If you are building your own fence, it’s important to know how much concrete you need for it. You don’t want too much or too little before you start your project. So, how much concrete for a fence post?
A fence post hole needs between one to four 50-lb bags of concrete mix (cement) to securely hold it in place. The amount of concrete will depend on the diameter and depth of the hole you will put into. Mix the cement with sand or any other aggregate instructed on the cement bag. Allow the fluid mixture to turn into hard concrete.
Concrete is the best material for securing fence posts, especially if the soil is sandy and porous. However, if you are using wooden fence posts, rotting will be a problem in the future because, unlike gravel, concrete does not offer drainage.
Read on to learn more about the amount of cement/concrete required to build fence posts, calculate the amount, and other essential information that will help you better understand this topic.
How Much Concrete for a Fence Post?
The majority of fence posts will need from 1 to 4 50-pound bags of cement and some aggregates. You need to use water to mix the cement with the aggregates to make ‘fluid’ concrete. Then you pour this fluid mixture into the hole with the fence posts and allow them to harden into solid concrete.
The amount of concrete you will need will depend on the number of holes and each hole’s depth and diameter. To calculate the appropriate diameter of a hole, you need to multiply the fence posts’ width 3 times. The depth of the hole should be from 1/3 to 1/2 the above-ground height of the post.
Concrete is the best for securing fence posts, especially if the soil is sandy. However, be aware that rotting may occur in the future because concrete is predominantly solid and does not offer drainage.
To save time and effort in mixing cement and aggregates with water to make fluid concrete, you can buy pre-mixed concrete of certain cement and aggregate proportion. All you need is to add water to the pre-mixed concrete according to the instruction on the bag. Then pour the material into the fence hole with the post, add water and allow it to harden.
How to Find the Amount of Concrete Required
As mentioned above, you can choose to use pre-mixed concrete for convenience. Pre-mixed concrete is available in bags by weight or by the cubic yard. Usually, when you order them, the supplier will deliver the goods to your project site.
To find the amount of concrete you will need, decide on the size of fence posts you want and the number of posts you will need. The diameter and the height of the posts will determine how big a hole you should dig.
General Rule of Thumb
- The diameter of the hole should be three times the diameter or width of the post.
- The depth of the hole should be from 1/3 to 1/2 the above-ground height of the post.
Step 1
First, compute the volume of the hole. And then, compute the volume for the post’s portion that you will insert into the hole. Be sure that you are using the same units of measurement. In the United States, the common unit of measurement is English, which means feet, inches, and yards.
Use the following formula for computing the volume of concrete for the post hole:
- For round posts – Volume = radius2 x π x depth, where (π = 3.14)
- For rectangular posts – Volume = length x width x depth
Step 2
When you have calculated the volume of the hole and the submerged post volume, subtract the post volume from the hole volume. The number you will get will give you the volume of concrete required for each hole.
Step 3
Multiply that number to the total number of holes in your fence, and you will get the total volume of cement or concrete that you will need to complete your project. Add a reasonable allowance to that volume for waste and spillage.
It will be good if you have uniform sizes of fence posts. In that way, you will only make one computation. However, if the fence posts are of different sizes, you need to compute their volumes individually. And then add up all their volumes to get the total volume of concrete or cement you will need.
The best way to illustrate how to do the calculation is by example. Let’s assume that you are building a 6-foot fence. One-third of 6 feet is 2 feet. So, 2 feet of the fence will be below the ground. All in all, you need 20 pieces of 4-inch diameter round posts. Actually, 4-inch posts have only 3 1/2 inch diameters.
Post Volume Calculation
- Convert the post diameter to feet: 3.5 inches / 12 = 0.2917 feet
- Calculate the radius: 0.2917 / 2 = 0.1458
- Calculate volume of round post: radius2 x π x depth, where (π = 3.14) = (0.1458 x 0.1458) x 3.14 x 2 feet = 0.0213 x 3.14 x 2 = 0.1336 cubic feet
Hole Volume Calculation
- 3 x radius of post = 3 x 0.1458 = 0.4374 feet
- Compute hole volume: radius2 x π x depth, where (π = 3.14) = (0.4374 feet)2 x 3.14 x 2 feet = 0.1913 x 3.14 v 2 feet = 1.2014 cu. ft.
Concrete Volume Calculation
- Hole volume – Post volume = 1.2014 – 0.1336 = 1.068 cu. ft. per post
Total Concrete Volume Calculation
- 1.068 x 20 posts = 21.36 cu. ft. of concrete
- Convert to cubic yards: 21.36 / 27 cubic yd./cu. ft. = 0.79 cubic yd.
- In terms of the number of bags, one bag of cement or concrete weighs about 40 to 50 pounds. One bag yields around 5.1 cubic feet of concrete. If each bag weighs around 40 pounds, the total bags you will need are about 80 bags. The majority of concrete or cement stores do not sell fractions of their products, but you will lose nothing if you try.
Approximate Volume of Cement/Concrete for Specific Fence Post Sizes
Here are the approximate volumes of cement or concrete mix that you will need for specific sizes of fence posts:
3-Inch Diameter Fence Post
If you want to use fence posts with 3-inch diameters, a post hole of 9-inch diameter with a depth of 1/3 the post height above ground is recommended. If the post height is 6 feet, you will need to bury 2 feet of it underground. That means you need to dig a hole two feet deep.
