How Many Bags of Concrete per Fence Post Do You Need?

Calculating how many bags of concrete you need per fence post is an important step for any DIY fence project. Using the right amount of concrete will ensure that you’ve properly installed your wooden posts so that they stay sturdy for the long haul. This guide will help you make your upcoming project a breeze by giving you all the math you need to figure out how much concrete you should use.

Bags of Concrete per Fence Post Based On Height

Each post hole can require anywhere from one to 10 bags of concrete, depending on the required post hole size. And, of course, your fence length will affect the number of posts you’ll need.

The chart below can help you understand the number of 50-pound concrete bags you’ll need for your project per post. As with any other aspect of fence installation costs, you need to know the fence length and the number of posts to get the most accurate estimates and budget accordingly.

*Post hole volume minus gravel volume minus post volume, assuming the circular post hole depth is 1/3 of the fence and fence post height.

Once you figure out how many bags of concrete you’ll need, you can move on to researching the type of concrete you should use. In most cases, the best option is a fast-setting concrete for fence posts.

Factors That Impact How Much Concrete You’ll Need

Running out of materials when you’re on a roll can quickly take the wind out of your sails. Avoid any last-minute trips to the hardware store by properly calculating how much concrete per fence post you’ll need before you start. Note that these calculations are a general target, and the actual amount you need can vary depending on your soil type.

Size and Depth of Your Post Holes

How deep should a fence post be? Accurately calculating this number is vital, whether you’re installing a new fence or trying to figure out how to replace a fence post.

First, find out your fence post’s diameter and height. For example, the standard wood post size for a 6-foot privacy fence is 4 inches in diameter. Your post hole should be at least three times the diameter of your post, so in this case, that would be 12 inches in diameter.

The depth of your post hole should be one-third to one-half of your fence and fence post height. So, in the example of a 6-foot post, the hole should be two to three feet deep. The table below simplifies this math based on the most popular fence post sizes.

Frost Lines

A frost line is the level at which groundwater freezes in your soil. If you install your fence post above the frost line, then, over time, your post will suffer from frost heaves.

For those unfamiliar with frost heaves, when the soil freezes, the expansion of water can push the non-frozen, moist soil that’s above the frozen layer upward, displacing the earth around your posts. These frost heaves are no joke, as they can even move your home’s foundation, so you can imagine the problems they would cause for a small wooden post.

You can find out where the frost line is in your area by looking into your local building codes. The frost line will be different depending on your location. Regardless of the math you did according to the fence height and post size above, you need to make sure to install your posts below the frost line.

So, even though your post size and fence height may only call for a post hole with a depth of 12 inches, you may need to dig even further. Some of the deepest frost lines in the United States are in parts of Alaska (100 inches), Minnesota (80 inches), and North Dakota (75 inches).

The variation in frost lines means that the cost of your wood fence can vary greatly depending on your location, typically increasing the further north you go.

Gravel

At the bottom of each post hole, you must add 6 inches of gravel before pouring in the concrete. That means that if your post hole needs to be 2 feet deep, you should subtract 6 inches from that to find out how much concrete you’ll need to pour.

As we get into our math below, we’ll show you how gravel comes into play in calculating post-hole volume and the amount of necessary concrete for a fence post.

How to Calculate the Amount of Concrete You Need per Post

One of the first things you’ll need to figure out during a DIY fence installation is how to install a fence post. A large part of getting started is knowing how much concrete per fence post you need.

Most of the calculations to figure out how much concrete you need require you to know the volume of the post hole. You can use the tried-and-true pencil and pen method with one of the formulas below, or you can opt to use a volume calculator. Whatever you choose, make sure that you’re working in the right measurements (inches versus feet). It’s easier to work in inches for the initial calculation.

Follow these steps to calculate how many bags of concrete per fence post you’ll need.

1. Determine the volume of space required for each post hole.

2. Determine the volume that the post will occupy in each hole.

3. Determine the volume that the gravel will occupy in each hole.

4. Subtract the volume of the post and gravel from the volume of space in each hole to determine how many bags of concrete per fence post you need.

1. Calculating Post Hole Volume

Calculate the volume of a rectangular post hole in cubic inches by multiplying the length, width, and depth. A rectangular post hole that’s 12 inches in width and length and 20 inches in depth would be 12 by 12 by 20 inches, for a total of 2,880 cubic inches.

You can calculate the volume of a circular post hole in cubic inches by multiplying the radius squared by pi (3.14) by the depth. The radius is half of the diameter. For a circular post hole that’s 9 inches in diameter (which equals a 4.5-inch radius) and 18 inches in depth, you would multiply 4.5 squared by pi by 18, which is about 1,144.5 cubic inches.

2. Calculating Post Volume

Next, determine the volume that the actual post requires.

