Concrete is a popular fencing material due to its durability and strength. Concrete posts require little to no maintenance and can survive for up to 30 years.
Underestimating the amount of concrete required can lead to issues, but you also don’t want to have several bags of concrete you won’t use. So, just how much concrete per fence post will you need?
You need to have an amount of concrete to fill a surface three times as wide as the actual post, and enough to fill a hole that is ⅓-½ as deep as the post above the ground height.
You need to determine the dimensions of the post within the hole, and the volume of the concrete required for the post hole itself.
However, there’s a lot more to consider as you first need to measure the fence posts to determine the size of each post hole.
Read on to learn more about the best type of concrete to use, the amount to purchase, and how to install concrete posts.
Determining How Much Concrete Per Fence Post to Use
You need to measure the diameter and depth of the hole. What’s more, you’ll need the height and diameter of the post to calculate the amount of concrete required for each fence post.
As a rule of thumb, the post hole diameter needs to be three times as big as the diameter of the post. For example, if you have a three-inch diameter post, you’ll need a 9-inch diameter hole.
Again, note that the depth of the hole needs to be ⅓-½ of the post’s height.
That means if you have a 12-foot post, you’ll need a hole that is 4-6 feet deep. You should also factor in the frost line and gravel layer to get an accurate depth.
To simply get the correct number of bags of concrete required for each fence post, you can use the formula: (Height of the fence post x (⅓-½ ) + (Width of the fence post x 3).
What’s the Best Type of Concrete for Fence Posts?
Although some people argue that you can use any concrete mix, experts recommend using fast-setting concrete for fence post holes.
Fast-setting concrete dries up quickly and allows you to set the posts in no time. Furthermore, you don’t need to mix it in a wheelbarrow or bucket.
On average, fast-setting concrete hardens in 30-60 minutes, depending on outside temperatures.
The concrete cures in about six hours, and you can proceed to install the posts.
When using fast-setting concrete, ensure that the outside temperature is between 15-32 degrees celsius to allow the concrete to cure evenly.
While you can pour concrete outside the recommended temperature range, it requires experience to get it right.
3” Diameter Fence posts 4” Diameter Fence Posts 6” Diameter Fence Posts
| Depth Bags | Depth Bags | Depth Bags |
| 10 inches 1 | 10 inches 2 | 10 inches 4 |
| 12 inches 1 | 12 inches 2 | 12 inches 4 |
| 14 inches 2 | 14 inches 2 | 14 inches 5 |
| 16 inches 2 | 16 inches 3 | 16 inches 5 |
| 18 inches 2 | 18 inches 3 | 18 inches 6 |
| 20 inches 2 | 20 inches 3 | 20 inches 6 |
| 22 inches 2 | 22 inches 3 | 22 inches 7 |
The estimation is based on a 50lb bag of concrete, depending on the hole diameter and depth.
A Quick Guide on How to Install Fence Posts in Concrete
To set fence posts in concrete, you’ll need to:
1. Measure the Post Holes
Start by measuring and marking the location of your holes, which should be every eight feet. Use spray paint or a stake for this work.
2. Dig the Post Holes
You can use a shovel and a posthole digger bar to make a few postholes. However, if you have a lot of postholes to dig, a powered auger is perfect and makes your work easier.
Remember that post hole depth should be at least ⅓-½ of the overall post length. Also, the posthole width needs to be three times the post’s width.
For example, a 3 x 3 fence post needs to have a 9 inches diameter hole.
3. Place the Post in the Hole
The next step is to position the posts inside the hole. You can either use a piece of lumber to hold the post in place or ask someone to help with the positioning.
Use a level to ensure the posts align before you pour in the concrete.
4. Fill the Post Holes with Concrete
Use the chart above to determine how much concrete you’ll need. Pour the concrete into the holes, ensure that it’s evenly spread at three to four inches from ground level.
Add in the recommended amount of water and allow the concrete to set.
That should take 30-40 minutes if you’re using fast-setting concrete.
However, if you want to get sturdy footing for your fence panels, allow the concrete to cure for 24 hours.
5. Slot the Fence Panels
Add fence panels, and you’re all set.
FAQs
Below we cover some frequently asked questions you may have.
Why Should I Use Concrete for My Fence Posts?
Concrete is suitable for fence posts as it is durable, sturdy, and can withstand high winds. Unlike timber, concrete doesn’t require regular maintenance and is resistant to rot.
How Much Sand and Cement Do I Need for Fence Posts?
You’ll need to mix four aggregate, two sand, and one cement for fence posts. Hand mixing works well, but a cement mixer will come in handy if you need to ensure an even consistency.
Can I Use Fast-Setting Concrete to Set a Fence Post?
Yes, fast-setting concrete is recommended as it doesn’t require any mixing, making your work easier and quicker.
How Much Concrete Will I Need for a 4×4 Fence Post?
A 4×4 fence post needs two-50 pound bags of concrete.
The hole should have a diameter that is the width of your post multiplied by three to give you the amount of concrete you’ll need.
Concreting your fence posts doesn’t have to be time-consuming and daunting.
Calculating the right amount of concrete for your fence posts requires you to know the number of posts, size of the posts, depth of the hole, and the diameter of the hole.
