The rule professionals use: bury one-third of the post’s above-ground height, with a 24-inch minimum — and always below your local frost line. A 6-foot privacy fence therefore needs posts about 30–36 inches deep, set in concrete.
Fence post depth chart
| Fence height | Hole depth | Post length to buy | Hole diameter |
|---|---|---|---|
| 4 ft | 24″ | 6 ft | 8″ – 10″ |
| 5 ft | 27″ | 8 ft | 10″ |
| 6 ft | 30 – 36″ | 8 – 9 ft | 10″ – 12″ |
| 8 ft | 42 – 48″ | 12 ft | 12″ |
Hole diameter should be roughly 3× the post width — a 4×4 post gets a 10–12″ hole so concrete surrounds it properly.
The frost line overrides everything
In freeze states, posts that don’t reach below the frost line get pushed up a little every winter (frost heave) until the fence waves like a ribbon. Frost depth runs 12–18″ in the South, 30–48″ across the northern tier — your county building department publishes the exact number. In Minnesota, that means even a 4-foot fence needs 42″ holes; in Georgia, 24″ covers everything.
How much concrete per post?
1.5–2 bags (80 lb) per standard post hole, 3–4 for gate and corner posts in bigger holes. For a full fence, that adds up fast — a 150-ft fence has ~20 posts ≈ 35–40 bags. Our wood fence calculator counts posts for your exact length, and the fence cost per foot comparison prices every material.
Setting posts that stay straight
Add 4–6″ of gravel at the hole’s bottom for drainage (posts rot from the bottom up in trapped water), set the post, brace it plumb both directions, then pour concrete to just above grade and slope the top away from the post so water sheds instead of pooling against the wood. Let posts cure 24–48 hours before hanging rails or panels — pushing on green concrete is how straight fences turn crooked.
Before you dig: two free calls
811 locates buried utilities (required by law in most states, free everywhere), and your building department confirms frost depth and any permit rules. Hitting a gas line costs infinitely more than the calls.