Here is a table that will show you the number of bags of cement/concrete you will need according to the depth of the hole, with each bag weighing 50 pounds. The figures below are based on a fence post that is 6 feet above the ground and two feet below the ground.
| Depth of Hole, Inches | Number of Cement/Concrete Bags Required |
| 10 to 12 inches | 1 bag |
| 14 to 26 inches | 2 bags |
| 27 to 36 inches | 3 bags |
4-Inch Diameter Fence Post
For a 4-inch post, you will need a hole that is 12 inches in diameter. The minimum depth of the hole should be 1/3 of the above-ground height of the post. Here is a table that will show you how many bags of cement/concrete will be required for every 12-inch diameter hole with a depth ranging from 10 to 36 inches.
| Depth of Hole, Inches | Number of Cement/Concrete Bags Required |
| 10 to 14 inches | 2 bags |
| 16 to 22 inches | 3 bags |
| 24 to 30 inches | 4 bags |
| 32 to 36 inches | 5 bags |
6-Inch Diameter Fence Post
The post hole of a 6-inch diameter post should be 18 inches in diameter with a minimum depth of 1/3 of the post’s above-ground height. Here is a table that will show how many bags of cement/concrete will be required for every 18-inch diameter fence post with depths ranging from 10 to 36 inches.
How much concrete for a 4×4 fence post? A 4×4 fence post will mostly require about 1-1/2 bag per hole of concrete.
| Depth of Hole, Inches | Number of Cement/Concrete Bags Required |
| 10 to 12 inches | 4 bags |
| 13 to 17 inches | 5 bags |
| 18 to 21 inches | 6 bags |
| 22 to 25 inches | 7 bags |
| 26 to 28 inches | 8 bags |
| 28 to 30 inches | 9 bags |
| 32 to 36 inches | 11 bags |
The Yield of Pre-Mixed Concrete Bags
It is good to know the average yield of every bag of pre-mixed concrete. Here is a table that will show you the approximate yield of pre-mixed concrete for the typical bag weights sold in the market.
| Bag Weight | Pre-mixed Concrete Yield (approx.) |
| 40 lb. (18.1 kg.) | 0.30 cubic ft. (8.5 L.) |
| 60 lb. (27.2 kg.) | 0.45 cubic ft. (12.7 L.) |
| 80 lb. (36.3 kg.) | 0.60 cubic ft. (17 L.) |
The Best Kind of Cement/Concrete for Fence Posts
There are several types of cement used to make different grades or classes of concrete. The best for building fence posts is the fast-setting cement or pre-mixed concrete. Moreover, these types of cement do not require mixing in a wheelbarrow or bucket.
You only need to add water to the cement or pre-mixed concrete. It will save you time in mixing the cement and aggregates before putting them into the post hole. Moreover, this cement mix will solidify or harden in about 20 to 40 minutes, speeding up all the fence posts.
Pour into Every Hole
Once you dig the post holes, pour into every hole around three to four inches of gravel. Then compact the gravel using a 2 x 4 wood plank. Set the post in place using a lever to ensure that it is truly vertical. Use two braces to stake the post in place so that it won’t move.
Then pour the dry concrete. Surround the post up to around three inches below the lip of the hole. Start pouring water slowly into the dry concrete until about 1 gallon of water is poured inside. Ensure that the concrete is fully saturated with water.
Curing Time
The curing time of the concrete will depend on the conditions on the project site. It is best to pour concrete when the temperature is from 50°F to 90°F. But you can still pour concrete even outside of this temperature range. It will affect the curing time of concrete, though.
Any standard concrete mix will work just fine if you are not in a hurry to finish your project. If you choose to use the regular cement/concrete mix, the concrete’s curing will take time. In other words, you can’t continue your fence work until the concrete has been set.
Ensure Posts Are Steady
Once the concrete is poured inside the post holes, you need to ensure that the posts will not be moved. You may need to put additional braces to the post for this purpose. Curing of regular cement/concrete mix usually takes 24 hours. So, the mix that you poured this morning will not be fully set until the next morning.
Give an Allowance
For any concreting project, it is wise always to make an allowance. That means you need to order a little more cement/concrete mix than you have calculated. The recommendation is to plan for an allowance of at least 10%.
This additional 10% will cover for the accidental spillage, grading inconsistencies, and settling. It is much better to do this than to have a bad pour where concrete poured in parts resulted in weak or separated mass.
Conclusion: How Much Concrete for a Fence Post?
It will require from one to four bags of cement to secure a fence post. You need to mix the cement with sand and some aggregates while adding water incrementally to make fluid concrete. Then you pour the fluid concrete into the hole where the fence post is and allow the mixture to harden into solid concrete.
The amount of concrete you will need to build a fence post will also depend on the depth and the diameter of the hole where you will insert the post into.
Again, how much concrete for a 4×4 fence post? A 4×4 fence post will mostly require about 1-1/2 bag per hole of concrete.
That’s it! Refer to the tables above for a more detailed cost depending on the size of the fence post you’re installing and the depth of the hole. Good luck!
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