How Much Concrete per Fence Post?
A typical 4×4 post for a 6 ft fence needs about 1.5–2 bags of 80 lb concrete.
Calculate concrete bags →
The calculation
- Hole volume = π × (diameter ÷ 2)² × depth.
- Subtract the post's volume in the hole.
- Convert cubic inches to cubic feet (÷ 1,728).
- Divide by bag yield: 0.6 cu ft for 80 lb, 0.375 cu ft for 50 lb.
Example: 10" × 24" hole with a 4×4 (3.5" actual): 1,885 − 294 = 1,591 in³ = 0.92 cu ft → 1.5 bags of 80 lb per post.
Hole size rules of thumb
- Diameter: about 3× the post width.
- Depth: one-third of the above-ground height, at least 24", below frost line in cold climates.
- Add 6" of gravel at the bottom for drainage.
All figures are estimates based on baseline U.S. assumptions, not contractor quotes.