Post Hole Concrete Calculator
Calculate how much concrete you need for fence posts, deck posts, or mailbox posts based on hole dimensions and number of posts.
Concrete Needed
Post hole concrete is calculated by finding the volume of each cylindrical hole (minus the post) and multiplying by the number of posts.
Hole Volume = pi x (Hole Radius)^2 x Depth
Post Volume = pi x (Post Radius)^2 x Depth
Concrete Per Hole = Hole Volume - Post Volume
Standard post hole guidelines:
- Hole diameter: 3x the post width (e.g., 4" post = 12" hole)
- Depth: 1/3 of total post length or below frost line
- A standard 50 lb bag of pre-mix fills about 0.375 cubic feet
- An 80 lb bag fills about 0.6 cubic feet
Typical depths by use:
- Fence posts (6 ft): 24 inches (60 cm)
- Deck posts: 36-48 inches (90-120 cm)
- Mailbox posts: 18-24 inches (45-60 cm)