Post Hole Concrete Calculator
Find how many 80-lb bags of concrete you need to fill one fence post hole from its depth and diameter, plus total cost per hole.
How this calculator works
A post hole is a small round excavation, so its volume follows the cylinder formula rather than a rectangular one: π × radius² × depth. This calculator converts your hole’s diameter (inches) to a radius in feet, multiplies by depth, and converts the result to 80-lb bags of concrete mix — per hole.
Formula: Cubic feet = π × (Diameter in ÷ 24)² × Depth ft. Bags = Cubic feet ÷ 0.6.
Worked example
A 10-inch diameter hole, 2 ft deep:
- Radius in feet: 10 ÷ 24 ≈ 0.4167 ft
- Cubic feet: π × 0.4167² × 2 ≈ 1.0908 ft³
- Bags: 1.0908 ÷ 0.6 ≈ 1.82 bags
- At $7/bag, that’s $12.73 — per hole.
Cost reference
| Hole size | Bags needed (80-lb) | Cost at $7/bag |
|---|---|---|
| 5 in dia × 1 ft | 0.23 bags | $1.61 |
| 10 in dia × 2 ft | 1.82 bags | $12.73 |
| 20 in dia × 4 ft | 14.54 bags | $101.78 |
How to measure your post hole
Measure the hole’s diameter in inches and its depth in feet after digging, since actual hole size often runs a bit larger than planned once you’re through rocky or loose soil. This result is per hole — multiply the bag count by your total number of post holes for the full material list, and round the total up to the nearest whole bag before ordering.
Frequently asked questions
How many bags of concrete does one post hole need?
It depends on hole size, but a common rule of thumb is 2-3 bags of 80-lb mix per post hole for typical fence-post dimensions. This calculator gives you an exact figure based on your actual hole depth and diameter rather than relying on the rule of thumb.
Does the post itself take up some of the concrete volume?
Yes, technically — the post displaces a small amount of concrete in the hole, but for a typical 4×4 or 6×6 post in a 8-12 in diameter hole, the post only accounts for a small fraction of the hole's cross-section. Most installers don't subtract for it, treating it as a small built-in safety margin rather than a source of meaningful overestimation.
How deep and wide should a post hole be?
A common guideline is a hole depth of about 1/3 to 1/2 the post's above-ground height, and a diameter roughly 3 times the post's width — so a 4×4 post often sits in a hole around 10-12 inches in diameter. Deeper holes (down to or below the local frost line) resist frost heave better in cold climates; check local code for any minimum depth requirement.
Should I round up the bag count for post holes?
Yes — bags are a whole-unit purchase, so round any fractional result up to the next full bag per hole, then multiply by your total number of holes. It's also common to buy one or two extra bags for the whole job as insurance against a hole that turns out deeper than expected.
Is dry-set concrete (poured dry, then watered) the same volume as mixed concrete?
Yes — whether you mix the concrete with water first or pour dry mix into the hole and water it in place (a common shortcut for fence posts), the bag count is the same since it's based on the dry mix's yielded volume either way. The dry-set method just skips the mixing step; it doesn't change how much material you need.
Estimates only. Verify quantities with your supplier before purchasing.