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How Many Bags Of Concrete Per Fence Post?

1 – 4 bags of concrete per fence post are typically what’s needed. This depends on the type of soil, fence design, height, post size and bag weight. In general the depth of the post’s hole needs to be 1/3 to 1/2 of the above ground height of the post. For example, a six foot high fence post should be buried 2-3 feet underground. This will vary based on the type of soil and design of the fence. The diameter of your post hole should be about 3 times the diameter of your post. So, if you’re using a four inch round or 4×4-inch square post, your post hole should be 12 inches in diameter.

A six foot high fence post with a 4 inch post would need a hole that’s 36 inches deep by 12 inches in diameter. To fill a hole this size takes 5 bags of 80lb concrete, but remember, you have to subtract the volume of the post your burying. When you account for the fence post what you need is 4 bags. The best way to determine the size of each fence post hole is:

  • The diameter of the hole is 3 times the width of the fence post
  • The depth of the hole is 1/3 to 1/2 the above ground height of the fence post

Once you’ve determined the size of your holes, you can use the chart below to figure out how many bags of concrete you need per fence post.

How Many Concrete Bags Per Fence Post Chart

Here’s an easy to follow chart to help you determine how many bags of concrete you need per fence post. I figured all the bags at 50 pounds to make work easier.

If you want to use different size bags it’s easy to figure out with some basic math. Add the total weight of the bags and then divide by your bag size. For example, a 4″ post with a 12″ hole requires a 10″ depth and 2 X 50 lb bags of concrete. Add them together and you get 100 pounds of concrete. 

When you divide the total weight by your bag size, lets say 80 pounds, it comes to 1 and a 1/4 bags.

3″ Post | 9″ Hole   4″ Post | 12″ Hole   6″ Post | 18″ Hole  
Depth Bags Depth Bags Depth Bags
10 1 10 2 10 4
12 1 12 2 12 4
14 2 14 2 14 5
16 2 16 3 16 5
18 2 18 3 18 6
20 2 20 3 20 6
22 2 22 3 22 7
24 2 24 4 24 7
26 2 26 4 26 8
28 3 28 4 28 9
30 3 30 4 30 10
32 3 32 5 32 10
34 3 34 5 34 10
36 3 36 5 36 11

What’s The Best Concrete Mix To Use On Fence Posts?

Fast setting concrete works great for installing fence posts since it comes premixed and is super easy to use.

In my opinion, the two best concrete mixes for setting fence posts are:

  • Quikrete fast setting concrete mix
  • Sakrete Fence post concrete mix

Both of these concrete mixes are a premixed and can be poured into the hole dry. This can save a lot of time and labor. And they both set up in around 20 to 40 minutes which can speed you up even more. Exact cure times depend on the outside temperature and humidity.

I recommend waiting at least 4-6 hours before installing your fence sections. Any standard concrete mix will work just fine for setting fence posts if your not in a hurry. I just prefer fast setting mixes because of how easy they are too use.

If you use a regular setting mix instead of fast setting you should wait until the next day to start building your fence.

I recommend pouring your concrete when the air temperature is between 50 and 90 degrees Fahrenheit and when its dry outside. The only water you want in the hole is what you pour in. If it’s too hot outside the concrete could crack. Too cold and it won’t cure properly. Both are bad outcomes because they weaken the concrete.

Should I Use Concrete For Fence Posts?

A fence is only as strong as it’s foundation. And concrete is very strong and durable. It’s a great way to anchor down fence posts so that your fence remains strong and level. This is especially important if you live in a windy area because a lot of force can be created by a wind gust.

It’s also a benefit to use concrete if you have a heavy fence. The more force that’s placed on the fence the stronger the posts need to be.

Setting a fence post in concrete ensures a strong stable base for your posts. This assumes you’ve dug the right size hole and used the correct amount of bags. Digging too shallow or not using enough bags may not provide the level of strength you’ll need.

Here are a few tips to consider when setting your posts in concrete:

  1. If your using a wooden fence post, pressure treated or wood that’s naturally weather resistant will last the longest.
  2. If you use a wooden post that’s not weather treated, use a preservative or the post will rot.
  3. Make sure to use a base layer of gravel under the concrete so rain water can drain.
  4. Taper the top of the concrete away from the fence post to help water drain away from the post.
  5. Caulk around the post where it comes out of the concrete. A small crack can let water in that can eventually weaken the post.
  6. Concrete works great for metal, concrete or wooden fence posts and all types of fences.

How Much Concrete For A 3×3 Fence Post?

A 3″ inch diameter fence post needs a post hole that’s about 9″ inch in diameter. The depth below ground should be about 1/3 the height of the post above ground. For example, a 6′ high post will need about a 2′ deep hole.

The chart below will show you how many bags of concrete you need for a 9″ diameter hole at varying depths.

