The number of bags of concrete required for a fence post depends on several factors, including the size of the post, the diameter and depth of the hole, and the type of fence post being installed. In general, you’ll need three 50-pound bags of fast-setting concrete to set a fence post in a hole that’s 3 times the size of the post (12 inches in diameter) and 1/2 to 1/3 to the height of the post (18 inches deep).
As a general rule of thumb, one 50-pound bag of fast-setting concrete typically fills about 0.375 cubic feet of volume. A 50lb bag of fast-setting concrete is ideal for installing fence post because it does not need to mix in bucket, and get harden just about 20 to 60 minutes. When you finished digging a hole, put 3 to 4 inches of gravel into bottom of hole and compact with the help of post. Then set your fence post in hole and use a level to ensure that it is perfectly vertical.
The standard fence post sizes for wooden fence posts range from 4×4 inches to 6×6 inches in dimension and are usually 8 feet long. For metal fence posts, common sizes include 2 3/8-inch and 2 7/8-inch diameters for residential applications, while larger diameters are used for commercial and industrial fencing.
To calculate how many bags you need, you’ll first need to determine the volume of the hole and volume of post. Subtracting the volume of the post from the volume of the hole gives you the amount of concrete required to fill the post hole of a fence. Then, divide that volume by the volume filled by one bag of concrete to get the number of bags required.
The general rule of thumb, the depth of the post’s hole needs to be buried 1/3 to 1/2 of the actual above-ground height of the post. So, six-foot-high fence posts ideally need to be buried 2 to 3 feet into the ground. The diameter of your post hole need to be 3 times the diameter of your post. So, if you’re using a 4×4-inch square post, your fence post hole will ought to be 12 inches in diameter, while for a 6×6 post, post hole will need to be 18 inches in diameter.
Typically, for a six-foot-high 4×4 fence post, you will need a post hole that is 24 inches deep and 12 inches in diameter, while for a six-foot-high 6×6 fence post, you will need a post hole that is 24 inches deep and 18 inches in diameter. In addition, for a six-foot-high 3×3 fence post, you will need a post hole that is 24 inches deep and 9 inches in diameter
For example, if your post hole is 9 inches in diameter and 24 inches deep, you’ll need 2 bags of 50lb fast-setting concrete, while a hole that is 12 inches in diameter 24 inches deep, you’ll need 4 bags of 50-pound fast-setting concrete. In addition, a hole that is 18 inches in diameter 24 inches deep, you’ll need 7 bags of 50lb fast-setting concrete.
How many bags of concrete for a fence post
In general, you’ll need two 50-pound bags of fast-setting concrete to set a six-foot-high 3×3 fence post in a hole that’s 9 inches in diameter and 24 inches deep, while for a six-foot-high 4×4 fence post in a hole (that’s 12 inches in diameter and 24 inches deep), you’ll need four 50-pound bags of fast-setting concrete. In addition, you’ll need seven 50-pound bags of fast-setting concrete to set a six-foot-high 6×6 fence post in a hole that’s 18 inches in diameter and 24 inches deep.
To calculate the number of bags of concrete needed for a fence post, you’ll first need to determine the volume of concrete required. Here’s a simple guide:
- Select size of fence post and hole: Lets for a six-foot-high 4×4 fence post, hole should be 12 inches (1 foot) in diameter and 24 inches deep (2 feet).
- Calculate the Volume of the hole: Use the formula for cylinder concrete, volume = (πD^2/4) ×h, such as V = (3.14× 1×1)/4 × 2 = 1.57 cubic feet
- Calculate volume of post: Lets a fence post measures 4 inches by 4 inches by 24 inches deep, so volume = (4x4x24)/ 1728 = 0.22 cubic feet (since there are 1728 cubic inches in a cubic foot).
- Subtracting the volume of the post from the volume of the hole gives you the amount of concrete required to fill the fence post hole, such as 1.57 _ 0.22 = 1.35 cubic feet
- Select bag size & find Bag Coverage: A 50lb bag of fast setting concrete yields volume about 0.375 cubic feet, while a 60lb bag yields 0.45 cubic feet volume
- Divide: Divide the total volume of concrete needed by the coverage per bag to get the number of bags required. Here’s the formula: Number of bags = Volume of concrete needed/Coverage per bag, so 1.35/0.375 = 3.6 bags (or rounded up to 4 bags).
How many bags of concrete for a 3×3 post
How much concrete do I need for a 3×3 fence post? You’ll need two 50-pound bags of fast-setting concrete to set a six-foot-high 3×3 fence post in a hole that’s 9 inches in diameter and 24 inches deep, while at 36 inches deep three bags are required. In addition, at 10 inches deep one 50-pound bag are needed for a 3×3 fence post.
You’ve determined the size of the hole you’ll need, you can use the chart below to determine how many 50-pound bags of concrete you’ll need for a 3×3 fence post.
| POST SIZE | HOLE SIZE | DEPTH OF HOLE | BAGS | |
| A 3X3 POST | DEPTH | DIAMETER | 10″ | 1 50lb bags |
| 24 inches | 9 inches | 12″ to 26″ | 2 50lb bags | |
| 28″ to 36″ | 3 50lb bags | |||
How many bags of concrete for a 4×4 post
How much concrete do I need for a 4×4 fence post? You’ll need four 50-pound bags of fast-setting concrete to set a six-foot-high 4×4 fence post in a hole that’s 12 inches in diameter and 24 inches deep, while at 36 inches deep five bags are required. In addition, at 10 inches deep two 50-pound bag are needed for a 4×4 fence post.
You’ve determined the size of the hole you’ll need, you can use the chart below to determine how many 50-pound bags of concrete you’ll need for a 4×4 fence post.
| POST SIZE | HOLE SIZE | DEPTH OF HOLE | BAGS | |
| A 4X4 POST | DEPTH | DIAMETER | 10″ to 14″ | 2 50lb bags |
| 24 inches | 12 inches | 16″ to 22″ | 3 50lb bags | |
| 24″ to 30″ | 4 50lb bags | |||
| 32″ to 36″ | 5 50lb bags | |||
How many bags of concrete for a 6×6 post
How much concrete do I need for a 6×6 fence post? You’ll need seven 50-pound bags of fast-setting concrete to set a six-foot-high 6×6 fence post in a hole that’s 18 inches in diameter and 24 inches deep, while at 36 inches deep eleven bags are required. In addition, at 10 inches deep four 50-pound bag are needed for a 6×6 fence post.
You’ve determined the size of the hole you’ll need, you can use the chart below to determine how many 50-pound bags of concrete you’ll need for a 6×6 fence post.
| POST SIZE | HOLE SIZE | DEPTH OF HOLE | BAGS | |
| A 6X6 POST | DEPTH | DIAMETER | 10″ to 12″ | 4 50lb bags |
| 24 inches | 18 inches | 14″ to 16″ | 5 50lb bags | |
| 18″ to 20″ | 6 50lb bags | |||
| 22″ to 24″ | 7 50lb bags | |||
Conclusions:-
In general, at 18 inches deep, you’ll need two 50-pound bags of fast-setting concrete for a 3×3 fence post, three 50-pound bags of fast-setting concrete for a 4×4 fence post, and six 50-pound bags of fast-setting concrete for a 6×6 fence post.