This page explains the measurement or planning assumption behind the result. When you need the project quantity itself, use Post Hole Concrete Calculator.
Why this planning detail matters
Post-hole concrete is a repeated-volume problem. Small diameter changes matter because circular area depends on the square of radius. A hole that is slightly wider than planned can use noticeably more concrete across many posts. Subtracting post displacement also prevents treating the hole as if it were entirely filled with concrete.
How to measure or choose the starting value
Measure hole diameter across the widest representative section and concrete depth from the bottom to the planned top of concrete. Convert diameter to radius and use cylinder volume πr²h. For a square or rectangular post, calculate the post cross-sectional area × embedded depth and subtract that from the hole volume when appropriate.
Adjustments that change the estimate
Bell-shaped bottoms, loose soil, over-augering, groundwater and irregular excavation can increase real volume. The correct hole diameter and depth may be governed by frost, soil, structural or code requirements; those are design inputs, not something a material calculator should choose. Post shape and whether the hole is fully backfilled with concrete also matter.
How to use the number before you buy
Calculate one representative hole, multiply by the number of matching holes, then convert to bag count using the selected mix yield. If holes vary substantially, group them by size instead of multiplying one average by every post. Keep spare bags or ordering allowance as a separate project choice.
Planning workflow
Confirm required hole dimensions
Use the project or code requirements.
Measure representative holes
Actual excavations can differ from nominal auger size.
Subtract post displacement
Only for the portion occupying the concrete volume.
Multiply and convert to bags
Use exact bag yield and round up.
Example: repeated holes
If one net hole requires 0.8 cu ft of concrete and there are 12 matching posts, the project volume is 9.6 cu ft before contingency. Bag count then depends on the yield of the selected concrete mix.
Reference table
| Planning item | Useful starting point | What to verify |
|---|---|---|
| Round hole | π × radius² × depth | Actual diameter/depth. |
| Post displacement | post area × embedded depth | Post shape and dimensions. |
| Multiple holes | net one-hole volume × count | Group different hole sizes separately. |
Frequently asked questions
Do I subtract the post from the hole volume?
If the post occupies space in the concrete-filled portion, subtract its displacement for a more detailed material estimate.
Can I use auger diameter as exact hole diameter?
Use it as a starting point, but actual soil and drilling can create larger or irregular holes.
Does this guide tell me how deep fence posts must be?
No. Depth is a project/design requirement; the calculator estimates material for the depth you provide.
Planning note: Product coverage, yield, package size, installation instructions and project requirements take priority over generic examples. Verify the exact product and project before purchasing.