Depth 50 Lb Bags
10″-12″ 1
14″-26″ 2
27″-36″ 3

Figures are based on a fence post that’s 6′ above ground and 2′ below.

How Much Concrete For A 4×4 Fence Post?

A 4″ diameter fence post needs a post hole that’s about 12″ in diameter. The depth below ground should be about 1/3 the height of the post above ground. For example, a 5′ high post will need about a foot and a half deep hole.

The chart below will show you how many bags of concrete you need for a 12″ diameter hole at varying depths.

Depth 50 Lb Bags
10″-14″ 2
16″-22″ 3
24″30″ 4
32″36″ 5

Figures are based on a fence post that’s 6′ above ground and 2′ below.

How Much Concrete For A 6×6 Fence Post?

A 6″ diameter fence post needs a post hole that’s 18″ in diameter. The depth below ground should be about 1/3 the height of the post above ground. For example, a 7′ high post will need about a two and a quarter foot deep hole.

The table below shows you how many bags of concrete you’ll need per fence post for an 18″ diameter hole that’s between 10-36 inches deep.

Depth 50 Lb Bags
10″-12″ 4
13″-17″ 5
18″-21″ 6
22″-25″ 7
26″-28″ 8
28″-30″ 9
32″-36″ 10

Figures are based on a fence post that’s 6′ above ground and 2′ below.

Do Soil Conditions Matter?

Yes. You can adjust the depth and hole size a little depending on the soil conditions. I’d say about 10% – 15% should be safe. But personally, I always overkill everything and go big. I’d rather my fence posts be filled with more than enough concrete then not enough.

As with any construction project, you have to make decisions based on site condition. These measurements are all just a general guide. Every fence is different. Every yard is different and so are the weather conditions your fence will face. Adjust as needed. I recommend you do what I do and go bigger when in doubt.

Let’s say your installing a fence post in really dense soil that’s rocky or lots of clay. You can get away with a smaller hole because the soil is so strong. In order the the fence post to topple over the soil will have to give way too and that’s harder with dense soil.

Now lets say you’ve got sandy soil that offers very little resistance or strength. In this case I’d go with a bigger and deeper hole. Since the soil won’t offer much strength you’ll need more to come from the concrete.

If you’ve got regular soil with some grass then I’d stick to the sizes listed on the charts.

Cement Or Concrete For Fence Posts?

Definitely use concrete instead of cement for a fence post.

Concrete is made by mixing cement, stone and sand with water. The stone and sand aggregate provide a lot of strength while the cement binds it all together. Concrete is extremely strong, durable and weather resistant. And it’s very heavy which makes it a great anchor for a post. There’s a reason why just about every large scale foundation, building, bridge and damn uses it.

Cement is weaker and prone to cracking. It’s a good binding agent, as a very thin coating or as a filler like grout. But I don’t recommend using cement for anything structural like holding up a fence post.

Is Quikrete The Same As Concrete For Fence Posts?

Yes. These charts are all figured for 50 pound bags of ready-mix concrete. Quikrete is the brand I typically use but Sakrete is another good one. There are lots of manufacturers that sell factory-mixed bagged concrete. Just make sure you buy the correct PSI for the job you’re doing. I like to use at least 3500 when setting a fence post.

I also recommend using fast setting concrete. It makes the job much easier because I can start setting my fence sections the same day.

Call Before You Dig

For most homeowners, any time you dig anywhere on your property you should call for a mark out. Unless you live somewhere rural with acres of property there may be some under ground utilities you could hit.

Hitting a gas or power line while digging can be very dangerous so make sure your local utility companies mark the lines before you dig. Here in New Jersey it’s a free service and only takes a few days so there’s really no reason not to call.

Also reach out to your local building inspector’s office to find out about zoning, permits or other requirements.

Summary: How Many bags Of Concrete Per Fence Post?

1 – 4 bags of concrete per fence post are typically what’s needed. This depends on the type of soil, fence design, height, post size and bag weight. In general the depth of the post’s hole needs to be 1/3 to 1/2 of the above ground height of the post. For example, a six foot high fence post should be buried 2-3 feet underground. This will vary based on the type of soil and design of the fence. The diameter of your post hole should be about 3 times the diameter of your post. So, if you’re using a four inch round or 4×4-inch square post, your post hole should be 12 inches in diameter. A six foot high fence post with a 4 inch post would need a hole that’s 36 inches deep by 12 inches in diameter.

To fill a hole this size takes 5 bags of 80lb concrete, but remember, you have to subtract the volume of the post your burying. When you account for the fence post what you need is 4 bags. The best way to determine the size of each fence post hole is:

  • The diameter of the hole is 3 times the width of the fence post
  • The depth of the hole is 1/3 to 1/2 the above ground height of the fence post

Once you’ve determined the size of your holes, you can use the chart above to figure out how many bags of concrete you need per fence post.

If you have any questions or comments about Quikrete E-mail any time.

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