Generally, the amount of concrete needed per fence post depends on the size of the post as well as the size of the hole i.e. the depth and diameter. In general practice, 1-4 bags of concrete per fence post is all you need.
Whether you’re upgrading your existing fence or are installing a new one for the outdoors; there’re times when you’re interested to know “how many bags of concrete needed per fence post”.
That’s where it is critical to understand how to estimate the size of the hole as foundation of the post and estimating the amount of concrete or number of bags of concrete needed for the post.
So, in our today’s blog, I’ll guide you with step by step guide on estimating concrete for your fence project.
A fence adds privacy to the home and defines the outdoor space as well. It not just protect your kids and pets by staying in front of your eyes but also protect your house from intruders.
Anyhow, I’m not going to delve deep in about the fence materials, or the other aspects. Rather I’m more interested in guiding you about estimating for the fence project.
Also Read: Fence Cost Calculator – Wood – Chain link – Vinyl – Privacy
Number of bags required for the fence post concrete
There are number of factors that can affect the number of bags required:
- Type of the soil
- Fence design
- Height
- Post size
- Concrete bag weight
Generally, the depth of the post’s hole needs to be 1/3-1/2 of the above ground height of the post.
For instance, a 12 feet high fence post should be buried 3-4 feet underground. The diameter of post hole should be about 3 times the diameter of the post.
To figure out how much concrete per fence post depends on the size of the hole and the size of the post.
While making the post, make sure to place about 6 inches of gravel in the bottom of the hole for proper drainage also it provides protection against frost heaves.
The frost lines refer to the depth at which the ground freezes completely during the winter season.
Normally, the frost line for most areas is about 3 feet down whereas in colder regions the post hole may need to dig much deeper with a depth of 6-7 feet deep.
Also Read: How much is a yard of concrete? – With Bonus tips to save money
1. How Many bags of concrete needed Per Fence Post?
To calculate how many bags of concrete required per fence post, first you need to calculate the volume of the portion of the post within the hole. There is different formula for different types of hole depending on their shape.
For Rectangular Post Hole:
Volume = Length x Width x Depth
For Cylindrical Post Hole:
Volume = Radius x π x Depth
Where π is equals to 22/7 or 3.14.
Concrete Amount:
After calculating all the values, simply subtract the post volume from the hole volume to find the total volume of concrete needed to fill the hole. The concrete volume can be converted to cubic feet or cubic yards as necessary.
Concrete Volume = Hole Volume – Post Volume
Assume we have a hole volume of 5040 cubic inches, to convert this value to cubic feet simply divide it with 1728. This will give you the volume in cubic feet which would be 2.91 cubic feet.
Calculation of Gravel Volume:
To calculate how much concrete for post hole is needed, we need to consider three things which includes total volume of the past hole, the volume of the gravel, and the volume of the post itself.
For rectangular post of 12 inches long hole, multiply the hole’s length by the width by 12 because we will be using 12 inches of gravel at the bottom of the post hole and that will give the gravel volume of 864 cubic inches or 0.5 cubic feet.
Whereas for a circular post hole, multiply the radius square by 3.14 by 6 inches and for a post hole with a 12 inches diameter the final value would be 678.6 cubic inches or 0.39 cubic feet.
Also Read: 10 Boundary Wall Designs That’ll Make You Go WOW!
Fence Post Concrete Calculator Estimate
Here’s an online calculator that helps you estimate the quantity of concrete required for the fence post as well the number of bags of concrete you need. Just select the shape of the post, enter the post dimensions like width, length and height. The calculator will give you the volume of hole, volume of concrete, and number of bags.
2. How much concrete is needed for a 3×3 fence post?
A 3 inches diameter fence post on average needs a post hole that’s about 9 inches in diameter. As mentioned in the start, the depth below ground should be about 1/3 the height of the post above ground. Simply put, a 6 feet post will need about a 2 feet deep hole. The table below will show how many bags of concrete needed for a 3×3 diameter hole at varying depths.
| Depth | 50 lb. Bags |
| 10-12 inches | 1 |
| 14-26 inches | 2 |
| 27-36 inches | 3 |
3. How much concrete is needed for a 4×4 fence post?
A 4 inches diameter fence post needs a post hole that is about 12 inches in diameter. Similarly, the depth below ground would be about 1/3 the height of the post above the ground. A 5 feet high post will need about 18 inches deep hole. The table below will show how many bags of concrete needed for a 4×4 diameter hole at varying depths.
| Depth | 50 lb. Bags |
| 10-14 inches | 2 |
| 16-22 inches | 3 |
| 24-30 inches | 4 |
| 32-36 inches | 5 |
4. How much concrete is needed for a 6×6 fence post?
A 6 inches fence post needs a post hole that is about 18 inches in diameter. Similarly, the depth below ground would be about 1/3 the height of the post above the ground. A 7 feet high post will need about 27 inches deep hole. The table below will show how many bags of concrete needed for a 4×4 diameter hole at varying depths.
| Depth | 50 lb. Bags |
| 10-12 inches | 4 |
| 13-17 inches | 5 |
| 18-21 inches | 6 |
| 22-25 inches | 7 |
| 26-28 inches | 8 |
| 28-30 inches | 9 |
| 32-36 inches | 10 |
5. Which one is better cement or concrete?
The answer to this question is very simple, concrete would be a best option for you post fence. As we know that concrete is a composition of cement, stone, and sand with water. The stone and sand aggregate provide strength to the structure whereas cement simply binds it all together. Concrete is proven extremely strong, durable, and it can resist extreme weather conditions. Also, it is heavy which makes it a great anchor and a perfect candidate for post fence.
Concrete is widely used in high rise buildings, water reservoirs, bridges, and for large scale foundation. Whereas cement is a weaker substance and it is prone to cracking as well. Having said that, cement is a good binding agent which helps in keeping the concrete material intact.
