How Many Bags of Concrete Do I Need for a 4x4 Post Hole?
A standard 4×4 post hole — 12 inches in diameter and 36 inches deep — holds about 2.36 cubic feet of concrete, which is 4 × 80-lb bags (or 6 × 60-lb bags). Dig below your local frost line, and buy about 10% extra for over-dig and spillage.
Key Takeaways
- Standard hole: 12 in diameter × 36 in deep = 2.36 cu ft.
- 4 × 80-lb bags (0.60 cu ft yield each).
- 6 × 60-lb bags (0.45 cu ft yield each).
- Hole depth should reach below the local frost line.
- Add 10% extra for over-dig and spillage.
How to Calculate Concrete for a Post Hole
A post hole is a cylinder. Find its volume in cubic feet, then divide by one bag’s yield — the same math works for any hole diameter or depth.
- 1Measure diameter and depth in inches.
- 2Convert to feet: 12 in = 1 ft, 36 in = 3 ft.
- 3Volume = π × (diameter ÷ 2)² × depth = 3.14 × 0.5² × 3 = 2.36 cu ft.
- 4Divide by bag yield: 2.36 ÷ 0.60 = 3.93 → 4 eighty-pound bags.
- 5Round up and add 10% — holes are rarely perfect cylinders.
The Post-Hole Volume Formula
Use feet for every measurement so the result comes out in cubic feet. For a square hole, use length × width × depth instead.
Volume (cu ft) = π × (diameter ÷ 2)² × depthWorked Example: 12 in × 36 in Hole
Full math for the standard 4×4 post hole, plus a chart for other common hole sizes.
Hole: 12 in diameter × 36 in deep.
Radius: 12 ÷ 2 = 6 in = 0.5 ft; depth: 36 in = 3 ft.
Volume: 3.14 × 0.5 × 0.5 × 3 = 2.36 cu ft.
80-lb bags: 2.36 ÷ 0.60 = 3.93 → 4 bags.
60-lb bags: 2.36 ÷ 0.45 = 5.24 → 6 bags.
| Hole size (diameter × depth) | Volume | 80-lb bags | 60-lb bags |
|---|---|---|---|
| 10 in × 24 in | 1.09 cu ft | 2 | 3 |
| 12 in × 24 in | 1.57 cu ft | 3 | 4 |
| 12 in × 36 in | 2.36 cu ft | 4 | 6 |
Common Mistakes
- Digging too shallow — footings must sit below the frost line or posts will heave.
- Bell-shaped holes that use far more concrete than the cylinder math says.
- Forgetting the post itself displaces some volume (a minor effect, but real).
- Pouring dry mix into the hole and hoping ground moisture cures it evenly.
- Not crowning the concrete above grade so water drains away from the post.
Practical Tips
- Call 811 (or your local utility locator) before you dig — it is free.
- Set the post, brace it plumb, then pour around it.
- Slope the top of the concrete away from the post to shed water.
- In cold climates, go 6 inches deeper than the published frost line.
- Estimate the full project with the NOVORY Concrete Calculator.
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