Understanding Post Hole Volume
The volume calculation is based on the shape of a **cylinder**, which is the ideal shape for a post hole dug with an auger or a cylindrical post-hole digger. Knowing the volume of the hole is essential for both determining the amount of concrete needed and the amount of **dirt (spoil)** that must be removed.
Cylinder Volume Formula:
$$\text{Volume} = \pi \times r^2 \times h$$ Where: $r$ is the **radius** of the hole (in feet), and $h$ is the **depth** of the hole (in feet). The calculator automatically converts your inches input into feet before running the formula.
Post Hole Best Practices
Depth and Diameter Rules
- **Frost Line:** The hole depth should always extend below the local **frost line** to prevent shifting (heaving) during winter freezes.
- **Hole Diameter:** For maximum stability, the hole's diameter should be at least **three times** the width of the post being set (e.g., a 4" post needs a 12" hole).
- **Drainage:** It's recommended to place **6 inches of gravel** at the bottom of the hole before pouring concrete to help with drainage and prevent the post from sitting in standing water. (Note: The depth input should reflect the concrete fill, *not* the total hole depth if using a gravel base.)
Concrete Yield and Waste
Bag Yield vs. Volume
- **Yield:** The **cubic feet (cu ft)** number on the bag is the *mixed* volume you get. This varies based on the product (standard concrete, fast-setting, etc.).
- **80 lb Bags:** Typically yield $\mathbf{0.6}$ cu ft.
- **60 lb Bags:** Typically yield $\mathbf{0.45}$ cu ft.
- **Waste:** Always round up to the nearest whole bag and consider purchasing **1-2 extra bags** to account for uneven hole digging and spillage.
Pro Tip: Crown the Concrete
Fill the hole so the concrete forms a slight **dome** (or crown) above the soil line. This crucial step ensures water drains away from the post, significantly prolonging the post's life by preventing wood rot at the ground level.