With this post hole calculator you can estimate the diameter, depth, and amount of concrete/gravel needed for post installation.
To calculate materials for building a fence, including post hole concrete/gravel, check out the fence calculator.
Note, this calculator is not meant to calculate post holes for any kind of structural footings. Footings for decks (as can be seen in this deck footing article), for example, typically will need to be larger than this calculator will estimate. Check with your local building code for those exact requirements.
POST HOLE CALCULATOR
How To Use This Calculator
First, enter the number of post holes you are digging.
Next, choose whether you will be installing square or round posts.
Enter the height of the posts. This is measured from the ground to the top of the posts.
Post Hole Digging Tips
- Use pressure treated lumber for posts since they will be in contact with the ground
- An 8″ auger will cut about a 9″ diameter hole which is plenty wide for a 4×4 post
- A 3″ base of gravel underneath each post allows for water to drain away from the base of the post, preventing wood rot
- The amount of post buried in the ground should be equal to 1/3 to 1/2 of the post/fence height for adequate wind resistance and resistance to freeze/thaw cycles
- Quikrete concrete for fence posts can be mixed in the hole, making for easier post setting and cleanup
- Slope the concrete 1″ up the post (above the ground) at about a 45 degree angle to shed water away
- Caulk the seam between the post and concrete with exterior grade caulk to prevent water from seeping in between the concrete and post
- This calculator will estimate the size of the hole and amount of concrete and gravel needed for a robust, long-lasting post installation. Note in some cases, you can use less concrete and cover it up with grass for a more aesthetic look. The method described above is the ideal method of installing posts but I’ve had success with both.