The math
Hole volume = π × (diameter ÷ 2)² × depth
Concrete = hole volume − post volume
A 10 inch hole 36 inches deep holds 78.5 × 36 ÷ 1,728 = 1.64 cubic feet. A 4x4 post takes up 12.25 sq in × 36 ÷ 1,728 = 0.26 cubic feet of that, leaving 1.38 cubic feet of concrete per post. That is a little over two 80 lb bags, since each yields 0.60 cubic feet.
Bags per post by hole size (4x4 post, 36 inches deep)
| Hole diameter | Concrete per hole | 80 lb bags | 60 lb bags | 50 lb bags |
|---|---|---|---|---|
| 8 in | 0.79 cu ft | 1.3 | 1.8 | 2.1 |
| 10 in | 1.38 cu ft | 2.3 | 3.1 | 3.7 |
| 12 in | 2.10 cu ft | 3.5 | 4.7 | 5.6 |
| 16 in | 3.93 cu ft | 6.6 | 8.7 | 10.5 |
Round up for the whole job, not per hole, so ten posts at 2.3 bags each need 23 bags before extra. The calculator does this for you.
Sizing the hole
- Diameter. A common rule is a hole about three times the width of the post, so 10 to 12 inches for a 4x4.
- Depth. Many fence builders bury about a third of the post length. Local frost depth and the manufacturer's instructions can require more.
- Base. A few inches of gravel under the post helps drainage, and the hole needs to be deeper by that amount.
For slabs and other shapes, use the concrete calculator, or see how bags convert to yards in the bags per cubic yard chart.
Frequently asked questions
How many bags of concrete for a fence post?
About 2 to 3 bags of 80 lb mix for a 4x4 post in a 10 inch hole 36 inches deep. The exact figure is 1.38 cubic feet, which is 2.3 bags.
How much concrete for a 12 inch hole?
A 12 inch hole 36 inches deep holds 2.36 cubic feet. A 4x4 post takes up 0.26, so you need about 2.10 cubic feet, or 3.5 bags of 80 lb mix.
How many bags of concrete for 10 fence posts?
About 26 bags of 80 lb mix for ten 4x4 posts in 10 inch holes 36 inches deep, with 10% extra.
Sources
Reference charts
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