Determining Concrete Need for Fence Post
Typically, one to four bags of concrete is required for each fence post. The post hole should be 1/3 to 1/2 the post’s height above ground.
Concrete Calculation Tips
- For reference, a 40-pound bag of concrete typically yields about 0.3 cubic feet, a 50-pound bag yields 0.37 cubic feet.
- To fill a hole this size takes 5 bags of 80lb concrete.
- As 133 lb concrete = 1 CF, so a bag of 50 lb concrete = 50/133 = 0.375 CF.
- A 50 lb bag of concrete approximately covers around 1.5 square feet area up to a standard depth of 3 inches for your patio and sidewalk.
Setting Fence Post with Concrete
1 – 4 bags of concrete are typically what’s needed to set a fence post in concrete. The best way to determine the size of the hole is: Diameter of the hole is 3 times the width of the fence post.
Example Calculation for 4×4 Fence Post
For example, a 6′ high fence post 4"x4" needs a hole 12" in diameter by about 2′ to 3′ deep. That would take four 50 lb. bags of fast-setting concrete mix per fence post.
Additional Tips for Post Installation
- Add concrete and water into the hole and around the 4” x 4” post. Let concrete set 24-48 hours.
- Quikrete fast setting mix needs about 1 gallon of water per 50 lb bag.
- While installing, place 6" gravel at the hole bottom for drainage and frost protection.
Calculating Concrete Needed
To calculate, first determine the volume of the post and hole. Volume of a rectangular post is length x width x depth. Volume of a cylindrical post is squared radius x pi x depth. Follow these steps: Count posts by measuring length and spacing. Calculate the volume of concrete by multiplying hole depth, area, and adding 10% for waste. Multiply the volume by posts to get the total concrete required.