Fence Post Concrete Calculator
Calculate how much concrete you need per fence post hole from the hole diameter, depth, and post width.
How this calculator works
A fence post hole isn’t filled edge to edge with concrete — the post itself takes up space in the middle of it. This calculator finds the full volume of the round hole as a cylinder, then subtracts the volume the post itself displaces, leaving the actual amount of concrete needed to fill the gap around the post.
Formula: Concrete needed = hole volume (a cylinder: π × (hole diameter ÷ 2)² × depth) minus the post’s own volume (approximated as a square post: post width² × depth).
Worked example
A 10-inch diameter hole, 2 ft deep, with a 3.5-inch square post:
- Hole volume = π × (10 ÷ 2 ÷ 12)² × 2 ≈ 1.091 ft³
- Post volume = (3.5 ÷ 12)² × 2 ≈ 0.170 ft³
- Concrete needed ≈ 0.921 ft³ per post
Notes
Multiply the per-post figure by your total post count to get the concrete needed for the full job, then convert to bags using the Concrete Bag Calculator — an 80-lb bag yields about 0.6 cubic feet once mixed.
Round post holes, typical for a post-hole digger or power auger, are the standard case this calculator assumes. A wider hole than the post needs meaningfully more concrete than you might expect, since volume grows with the square of the radius, not in a straight line with diameter.
How to use
Enter your hole diameter, hole depth, and post width, and the calculator returns the concrete volume needed for that single post hole. Multiply by your post count for a full-job estimate, or convert straight to bag count.
A general construction guideline. Local frost depth and code requirements vary — confirm exact post depth requirements with your local building department.
Frequently asked questions
Why does the formula subtract the post's volume?
The post itself occupies space inside the hole, so concrete only fills the gap around it, not the full hole volume. Subtracting the post's volume from the hole's volume gives the actual amount of concrete you need to pour, rather than overestimating by the post's own footprint.
Why does a wider hole need so much more concrete?
Hole volume is based on the radius squared, so small increases in diameter grow the volume faster than you'd expect. A modestly wider hole at the same depth can meaningfully increase the concrete needed, which is why it's worth using your actual planned hole diameter rather than rounding it down.
How do I convert cubic feet of concrete into bags?
An 80-lb bag of concrete yields about 0.6 cubic feet once mixed. Divide your total concrete volume in cubic feet by 0.6 to estimate the number of 80-lb bags needed, or use the Concrete Bag Calculator directly for other bag sizes.
Does this work for round or square post holes?
This calculator assumes a round hole, which is the standard result from a post-hole digger or power auger. If you're forming a square hole instead, the volume formula would need to use a square cross-section rather than a cylinder, so the numbers won't match exactly.
Estimates only. Verify quantities with your supplier before purchasing.