Post hole concrete calculator
Calculate how much concrete you need for fence posts, deck posts or any post in a hole.
Post hole concrete calculator
Calculate how much concrete you need for fence posts, deck posts or any post in a hole.
Enter your post hole dimensions to see the concrete estimate.
Volume needed
Project summary
How it works
These are volume estimates for planning. Actual concrete requirements depend on hole conditions and post dimensions.
Post hole concrete calculator
The hole is not solid concrete
The most common mistake here is treating the entire hole as concrete volume. A post sits in the middle of the hole, and that post displaces real volume. A 4x4 post in a 12-inch hole that is 36 inches deep displaces about 0.38 cubic feet of concrete. Across 10 fence posts, that is almost 4 cubic feet you would otherwise pay for and not use. The calculator subtracts the post volume when you enter the post dimensions.
How deep should the hole be?
A common rule of thumb is one-third of the post height above ground. A 6-foot fence needs a 2-foot burial, so a 36-inch hole. In frost-prone areas, the hole should go below the frost line to prevent heaving. Local building codes specify this depth, and it varies from 12 inches in mild climates to 48 inches in cold ones. The calculator does not know your frost line, so it uses whatever depth you enter.
Bags versus ready-mix for post holes
For a few posts, bags are the practical choice. You mix each hole separately, tamp the concrete around the post, and you are done. For a long fence line with many posts, a ready-mix delivery may be cheaper per hole, but it requires a mixer and enough hands to pour before the concrete sets. Fifteen bags of 80-pound mix fills about one cubic foot, which is enough for roughly two standard post holes.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a standard 4x4 post in a 12-inch diameter hole, 36 inches deep, you need about 0.78 cubic feet of concrete around the post. That is roughly 2 bags of 80-pound mix or 3 bags of 60-pound mix. For a 4x4 in a 10-inch hole, 24 inches deep, you need about 0.36 cubic feet, which is just over one 80-pound bag. Enter your own hole and post dimensions above.
How deep should a fence post hole be?
About one-third of the total post length is the general guideline. A 6-foot fence uses an 8-foot post buried 2 feet deep. In areas with frost, the hole should be below the frost line, which can be 36 to 48 inches in cold climates. Check your local building department for the specific requirement.
How many bags of concrete per post hole?
A standard 10 inch diameter hole that is 3 feet deep takes about two 60 pound bags of concrete mix. The calculator gives you the exact amount based on your hole diameter and depth.
What size post hole do I need for a 4 by 4 post?
A 10 to 12 inch diameter hole is standard for a 4 by 4 post. The hole should be 3 times the width of the post. The calculator lets you enter any diameter.
Can I use fast setting concrete for posts?
Yes, fast setting concrete sets in 20 to 40 minutes and is convenient for fence posts. It costs a bit more than regular mix. The calculator shows volume regardless of concrete type.
How much does it cost to set a fence post?
Each post needs about 2 bags of concrete at 5 to 7 dollars per bag, so roughly 10 to 14 dollars in materials per post. The calculator gives you the total bags needed.
Do I need gravel at the bottom of the post hole?
A few inches of gravel at the bottom provides drainage and prevents the post from sitting in water. This extends the post life. The calculator focuses on concrete volume for the hole.
How many bags of concrete do I need for a fence post?
A typical fence post hole is 10 to 12 inches in diameter and 36 inches deep, requiring about 0.2 to 0.3 cubic feet of concrete per hole. One 50-pound bag yields about 0.37 cubic feet, so one bag per post is usually enough for standard posts. The calculator works this out from your hole dimensions and shows the bag count for your specific project.