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 |
Decision guide
Calculate the annulus around each post
A post hole is usually a cylinder with a post occupying part of that volume. For a closer estimate, calculate hole volume, subtract the embedded post volume when material, multiply by the number of holes, then apply an allowance for irregular excavation and spillage. Follow local frost-depth, wind-load, and utility-location requirements.
Ten 12-inch holes, 3 feet deep
The gross cylindrical volume is about 23.56 cubic feet, or 0.87 cubic yard, before subtracting posts. A 10% planning allowance on the gross figure gives about 0.96 yard. Bag calculations must then use the exact product yield and round up to whole bags.
Before you decide
- Locate underground utilities before digging.
- Measure actual hole diameter and depth.
- Subtract post volume only when it is significant.
- Confirm the selected mix is intended for post setting.
Method and limitations
Manufacturer post calculators are useful cross-checks, but yields remain approximate and holes are rarely perfect cylinders. Product instructions control water, placement, bracing, and cure time. For gates, tall fences, high wind, or structural posts, obtain appropriate local design guidance.