Fence Post Concrete Calculator
Enter your number of fence posts and each hole's diameter and depth to calculate the total concrete volume and number of bags needed to set them.
How the Fence Post Concrete formula works
Cylinder volume per hole, summed across all posts:
Volume per hole (ft³) = π × (diameter/2)² × depth / 1728 Total bags = ceil( Volume per hole × Number of posts / Bag yield )
Step-by-step calculation
- Calculate one hole's volume using the cylinder formula, converting from cubic inches to cubic feet.
- Multiply by the total number of posts.
- Divide by a single bag's yield (commonly about 0.45 ft³ for a 60 lb bag) and round up.
Worked example
10 posts, each hole 10 inches in diameter and 24 inches deep: Volume per hole = π × 5² × 24 / 1728 ≈ 1.09 ft³. Total volume = 10.9 ft³. At 0.45 ft³ per 60 lb bag, that's ceil(10.9/0.45) = 25 bags.
Frequently asked questions
How deep should a fence post hole be?
A common rule of thumb is burying about one-third of the post's total length, though local frost depth and soil conditions can require deeper holes in colder climates to prevent frost heave.
Should I subtract the post's own volume from the hole?
This calculator estimates based on total hole volume for simplicity, which provides a reasonable buffer since posts don't perfectly fill a cylindrical hole and some extra concrete is typically welcome for stability.