Concrete for a fence post: 12-inch hole, 36 inches deep
Setting a 4x4 post in a 12-inch hole 36 inches deep takes 2.10 cubic feet of concrete: about 6 bags of 50 lb fast-setting mix per post.
| Bag size | Per post | For 10 posts |
|---|---|---|
| 80 lb | 4 | 36 |
| 60 lb | 5 | 47 |
| 50 lb | 6 | 57 |
| 40 lb | 8 | 71 |
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.