Fence Post Depth for a 24 inch Frost Line
Find how deep to set fence posts and what length of post to buy.
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Where the ground freezes, the bottom of the post should sit below the frost line so frost heave cannot lift it. Your local building department can confirm the frost depth.
How the depth is worked out
Depth in the ground = the greater of half the height above ground (the one-third rule) and the frost depth. Post length = height above ground + depth, rounded up to a stock length.
Example: a 6 ft fence with a 24 in frost depth needs posts 36 in into the ground, so buy 10 ft posts and dig 42 in deep.
Post depth at a glance
Worked out with the same settings as the calculator above, changing only the fence height.
| Fence height | Depth in ground | Post to buy | Hole depth |
|---|---|---|---|
| 4 ft | 24 in | 6 ft | 30 in |
| 5 ft | 30 in | 8 ft | 36 in |
| 6 ft | 36 in | 10 ft | 42 in |
| 8 ft | 48 in | 12 ft | 54 in |
Rules of thumb
| Item | Typical |
|---|---|
| Depth in the ground | ⅓ of the post length, or below the frost line |
| Hole width | About 3 × the post width |
| Wood post spacing | 6 to 8 ft |
| Chain link line posts | No more than 10 ft |
| T-posts for barbed wire | 8 to 12 ft |
| Field fence | Wood posts 8–10 ft, T-posts 6–8 ft |
Common questions
How deep should a fence post be?
A common rule is to bury a third of the post's total length, which is half the height above ground. Quikrete's example is a post 6 ft above ground set 3 ft deep in a 9 ft post.
What about the frost line?
Where the ground freezes, set the bottom of the post below the frost line so frost cannot heave it. Frost depth ranges from almost nothing in the southern US to several feet in northern states and Canada; your building department can confirm it.
How wide should the hole be?
About three times the width of the post: roughly 10½ to 12 in for a 4×4 and 16 in for a 6×6.
Sources
- Quikrete, project calculator (post depth example): https://www.quikrete.com/calculator/