635 ÷ 0.60 = 2.72 bags. So, you should buy 3 bags per hole.
Fast-Setting vs. Standard Concrete
When setting posts, you have two primary options for bagged concrete:
- Standard Concrete: Requires you to mix the concrete with water in a wheelbarrow or tub before shoveling it into the hole. It is cheaper, stronger, but labor-intensive. You must brace the post while it cures.
- Fast-Setting Concrete (e.g., Quikrete Fast-Setting): You can pour the dry mix directly from the bag into the hole, and then pour water on top. It sets in 20-40 minutes, meaning you often don't even need to brace the post. It is slightly more expensive but saves massive amounts of time on large fence projects.