Determining the Minimum Post Depth
The depth a fence post needs to be set is critical for resisting wind load and preventing frost heave. The common professional standard is based on the $\mathbf{1/3}$ rule.
The 1/3 Rule Formula:
$$\text{Post Depth} = (\text{Height Above Ground} \times \mathbf{0.33}) + \text{Safety Buffer}$$ The safety buffer is typically $\mathbf{6}$ inches, or the local frost line, whichever is deeper.
Calculating Concrete Volume
Volume Per Hole
- **Formula:** The hole is approximated as a cylinder, so the volume ($V$) is $V = \pi r^2 h$. $r$ is the hole radius, and $h$ is the calculated depth.
- **Hole Size:** The hole should be $\mathbf{3}$ times the width of the post itself for adequate concrete coverage. (e.g., $4 \times 4$ post needs a $\mathbf{12}$-inch hole).
Concrete Bag Quantity
Yields (Approximate)
- **80 lb Bag:** Yields approximately $\mathbf{0.60}$ cubic feet (cu ft) of mixed concrete.
- **60 lb Bag:** Yields approximately $\mathbf{0.45}$ cubic feet (cu ft) of mixed concrete.
- **Always Check the Bag!** Exact yield varies by brand and mix type. Always round your required bag count up.
Frost Line Consideration:
In cold climates, your post must be set below the $\mathbf{local\ frost\ line}$ to prevent the ground from lifting the post out of the hole when it freezes. This will often override the $1/3$ rule, requiring a deeper set.