How do you calculate concrete for a post hole?
First calculate the cylindrical hole volume, then subtract the space taken by the buried post and any gravel layer. Divide the remaining concrete volume by the yield printed on the concrete bag, then round up to whole bags.
Hole volume uses pi x radius squared x fill depth. The calculator uses inches internally and converts the result to cubic feet before dividing by bag yield.
Why does hole diameter change the answer so much?
A small change in diameter can add a surprising number of bags because round-hole volume depends on radius squared. A 12 inch hole is not just a little bigger than a 10 inch hole; it has about 44% more cross-sectional area before post displacement.
Should line posts, corner posts, and gate posts use the same inputs?
Usually not. Line posts carry repeated fence panels or fabric, while corners, ends, and gates resist turning, tension, latch force, and moving weight. That is why project mode lets you calculate each group separately instead of pretending every hole is identical.
What concrete bag yields does this calculator use?
Common concrete mix yields are roughly 0.30 cu ft for 40 lb, 0.375 cu ft for 50 lb, 0.45 cu ft for 60 lb, and 0.60 cu ft for 80 lb bags. These are planning values. The bag label is the source of truth for the mix you buy.
What should you check before setting fence posts?
Use this page as a material estimator, then check local code, frost depth, soil conditions, fence height, and gate load before digging. For broader concrete planning, compare this result with the concrete bag calculator. If you are also estimating pickets, rails, or post spacing, use the fence board calculator from the tools list when it goes live.