How Much Concrete per Fence Post?
Quick Answer
A standard 4×4 fence post set in a 10-inch-wide hole, 2 to 2.5 feet deep, needs 1 to 2 80-lb bags of concrete per post. Larger 6×6 posts or deeper holes need 3 or more bags each.
- 4×4 post, 24-inch hole: 1–2 80-lb bags
- 4×4 post, 36-inch hole: 2–3 bags
- 6×6 post, 36-inch hole: ~3 bags
- Rule of thumb: hole depth = 1/3 of above-ground post height
Formula
Calculate the cylinder volume of the hole (π × radius² × depth), then subtract the volume of the post that sits in the hole. An 80-lb bag yields about 0.6 cubic feet of concrete.
Step-by-Step Calculation
- 1
Hole volume (10-in dia, 30-in deep)
3.14 × 0.417² × 2.5 = 1.36 cubic feet
- 2
Post volume (4×4, actual 3.5-in)
0.292 × 0.292 × 2.5 = 0.21 cubic feet
- 3
Concrete needed
1.36 − 0.21 = 1.15 cubic feet
- 4
Bags (80-lb = 0.6 cu ft)
1.15 ÷ 0.6 = ~2 bags per post
Concrete Bags per Fence Post
| Post Size | Hole Diameter | Hole Depth | Concrete | 80-lb Bags |
|---|---|---|---|---|
| 4×4 | 10 inches | 24 inches | 0.92 cu ft | 1–2 bags |
| 4×4 | 10 inches | 36 inches | 1.38 cu ft | 2–3 bags |
| 6×6 | 12 inches | 36 inches | 1.73 cu ft | 3 bags |
| 4×4 (gate post) | 12 inches | 36 inches | 2.10 cu ft | 3–4 bags |
Fast-setting concrete (poured dry into the hole, then water added) is the most common choice for fence posts. One 50-lb bag of fast-set yields about 0.375 cubic feet.
Frequently Asked Questions
How deep should a fence post hole be?
The hole depth should be about one third of the above-ground post height, plus 6 inches of gravel at the bottom for drainage. For a 6-foot fence, dig holes 24 to 36 inches deep and below the frost line in cold climates.
Do I need to mix concrete for fence posts?
Not with fast-setting concrete. You can pour the dry mix straight into the hole around the post, then add water — it sets in 20 to 40 minutes. For standard concrete, mix to a firm consistency before placing.
How wide should a fence post hole be?
The hole diameter should be about three times the post width. A 4×4 post needs a 10 to 12 inch hole; a 6×6 post needs a 12 to 18 inch hole. Wider holes hold posts more securely.