Posts that are too shallow lean over time. This tool gives a safe hole depth from your fence height, your local frost depth and your soil, and tells you what post length to buy.
Your shopping list
How to use the Fence Post Hole Depth Calculator
- Pick the height of the fence above the ground.
- Enter the frost depth in your area. Use 0 if the ground never freezes.
- Choose your soil type.
- Read the hole depth, hole width and the post length to buy.
The math behind it
Depth = the biggest of: half the fence height, the frost depth, or 24 in
Soft soil: depth x 1.2
Hole width = 3 x post width
Post length = fence height + hole depth
A worked example
What we typed in
- Fence height above ground: 8 ft
- Frost depth in your area: 48 in
- Post size: 4×4 post
- Soil type: Loose (sand or wet soil)
What we got
- Hole depth: 58 in
- Hole width: 10.5 in
- Post length to buy: 13 ft
- Gate and corner posts depth: 64 in
Second worked example
For a 6 ft fence, 36 in of frost depth, firm soil and a 4×4 post, the calculator estimates a 36 in hole depth and a 10.5 in hole width. Confirm the required depth for your location and fence system before digging.
Common sizes and example outputs
| Actual post width | Hole width at three times the post width |
|---|---|
| 3.5 in | 10.5 in |
| 5.5 in | 16.5 in |
Tips from the job site
- Ask your city building office for the local frost depth. It is a free call.
- Dig gate and corner posts 6 in deeper.
Common questions
Why does frost depth matter?
Frozen ground pushes posts up, a bit more every winter. Going below the frost line stops this.
What is the minimum depth?
24 in is a common minimum even on a short fence.
Last checked: (worked example and recalculation).