Fence Post Concrete Calculator
Enter the fence run length, post spacing, the post size, and the hole diameter and depth. The calculator counts the posts for a straight run, then works out the concrete that fills the space around each post.
Corners, gates and end posts add extra posts. Count those separately and add them to the result.
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Worked example
Inputs: fence run length 100 ft; post spacing 8 ft; post width 3.5 in; hole diameter 10 in; hole depth 24 in.
- Sections: 100 ÷ 8 = 12.5, round up to 13. Posts = 13 + 1 = 14.
- Hole volume: π × (10 ÷ 2 ÷ 12)² × (24 ÷ 12) = π × 0.1736 × 2 = 1.091 cubic feet.
- Post volume: (3.5 ÷ 12)² × 2 = 0.1701 cubic feet.
- Concrete per post: 1.091 − 0.170 = 0.921 cubic feet.
- Total: 14 × 0.921 = 12.89 cubic feet. At about 0.60 cubic feet per 80 lb bag: 12.89 ÷ 0.60 = 21.5, round up to 22 bags.
Result: 14 posts needed.
Where the formula comes from
Cylinder and rectangular-prism volumes. Bag yields are typical published figures for premixed concrete (about 0.60 ft³ per 80 lb bag and 0.45 ft³ per 60 lb bag); check your bag. Hole depth rules vary with frost line and fence height, so follow local code and the fence manufacturer.
Estimates only. Results are planning figures, not engineering or code advice. Check your local building code and confirm quantities with your supplier.
Frequently asked questions
How deep should fence post holes be?
It depends on fence height, soil, wind exposure and your local frost depth. A common rule of thumb buries about a third of the post, but local requirements and the fence manufacturer take priority.
Why does the post size matter?
The post takes up room in the hole, so less concrete is needed than the full hole volume. Entering the actual post width (a 4x4 is about 3.5 in) avoids overbuying.
Does this include corner or gate posts?
No. It assumes a straight run with a post at each end. Add extra posts and concrete for corners, gates and any heavier end posts.
Should I add extra bags?
It is sensible to buy one or two spare bags, since holes are seldom exactly the size you planned.