Concrete for a fence post: 10-inch hole, 36 inches deep
Setting a 4x4 post in a 10-inch hole 36 inches deep takes 1.38 cubic feet of concrete: about 4 bags of 50 lb fast-setting mix per post.
| Bag size | Per post | For 10 posts |
|---|---|---|
| 80 lb | 3 | 24 |
| 60 lb | 4 | 31 |
| 50 lb | 4 | 37 |
| 40 lb | 5 | 47 |
Subtracts the space a 4x4 post (3.5 inches square) takes up. A common rule: hole depth about one-third of the post's height above ground, and hole width three times the post width.