How Deep Should Fence Posts Be?
Short answer: bury about one third of the post’s overall length, and dig the hole 6 inches deeper for a gravel base. That’s Quikrete’s rule for setting posts in concrete. For a 6 ft fence, use 9 ft posts with 3 ft in the ground, in a hole about 3½ ft deep.
For the posts, concrete, rails and pickets for a whole fence, use the fence calculator.
Post depth by fence height
| Fence height | Post length (at least) | Buried | Hole depth with 6 in gravel |
|---|---|---|---|
| 3 ft | 4.5 ft | 1.5 ft | 24 in |
| 4 ft | 6 ft | 2 ft | 30 in |
| 5 ft | 7.5 ft | 2.5 ft | 36 in |
| 6 ft | 9 ft | 3 ft | 42 in |
| 8 ft | 12 ft | 4 ft | 54 in |
If a third of the post is in the ground, the post is 1.5 × the fence height. Buy the next stock length up and trim the tops level after setting.
How wide should the hole be?
About 3 times the post’s width (Quikrete): roughly 10 to 11 inches for a 4 × 4 (3½ in actual) and about 16½ inches for a 6 × 6 (5½ in actual).
Setting the post
- Dig the hole to the depth above.
- Add 6 inches of gravel and tamp it, so water drains away from the bottom of the post.
- Set and brace the post plumb.
- Fill with concrete to about 3 to 4 inches below the top of the hole, then slope the top away from the post.
Go deeper when…
- The ground freezes: Quikrete says structural posts should go at least 6 inches below the frost line, or frost can push them up. Your building department can tell you the local frost depth.
- Gate and corner posts: they take the most load. Use larger posts or deeper holes.
- Soft or sandy soil, or windy sites: extra depth adds stability.
Before digging, call 811 to have underground utilities marked.
Common questions
How deep should a 6 ft fence post be?
3 ft in the ground (a 9 ft post), in a hole about 42 inches deep including 6 inches of gravel.
How many bags of concrete per fence post?
For a 4 × 4 post in a 10.5 in hole with 3 ft buried, about 1.4 cu ft: roughly 2.3 bags of 80 lb mix.