Fence Post Concrete & Spacing Calculator

Como o cálculo funciona

Two calculations sit behind the result. Post count is ceil(fence length ÷ post spacing) + 1, because a fence always carries one more post than it has spans. Each hole is treated as a cylinder: π × (diameter ÷ 2)² × depth in cubic inches, divided by 1,728 to reach cubic feet. That per-hole volume is multiplied by the post count, divided by your chosen bag yield and rounded up to whole bags, then multiplied by the price per bag.

Hole diameter is the strongest lever because volume scales with its square: widening a hole from 10 to 12 inches raises the concrete needed by 44 percent, while depth only scales linearly. The bag select carries real yields — 40 lb at 0.30 ft³, 50 lb at 0.375, 60 lb at 0.45, 80 lb at 0.60 — and defaults to the 50 lb bag. On the defaults, a 100 foot fence at 8 foot spacing needs 14 posts and 41 bags.

The cylinder volume is the whole hole; the post standing in it is not subtracted. For a 4×4 in a 10 inch hole that is about 16 percent of the volume, which conveniently absorbs spillage but does mean you are over-ordering slightly on wide posts. There is no bell or flare at the base, no gravel drainage layer, no extra allowance for gate and corner posts that usually want deeper holes, and no metric mode — lengths are feet and hole dimensions inches.

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