How Deep Should Fence Posts Be?
Depth by fence height
| Fence above ground | Hole depth | Post to buy |
|---|---|---|
| 4 ft picket | 24 in | 6 ft |
| 5 ft | 24–30 in | 8 ft |
| 6 ft privacy | 30–36 in | 8 ft |
| 8 ft | 36–42 in | 10 or 12 ft |
| Gate and corner posts | add ~6 in | next length up if needed |
The rule behind the table: bury at least one-third of the fence height, never less than 24 in. The fence calculator's worked example — a 6 ft privacy fence in 10 in holes 30 in deep — sits right on it. An 8 ft post buried 30 in leaves 5 ft 6 in of post above grade; on most 6 ft privacy fences that is fine, because the pickets run a few inches above the post tops. If your design needs full-height posts, buy the next length up and trim.
The frost line overrides the table
Where the ground freezes, water in the soil freezes and expands, and a footing that sits within the frozen layer gets lifted a little every winter — frost heave walks posts up and out of plumb year by year. The fix is to bear below the local frost line, which ranges from a few inches in the South to 48 in across the northern states. Your local building department publishes the number. Use whichever is deeper: the one-third rule or the frost line.
Why depth is what keeps a fence straight
A fence post is a lever, and the wind on a solid privacy panel is the hand pushing it. The soil around the buried portion is the fulcrum, so every extra inch of depth shortens the lever arm above and lengthens the resistance below — which is also why a 6 ft solid fence needs more depth than a 6 ft picket fence that wind blows through, and why exposed, windy runs justify both deeper holes and closer post spacing.
Hole width and what goes in it
- Diameter: three times the post width. A 4x4 (3.5 in actual) gets a 10 to 12 in hole; a 6x6 gets 14 to 18. The strength is in the concrete collar around the post, and a tight hole leaves no room for it.
- 4 to 6 in of gravel at the bottom, compacted, so the post end grain drains instead of standing in a concrete cup of water.
- Concrete to just above grade, crowned so water sheds away from the wood.
Straight sides beat a cone: a hole that tapers wider at the top gives frost a shoulder to grab. If anything, bell the bottom slightly — it anchors against uplift.
Gravel-set vs concrete-set
Line posts on a light, open fence can be set in compacted gravel alone: drainage is better for the wood and a leaning post can be nudged back later. Anything carrying real load — solid panels, corners, ends and every gate post — earns concrete. Gates especially: a sagging gate is almost always a moving gate post, and the cure at build time is one size deeper and one bag more.
Enter the run length and post spacing to get posts, sections, rails and pickets — plus the concrete needed to set every post, which is usually the item people underestimate.
Frequently asked questions
Do fence posts need to be below the frost line?
In climates where the ground freezes, yes — otherwise frost heave lifts the posts a little each winter and the fence goes out of line. In frost-free regions the one-third rule with a 24 inch minimum governs instead.
How wide should a fence post hole be?
About three times the post width: 10 to 12 inches for a 4x4, 14 to 18 inches for a 6x6. The concrete collar around the post is what resists the wind load, so it needs room to exist.
Can you set fence posts without concrete?
Light line posts can be set in well-compacted gravel, which drains better and is easier to adjust. Corner, end and gate posts, and any solid privacy fence in wind, should be set in concrete.
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