By Zencalcy Editorial TeamLast reviewed About us
A post hole is a small excavation with a surprisingly large volume once depth is counted. Whether you are filling it with concrete, gravel or the spoil you dug out, the volume is the same: area × depth, with a small bulking factor for loose soil.
Volume of a dug hole, and the concrete to fill it.
Round: V = π × r² × d Rectangular: V = L × W × d
Volume is computed from inside dimensions. Loose excavated soil bulks about 20–30% more than its in-ground volume, so a hole that holds 1 cu ft will produce 1.25 cu ft of spoil.
| Symbol | Meaning |
|---|---|
| r | Hole radius. |
| L, W | Rectangular hole dimensions. |
| d | Depth of the hole. |
π × 0.5² × 3 = 2.36 cu ft, so about four bags of 80 lb concrete per hole.
Continue with the Cement Calculator, Concrete Calculator, Concrete Block Fill Calculator, or the Concrete Column Calculator — or browse more cement & concrete and all construction calculators.
A 12-inch hole 3 ft deep takes about 2.4 cu ft, or four 80 lb bags. A 10-inch hole 3 ft deep takes 1.6 cu ft.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026