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Fence Post Depth Calculator

Fence height and frost depth to how deep to set posts, and concrete per hole.

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Post depth

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Below the frost line, or the post moves

In freeze-thaw climates, a post set above the frost line gets pushed upward as the ground around it freezes and expands — frost heave. Setting the base of the hole below the frost depth is often more important than the general one-third rule.

A common rule of thumb sets a fence post one-third of its above-ground height into the ground — a 6 ft fence gets roughly a 2 ft hole. But in any climate with a real winter, the frost depth in your area (the depth to which the ground actually freezes) usually matters more: the hole should reach at least 6 in below that line so frost heave can't lift the post.

Soil type shifts the math slightly. Loose, sandy soil holds a post less securely and often calls for a deeper or wider hole; dense clay grips more tightly and can sometimes get away with slightly less depth, though frost heave risk in clay is often worse, not better.

Concrete volume per hole is a simple cylinder: the hole's cross-sectional area times its depth. A 4 in diameter post typically sits in an 8 to 10 in diameter hole so there's concrete on every side, not just packed dirt.

Where these numbers come from

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

What's my local frost depth?

Your county building department or a local frost depth map can tell you the standard frost line for your area; it ranges from 0 in the deep South to 5+ ft in the northern US.

Do I need concrete, or can I just tamp gravel?

Concrete gives the strongest, most permanent hold and is standard for privacy fences and anything load-bearing. Tamped gravel drains better and works for lighter fencing in well-drained soil, but it's a weaker hold overall.

Should every post go to the same depth?

Corner, end, and gate posts carry more sideways load than line posts and are often set several inches deeper, or in a wider-diameter hole, even when the fence height is the same throughout.

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