How Much Concrete is Needed for a Metal Fence Post?

Preparation

• Before you can calculate your concrete amount, you will need to dig at least one post hole. The simplest tool for this job is a post hole digger. A post hole digger creates a narrow hole ideal for setting posts. Always review the manufacturer’s instructions for the exact size of the required hole. Holes too shallow will not adequately support the post.

Measurements

• Now that you have at least one of your holes, it is time to get some measurements. The first measurement you need is the radius of your hole. To get an accurate radius, measure your hole’s width and divide by two. Thus, if your post hole is 3 inches wide, the radius is 1 1/2 inches. The next measurement you will need is the depth of the hole. For this, set the tape measure all way down to the bottom of the hole and this is the measurement. For this example, the hole is 24 inches deep.

Formula

• Now that you have the measurements, plug them into a basic geometric formula and determine the necessary square footage of concrete. The formula is pi times the squared post hole radius times the depth of the post hole. To convert that from cubic inches to feet, divide all by 1,728. So, for this example, you would write out pi (1.5^2)*(24), which is 169.64. Next, that is divided by 1728, for a total of .098 cubic feet of concrete.

Finish up the Calculation

• Once you have the amount per post hole, multiply this by the total amount of post holes you will be filling. In this example, if you want to build a fence with 12 posts, this would require about 1.18 cubic feet of concrete for the entire project. Keep in mind that while the amount is precise, you should always allow for some margin of error during installation and order some extra concrete. You don’t have to mix it up unless you need it and can save it for your next project.