How many bags of concrete per fence post?
A 4x4 post in a 10 inch wide, 2 foot deep hole needs about 1.5 bags of 80 lb concrete, or about 2.5 bags of 50 lb fast-setting mix.
Concrete fills the hole around the post: the hole’s volume minus the post. The table is for a 4x4 post (3.5 x 3.5 in actual size). Round up to whole bags per post, or add them up across all posts and round once.
| Hole | Concrete | Bags of 80 lb | Bags of 50 lb fast-set |
|---|---|---|---|
| 8 in wide, 2 ft deep | 0.53 cu ft | 0.9 | 1.4 |
| 10 in wide, 2 ft deep | 0.92 cu ft | 1.5 | 2.5 |
| 10 in wide, 3 ft deep | 1.38 cu ft | 2.3 | 3.7 |
| 12 in wide, 3 ft deep | 2.1 cu ft | 3.5 | 5.6 |
Sizing the hole
- Width: about three times the post width, so 10 to 12 inches for a 4x4.
- Depth: a common rule is one third of the post’s height above ground, and below your frost line in cold climates. Check local code.
- Put a few inches of gravel in the bottom for drainage before setting the post.