What this calculates
Each hole is treated as a cylinder. The square post takes up some of that space, so its footprint is subtracted before multiplying by the number of holes.
Formula
Per hole = (π × (diameter ÷ 2)² − post width²) × depth
Total = per hole × number of holes
Worked example
10 holes, 10 in wide × 36 in deep, 4×4 posts (3.5 in actual)
- 1.Hole area: π × 5² = 78.5 sq in; post: 3.5² = 12.25 sq in
- 2.(78.5 − 12.25) × 36 = 2,386 cu in = 1.38 cu ft per hole
- 3.× 10 holes = 13.8 cu ft; +10% = 15.2 cu ft → 26 × 80 lb bags
Common mistakes
- Using nominal lumber size. A 4×4 is actually 3.5 × 3.5 in; a 6×6 is 5.5 × 5.5 in.
- Forgetting any gravel placed in the bottom of the hole, which reduces concrete depth.
Frequently asked questions
How many bags of concrete per fence post?
It depends on hole size. A 10 in × 36 in hole with a 4×4 post needs about 1.4 cu ft — 3 whole 80 lb bags at an example yield of 0.60 ft³ per bag, before waste.
How wide should a post hole be?
A common rule of thumb is about three times the post width, but follow the post manufacturer and local guidance.