Finding the right amount of concrete for any project is important. Too little concrete, and it will crack or even break. Too much concrete is a waste of money and may cause unexpected sinking. This becomes especially important for fence posts, as both broken concrete and sinking could cause strain on the fence that may eventually break it.
Calculating the Amount of Concrete Needed
Measure the radius and depth of the fence post holes. Keep in mind that these holes should be about three times the diameter of the fence post, and a third of the height, plus six inches.
Use these numbers to calculate the volume of the holes.
Note that fence post holes are cylinders. To find the volume of a cylinder, multiply radius squared by depth (sometimes referred to as height) by pi (3.14).
For example, a post hole that is six inches across and 36 inches deep would have a volume of 3^2 x 3.14 x 36 = 1017.36 cubic inches.
Convert cubic inches into cubic feet. A cubic foot equals 1,728 cubic inches. So divide the number of cubic inches by how many inches are in one cubic foot. In the above example, we would have .59 cubic feet (1,017.36 divided by 1,728).
To calculate how much concrete you will need for all the fence post holes, multiply the number of cubic feet in one hole by the number of holes you have (all the fence post holes should be the same size). Bags of concrete will say how many cubic feet they will be good for.