Concrete for a fence post: 8-inch hole, 24 inches deep
Setting a 4x4 post in a 8-inch hole 24 inches deep takes 0.53 cubic feet of concrete: about 2 bags of 50 lb fast-setting mix per post.
| Bag size | Per post | For 10 posts |
|---|---|---|
| 80 lb | 1 | 9 |
| 60 lb | 2 | 12 |
| 50 lb | 2 | 15 |
| 40 lb | 2 | 18 |
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.