How Deep Should Fence Posts Be Set?
The standard rule used by fence contractors and recommended by the National Association of Home Builders is simple: bury one-third of the post's total length in the ground, with a minimum burial depth of 24 inches regardless of post height. This is known as the 1/3 rule.
So a 6-foot fence post needs 2 feet in the ground — totalling an 8-foot post at purchase. A 4-foot post still needs the 2-foot minimum, requiring a 6-foot post. The minimum exists because anything shallower is prone to leaning and frost heaving, regardless of how short the above-ground section is.
Common Post Heights and Depths
- 4 ft fence — 24 in burial (minimum), 6 ft total post
- 6 ft fence — 24 in burial (minimum), 8 ft total post
- 8 ft fence — 32 in burial, 10 ft 8 in total post
- 10 ft fence — 40 in burial, 13 ft 4 in total post
Frost Line: The Variable That Overrides Everything
In regions with ground frost, the 1/3 rule is a minimum, not a ceiling. Posts must be set below the frost line — the maximum depth to which the ground freezes in winter — or the soil will expand and push them out of the ground over successive freeze-thaw cycles.
Frost lines vary enormously by location. Miami has essentially no frost line. Minneapolis is around 42 inches. Parts of Montana and Canada reach 60 inches or more. Your local building department can confirm the requirement for your specific area, and it's often required for permitted fence projects anyway.
If your local frost line is deeper than the 1/3 rule would give you, the frost line wins. This calculator applies that override automatically when you enter your frost depth.
Should You Use Concrete?
For most residential fence posts, yes. Concrete anchors the post firmly, prevents side-to-side movement, and dramatically extends post life by keeping the base away from the moist soil that accelerates rot.
The hole should be three times the diameter of the post. A 4-inch post needs a 12-inch hole. Use fast-setting concrete — Quikrete or Sakrete 50 lb bags work well. You do not need to mix it; pour it dry into the hole and add water on top. It sets firm in 20 to 40 minutes.
Line posts (the ones between corners and gate posts) can sometimes get away without concrete in firm, undisturbed soil with good drainage. Corner posts, gate posts, and end posts should always be set in concrete — they bear the most lateral load.
Wood vs Metal Posts
The 1/3 rule applies to both. For wood posts, use pressure-treated lumber rated for ground contact (UC4B or better). Posts rated for above-ground contact only will fail below grade within a few years regardless of depth or concrete. For metal posts — T-posts for wire fencing, hollow steel for ornamental — the depth requirements are the same, though metal is far more resistant to decay.