Rectangular post volume: To calculate the volume for a rectangular post, follow these steps.

1. Determine the proper depth of the hole in inches.

2. Subtract the 6 inches of gravel in the bottom to determine how far the post must go into the ground.

3. Multiply the length and width of the post by the number of inches it must go into the ground.

For a 4-by-4-inch post that is in a 20-inch hole, you’d multiply 4 by 4 by 14, yielding 224 cubic inches as the volume of the post.

Circular post volume: To calculate the volume for a circular post, follow these steps.

1. Determine the proper depth of the hole in inches.

2. Subtract the 6 inches of gravel in the bottom to determine how far the post must go into the ground.

3. Square the radius (half the diameter) of the post.

4. Multiply the result by pi and by the number of inches the post must go into the ground.

For a 3-inch diameter post in an 18-inch hole, you’d multiply 1.5 squared (which is 2.25) by pi (3.14) by 12, yielding about 85 cubic inches as the volume of the post.

3. Calculating Gravel Volume

Determine how much volume the gravel will occupy in the bottom of the hole.

Rectangular hole gravel volume: To calculate the volume of the gravel in a rectangular hole, follow these steps.

1. Measure the length and width in inches of the rectangular hole.

2. Multiple the length and width by 6 for the depth of the gravel.

For a 4-by-4-inch post, you’d have a 12-by-12-inch hole. Multiply 12 by 12 by 6 to determine the volume of the gravel, which is 864 cubic inches.

Circular hole gravel volume: To calculate the volume for a circular post, follow these steps.

1. Measure the diameter of the hole to determine the radius.

2. Square the radius and multiply the result by pi and by 6 for the depth of the gravel.

For a 3-inch diameter post, you’d have a 9-inch diameter hole or a 4.5-inch radius. Multiply 4.5 squared (which is 20.25) by pi (3.14) by 6, yielding about 381.5 cubic inches as the volume of the gravel.

4. Calculating Concrete Volume

Finally, you can calculate the volume of concrete you need in cubic inches. Just subtract the calculations you made in step two and step three from the number you calculated in step one. Continuing with our examples we’ve been using:

• Rectangular 4-by-4-inch post and hole: Subtract 224 and 864 from 2,880, yielding 1,792 cubic inches of concrete.

• Circular 3-inch diameter post and hole: Subtract 85 and 381.5 from 1,144.5, yielding 678 cubic inches of concrete.

Converting the Concrete Volume to Cubic Feet

As the final step in your volume calculations, divide your concrete volume in cubic inches by 1,728 (the number of cubic inches in a cubic foot) to calculate the concrete volume in cubic feet. Doing so will make it easier for you to figure out how many bags of concrete to buy, because concrete is most often measured in cubic feet, not inches.

So if the rectangular post hole requires 1,792 cubic inches of concrete, it will need 1.04 cubic feet of concrete. A circular post hole that needs 678 cubic inches of concrete will require 0.39 cubic feet of concrete.

Calculate the amount of concrete you need per post and multiply that by the total number of posts you’re planning to install. From there, you can use the chart below to figure out approximately how many bags of concrete you’ll need. You may want to add about 10% to your final calculation to account for potential waste and to ensure you have enough material to do the job without having to return to the hardware store.

Doing the math and calculations accurately here is important so you know how many bags of concrete to purchase. When bagging concrete, product manufacturers measure it by the cubic foot, not by the cubic inch. 

Building a Fence Yourself vs. Hiring a Pro

Now that you know how many bags of concrete per fence post you’ll need, the final choice is up to you: DIY or call a pro.

Although you may be able to handle this job yourself, it can be a challenge for a novice. Beyond squaring everything up, you have to answer questions like, how far apart should fence posts be and should you use fence post foam vs. concrete to install your posts? A pro fence installer will have all these answers for you.

The pro can shorten the amount of time it takes to build your fence by two to four times vs. what it takes you to build it as a DIY project. The pro will also know which materials to order, which saves time versus having to make multiple trips to the hardware store.

The primary advantage of building a fence yourself is saving money. The average cost to build a fence is $1,700 to $4,600, although it can be quite a bit higher for a large fence with high-end materials. You could save up to $1,700 on labor costs on a fence at the top of the average range if you do the work yourself.

If you need to install gates, this can be especially tough for a novice because of the precise work that needs to happen. A gate installation cost can be anywhere from a few hundred dollars for a simple swing-open design to several thousand for a high-end security gate.

If this all sounds like too much work to take on as a DIY job, you can hire a local fence company to handle the project from start to finish. If you live in an area where fence-building companies are scarce, you have a few other options for who to hire to build a fence. Consider hiring a local handyperson because this type of professional can often do the job. However, you’ll want to be sure anyone you hire has experience with building fences.

Mizuki Hisaka contributed to this piece.