Fence Post Depth Calculator
Find the recommended fence post set depth from the post's above-ground height and your local frost depth.
How this calculator works
A fence post needs enough depth in the ground to stay stable under wind and impact load, and in cold climates, enough depth to sit below the frost line so seasonal freezing doesn’t heave it out of the ground. This calculator checks three separate depth requirements and recommends the deepest one, since setting a post too shallow under any of them risks instability or frost heave over time.
Formula: Recommended depth = the greatest of: post height ÷ 3, local frost depth (converted to feet), or a 2 ft minimum.
Worked example
A 6 ft tall fence post, with a 36-inch local frost depth:
- Height ÷ 3 = 2.0 ft
- Frost depth = 36 ÷ 12 = 3.0 ft
- Minimum = 2.0 ft
- Greatest of the three = 3.0 ft
Notes
Setting a post below the frost line matters more than the simple one-third rule in cold climates. Frost heave can push a shallow post up and out of the ground over repeated freeze-thaw cycles, even if the one-third rule alone would have said the post is deep enough for basic stability.
Always check your local frost depth, available from your building department or a regional frost-depth map, rather than relying on the one-third guideline alone if you’re in a climate that freezes.
How to use
Enter your post’s above-ground height and your local frost depth, and the calculator returns the recommended set depth — the greatest of the one-third rule, the frost depth, and the 2 ft minimum. Use this depth to plan your hole depth before ordering posts or concrete.
A general construction guideline. Local frost depth and code requirements vary — confirm exact post depth requirements with your local building department.
Frequently asked questions
Why compare three different numbers instead of using one rule?
The one-third rule (post height divided by 3) works for stability in most soils, but it doesn't account for frost heave in cold climates, and it can go below a practical minimum for short posts. Taking the greatest of the one-third rule, the local frost depth, and a 2 ft minimum ensures the post is deep enough for whichever condition is the strictest for your situation.
What is frost heave and why does it matter for post depth?
Frost heave happens when moisture in the soil freezes and expands, pushing everything above the frost line upward. If a post is set shallower than the frost line, repeated freeze-thaw cycles can gradually push it up and out of the ground, loosening or tilting it over several seasons.
Where do I find my local frost depth?
Your local building department can tell you the required frost depth for your area, and many regions publish frost-depth maps online. Frost depth varies significantly by climate, so don't assume a figure from another region applies to your location.
Does the one-third rule ever give a deeper result than frost depth?
Yes — for taller posts in regions with shallow frost depth, one-third of the post height can exceed the frost depth requirement. That's exactly why the calculator takes the greatest of all three values rather than defaulting to frost depth alone.
Estimates only. Verify quantities with your supplier before purchasing.