How To Secure 4x4 Posts For Garden In The Ground

How Many Bags of Concrete for a Fence Post?

Knowing how deep your fence post needs to be set is key to answering the question. Learn how to set a post and figure out how much concrete you'll need.

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Image courtesy of Brian Patrick Flynn

The general rule of thumb when setting a post is that the depth of the post's hole needs to be 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 three feet into the ground. The diameter of your post hole should be three times the diameter of your post. So, if you're planning on using a four-inch round or 4x4-inch square post, your post hole will need to be 12 inches in diameter. For a six-foot-high fence post, we would need a hole that's 36 inches deep and 12 inches in diameter.

Now that 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. Our example post will require two bags for every post if you're using fast-setting concrete.

Fast-setting concrete is ideal for installing fence posts since it doesn't need to be mixed in a bucket or a wheelbarrow. Once you've finished digging your post holes, add about three to four inches of gravel into the bottom and compact it using a post or a 2x4. Then, set your post in place and use a level to ensure that it's perfectly vertical. Stake your post in place with two braces and pour your dry concrete into the hole, surrounding the post up to about three inches below the lip. Slowly fill the hole in with about a gallon of water, enough so that you've saturated all of the concrete.

Fast-setting concrete is generally hardened off in about 20 to 60 minutes, depending on the outside temperature, and is usually cured enough to begin work in about four to six hours. Remember that although you can pour concrete in a wide range of conditions, the safe window for the average homeowner to pour concrete is when the air temperature is between 50 and 90 degrees Fahrenheit. While it's technically possible to pour in conditions outside of those ranges, it requires a fair amount of experience. Your concrete will cure evenly when temperatures are mild and the water is at a tepid temperature.

Remember, any time you're planning on digging a deep hole in your yard, it's a good idea to call your local utilities to come mark your underground wires and pipes. Hitting a gas line or buried power line while digging can be extremely dangerous and even fatal. If you're curious about what permits you might need for a fence project, you can reach out to your local building inspector's office or read our handy reference here.