The formula estimates the entered concrete cylinder. It does not choose the required diameter or depth and does not automatically subtract a post, because installation details differ.
- Radius is half the diameter.
- Use concrete fill depth, not an unrelated excavation dimension.
- Calculate unequal holes in separate groups.
How this answer is worked out
π × (diameter ÷ 2)² × depth × count ÷ bag yieldPost-hole input checks
| Input | Correct meaning | Common mix-up |
|---|---|---|
| Diameter | full concrete cylinder width | radius |
| Depth | planned concrete fill | entire excavation by default |
| Count | same-size holes | all mixed sizes |
| Yield | bag size you chose | a generic bag value |
Put it into practice
Use the approved detail
Get hole and fill geometry from the actual fence, deck, or equipment plan and local requirements.
Normalize units
Convert diameter and depth to compatible units before volume.
Select and round
Divide by the exact current bag yield and round only the final quotient.
Before you start
- Use the utility-location service before digging.
- Local soil, frost, wind, drainage, and structural needs control the installation detail.
Sources and notes
Source links reviewed July 16, 2026. A review date is not the document's publication date.
- QUIKRETE: Concrete Mix No. 1101 Technical Data SheetNorth America · manufacturer data sheet
Bag counts are based on stated approximate yield and must be rounded up to whole bags.
- National Institute of Standards and Technology: NIST Guide to the SI, Appendix B — Conversion FactorsUnited States · government standard
Code retains exact defining constants where NIST identifies an exact relationship.