How many bags of concrete per fence post?
Short answer
- A 4 × 4 post for a 6 ft fence, set 3 ft deep, takes about 4 bags of 50 lb fast-setting concrete (1.42 cubic feet).
- A 6 × 6 post at the same depth takes about 10 bags (3.51 cubic feet).
- Holes are 3 times the post width, with 6 in of gravel in the bottom, filled to about 3 in below grade (Quikrete).
Bags per post by fence height
Depth follows Quikrete's rule of 1/3 of the post's total length, which is half the height above ground. Bags are 50 lb Quikrete Fast-Setting Concrete Mix at about 0.375 cubic feet each.
| Fence height | Post depth | Post length | 4 × 4 post | 6 × 6 post |
|---|---|---|---|---|
| 3 ft | 1.5 ft | 4.5 ft | 2 bags | 5 bags |
| 4 ft | 2 ft | 6 ft | 3 bags | 6 bags |
| 5 ft | 2.5 ft | 7.5 ft | 4 bags | 8 bags |
| 6 ft | 3 ft | 9 ft | 4 bags | 10 bags |
| 8 ft | 4 ft | 12 ft | 6 bags | 13 bags |
If your local code or HOA sets a different depth, follow that. In cold climates, go below the frost depth your local building department lists, and call 811 before you dig. The fence calculator works out posts, rails, pickets and concrete for your exact fence, and the concrete calculator handles any hole size.
How the math works
Concrete per post = the hole (a cylinder 3 times the post width across) minus the post itself, from the top of the gravel to about 3 in below the surface. For a 4 × 4 post set 36 in deep: a 10.5 in hole over 33 in of fill is 1.65 cubic feet, minus the post's 0.23 cubic feet, leaves 1.42 cubic feet. At 0.375 cubic feet per bag that rounds up to 4 bags.
Common questions
How many bags of concrete per fence post?
For a 6 ft fence with 4 × 4 posts set 3 ft deep (Quikrete's 1/3 rule), about 4 bags of 50 lb fast-setting concrete per post. A 6 × 6 post at the same depth takes about 10 bags, because the hole is three times the post width.
How many bags for 10 fence posts?
Multiply the per-post count by the number of posts. Ten 4 × 4 posts for a 6 ft fence take about 40 bags of 50 lb fast-setting concrete.
How wide should a fence post hole be?
Quikrete recommends a hole about 3 times the width of the post: about 10.5 inches for a 4 × 4 (3.5 in actual) and about 16.5 inches for a 6 × 6 (5.5 in actual).