How Much Concrete per Fence Post Hole?
Quick answer: about 2 bags of 80lb mix per hole for a standard 10-inch-diameter hole 24 inches deep. Hole volume minus post volume ≈ 1.2 cu ft; an 80lb bag makes 0.6 cu ft.
- Hole size rules
- Depth rules
- Volume: hole minus post
- Bags by hole size
- Fast-setting mix
- Worked example: 10-inch hole
- FAQ
Hole size rules
The hole should be about 3× the post width: a 4×4 post gets a 12-inch hole, a 6×6 post an 18-inch hole. The concrete must surround the post on all sides — that ring of concrete is what resists leaning and frost heave.
Depth rules
Dig 1/3 of the post's above-ground height, and at least 24–30 inches. A 6-foot fence post wants a 24-inch hole; an 8-foot post wants 30–32 inches. Depth drives concrete volume — deeper holes need more bags.
Volume: hole minus post
Concrete volume = hole volume − post volume. A 10-inch hole 24 inches deep is about 1.31 cu ft (π × 0.417² × 2); the 4×4 post section takes about 0.11 cu ft — leaving ≈1.2 cu ft of concrete per hole.
| Hole size | Depth | 80lb bags per hole |
|---|---|---|
| 10 in (4×4 post) | 24 in | 2 |
| 10 in (4×4 post) | 30 in | 2–3 |
| 12 in (6×6 post) | 30 in | 3–4 |
| 12 in (6×6 post) | 36 in | 4–5 |
Fast-setting mix
Fast-setting post mix sets in 20–40 minutes and is designed for fence posts: pour the dry mix into the wet hole, add water, and it holds the post straight while you work. It's the standard choice for fencing; use it per the bag instructions and plumb each post before it sets.
Worked example: a 10-inch hole, 24 inches deep
- Hole volume = π × 0.417² × 2 ≈ 1.31 cu ft
- − post volume ≈ 1.2 cu ft
- 1.2 ÷ 0.6 (80lb bag) = 2 bags
- 12 posts → about 24 bags for the fence
FAQ
About 2 bags of 80lb mix per hole for a standard 10-inch-diameter hole 24 inches deep. Hole volume minus post volume ≈ 1.2 cu ft; an 80lb bag makes 0.6 cu ft.
One-third of the post height above ground, and at least 24–30 inches. A 6-foot fence post wants a 24-inch hole; an 8-foot post wants 30–32 inches.
About 2 bags per post for a typical 10-inch hole at 24 inches deep. Deeper or wider holes take 3 bags. A 100-foot fence with posts every 8 feet needs about 24 bags.
About 3 times the post width — a 4×4 post gets a 12-inch hole, a 6×6 post an 18-inch hole. The concrete must surround the post on all sides.
Yes — fast-setting post mix sets in about 20–40 minutes and needs no separate mixing: pour the dry mix into the wet hole and add water.