Key Takeaways
- The formula for a round post hole is pi x radius squared x depth
- A typical fence post hole is 8 to 12 inches in diameter and 24 to 36 inches deep
- An 80-pound bag of premix concrete fills about one standard post hole
- Concrete should fill the hole to about 2 inches below grade so soil covers the top
- Frost depth determines minimum hole depth in cold climates
Frost depth is the depth to which soil freezes in winter, which determines how deep footings must extend to prevent frost heave.
Bearing capacity refers to the soil’s ability to support the load transferred through a post or footing.
Setting a post in concrete is one of the most common small-pour tasks on a residential property. Fence posts, deck footings, mailbox posts and sign posts all need concrete to stay plumb and stable. The math is simple, but the dimensions matter more than people expect because even small errors in diameter or depth change the volume significantly.
Here is how to calculate the right amount.
The formula for a round hole
The volume of a cylinder is pi times the radius squared times the height. For a post hole, the height is the depth of the hole.
V = pi x r^2 x d
For a post hole that is 10 inches in diameter and 30 inches deep:
r = 5 inches V = 3.14 x 25 x 30 = 2,355 cubic inches
Convert to cubic feet by dividing by 1,728 (12 x 12 x 12):
2,355 / 1,728 = 1.36 cubic feet
Converting to bag counts
An 80-pound bag of premix concrete yields about 0.6 cubic feet of wet concrete. A 60-pound bag yields about 0.45 cubic feet.
For that 10-inch by 30-inch hole:
1.36 / 0.6 = 2.27 bags
You would need about 2.5 bags of 80-pound premix for one hole. Multiply by the number of posts to get your total.
For a 30-foot fence with posts every 8 feet, that is 5 posts (including the end posts), requiring about 13 bags of 80-pound premix.
Common post hole sizes
4-inch diameter is typical for a line post on a chain-link or picket fence. The hole is just wide enough to hold the post with concrete around it.
6 to 8-inch diameter works for most 4x4 fence posts and mailbox posts. This is the most common residential post hole size.
10 to 12-inch diameter is used for 6x6 posts, gate posts and structural deck posts. The extra width provides more concrete volume and more lateral stability.
12 to 16-inch diameter is typical for deck footings and larger structural posts that need to meet building code requirements.
How deep should the hole be?
The minimum depth depends on the application and your local frost line. For a fence post in a temperate climate, 24 to 30 inches is common. In cold climates with deep frost lines, footings may need to extend 36 to 48 inches or more.
For deck posts, your local building code specifies the minimum footing depth. This is not optional. Frost heave can lift a footing out of the ground if it sits above the frost line, which means the entire deck structure moves with seasonal freeze-thaw cycles.
A general rule for fence posts is that one-third of the above-ground height should be below ground. A 6-foot fence has about 2 feet in the ground. An 8-foot fence has about 2.5 to 3 feet in the ground.
The concrete fill level
Do not fill the hole all the way to the top. Leave about 2 inches of space below grade so that soil can cover the concrete. This hides the concrete from view and prevents water from pooling on the flat concrete surface, which can freeze and crack the top layer.
For fence posts, the concrete typically flares out slightly at the bottom, creating a bell shape that resists uplift. This is optional but adds stability, especially for tall or wind-loaded posts.
Square versus round holes
Most post holes are round because they are drilled with an auger. But square holes are sometimes used for larger footings or when pouring from a form. The volume calculation for a square hole is width x length x depth.
A 12-inch square hole that is 36 inches deep:
12 x 12 x 36 = 5,184 cubic inches = 3 cubic feet
That is 5 bags of 80-pound premix, versus about 3.5 bags for a round hole of the same width and depth. The difference comes from the corners of the square adding volume.
Pre-mix versus site-mix
For a handful of fence posts, premix bags are the practical choice. You buy exactly what you need, mix it in a wheelbarrow or the hole itself, and set the post. No scheduling, no minimum order, no leftover concrete.
For a project with many posts like a long fence line or a large deck with multiple footings, ready-mix concrete delivered by the truck may be more efficient. The break-even point is typically around 5 to 10 cubic feet, depending on your local pricing.
The post hole concrete calculator determines the volume for round and square holes and converts it to bag counts or cubic yards for ready-mix orders.
Editorial note: This article is reviewed for accuracy and updated regularly. All calculations use standard industry formulas. For the latest frost depth data, consult your local building department directly.
Frequently asked questions
How many bags of concrete do I need for a fence post?
For a standard 4x4 fence post in an 8-inch diameter hole that is 30 inches deep, you need about 2 bags of 80-pound premix. For a 6x6 post in a 12-inch diameter hole, you need about 4 to 5 bags.
How deep should post holes be for a fence?
For most residential fences, 24 to 30 inches deep is standard. In cold climates, extend the hole below the frost line to prevent heave. A general rule is one-third of the above-ground fence height should be below ground.
Can you pour dry concrete into a post hole?
You can, but it is not ideal. Dry-pouring means adding water after the concrete is in the hole, which often results in uneven mixing and voids. Pre-mixing in a wheelbarrow before pouring gives a more consistent set.
How long does concrete take to set around a post?
Concrete reaches initial set in 20 to 40 minutes and full strength in 24 to 48 hours. You can backfill the hole after the concrete has set enough to hold the post plumb, usually the next day.