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Concrete & MasonryFence Post Concrete Calculator — How Many Bags Per Hole?
Enter your hole diameter, depth, and how many posts you’re setting to get the bags of concrete needed. Sizes are in inches — a typical hole is about 3× the post width across and a third of the post’s height deep.
How this calculator works
We treat each hole as a cylinder (π × radius² × depth), add them up, and divide by 0.6 cubic feet — the yield of one 80-lb bag. This fills the whole hole; the post itself displaces a little, so you may use slightly less.
What changes how much you need
- Rule of thumb: hole diameter ≈ 3× the post width, depth ≈ 1⁄3 of the above-ground height plus 6 inches for the gravel base.
- Add a few inches of gravel at the bottom for drainage before you set the post.
- Fast-setting (no-mix) concrete pours dry into the hole — same volume math, ~0.6 ft³ per 50-lb bag.
- Buy an extra bag or two; coming up short with posts braced and waiting is no fun.
Frequently asked questions
How many bags of concrete per fence post?
For a typical 10-inch-wide, 36-inch-deep hole, about 2–3 bags of 80-lb concrete. Bigger holes or deeper sets need more.
How deep should a fence post hole be?
About one-third of the post’s above-ground height, plus ~6 inches of gravel base. For a 6-ft fence, that’s roughly 24–36 inches deep.
Do I subtract the post from the concrete?
You can — the post displaces some volume — but most people fill the full hole and accept a little extra. This calculator estimates the full hole.