How Much Concrete Per Fence Post
Concrete quantity per fence post depends on the hole dimensions (diameter and depth) and the post size. Here's how to calculate it.
The Calculation
Concrete fills the space between the post and the hole wall. The formula:
- Calculate hole volume (cylinder): π × (hole radius)² × depth
- Calculate post volume (cylinder approximation): π × (post radius)² × depth
- Subtract: concrete per post = hole volume − post volume
- Convert to bags: ceil(total concrete ÷ bag yield)
Example: 4×4 Post, 12" Hole, 24" Deep
- Hole volume = π × (6/12)² × (24/12) = π × 0.25 × 2.0 = 1.571 cu ft
- Post volume = π × (2/12)² × 2.0 = π × 0.0278 × 2.0 = 0.175 cu ft
- Concrete per post = 1.571 − 0.175 = 1.396 cu ft
- 60 lb bags (0.449 cu ft yield): ceil(1.396 ÷ 0.449) = 4 bags per post
Quick Reference: Bags per Post
| Post Size | Hole Diameter | Hole Depth | Concrete/Post | 60 lb Bags |
|---|---|---|---|---|
| 4×4 | 12" | 24" | 1.40 cu ft | 3-4 |
| 4×4 | 12" | 30" | 1.74 cu ft | 4-5 |
| 6×6 | 18" | 30" | 3.93 cu ft | 9-10 |
| 6×6 (gate) | 18" | 36" | 4.71 cu ft | 11-12 |
Don't Forget the Gravel Base
Add 6" of gravel at the bottom of each hole before setting the post. This improves drainage and prevents post rot. The Fence Post Calculator includes the gravel estimate automatically.
Use the Calculator
The Fence Post Calculator handles the full calculation — just enter fence length, post spacing, hole dimensions, and bag weight. It also estimates total gravel needed.