Fence Post Concrete Calculator — Bags Per Post
Enter your post size and hole dimensions to get how much concrete you need to set your fence posts — total bags and bags per post.
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How it works
concrete per hole = (π × (hole_dia ÷ 2)² − post area) × hole_depth ÷ 1728; bags = ceil(total ÷ bag yield)
Concrete per fence post hole (50 lb fast-setting bags)
| Post / hole size | Concrete per post | 50 lb bags per post |
|---|---|---|
| 4×4 post · 8 in hole · 24 in deep | 0.53 cu ft | 2 |
| 4×4 post · 10 in hole · 24 in deep | 0.92 cu ft | 3 |
| 4×4 post · 10 in hole · 30 in deep | 1.15 cu ft | 4 |
| 4×4 post · 12 in hole · 36 in deep | 2.10 cu ft | 6 |
| 6×6 post · 12 in hole · 36 in deep | 1.73 cu ft | 5 |
Concrete per post = hole volume − post volume. A 50 lb bag of fast-setting mix yields about 0.375 cu ft (60 lb ≈ 0.45, 80 lb ≈ 0.6). Dig the hole about 3× the post width and one-third to one-half the post height deep, below the frost line.
Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10-inch-wide hole 2 feet deep, about 2–3 bags of 50 lb fast-setting concrete (or 2 of the 80 lb bags). Wider or deeper holes need more — a 12-inch hole 3 feet deep takes about 6 of the 50 lb bags.
How deep should a fence post hole be?
Bury about one-third of the post’s total length, and always below your local frost line — commonly 24–36 inches for a 6 ft fence. Dig the hole about three times the post’s width.
Do I need gravel at the bottom of the post hole?
A few inches of gravel under the post helps drainage and reduces rot on wood posts. Set the post on the gravel, then pour concrete around it — not underneath it.