Post-hole formula
Round holes use pi x radius x radius x depth x count. Convert the diameter to feet, divide by two for radius, and keep depth in feet.
An example 12 in diameter hole that is 2.5 ft deep is about 1.96 cubic feet before waste.
Round post holes
Fence Post Concrete Calculator
Use post-hole mode to estimate concrete for fence posts, deck footings, mailbox posts, and round tubes. Enter diameter, depth, and the number of holes.
Why buffer matters
Post holes are rarely perfect cylinders. Loose soil, bell-shaped bottoms, rocks, water, and auger wobble can increase the real volume.
For small post jobs, adding one or two extra bags is often easier than stopping mid-set.
Post-hole examples
| Hole size | Depth | Concrete per hole | 80 lb bags with 10% waste |
|---|---|---|---|
| 8 in diameter | 2 ft | 0.70 ft3 | 2 bags |
| 10 in diameter | 2.5 ft | 1.36 ft3 | 3 bags |
| 12 in diameter | 2.5 ft | 1.96 ft3 | 4 bags |
| 18 in tube | 3 ft | 5.30 ft3 | 10 bags |