How to use
- Count posts that share the same hole and post dimensions.
- Measure the round hole from grade to its bottom, including the gravel layer.
- Enter the unfilled gap at the top and the actual post size, then select square or round.
- Use a package-yield reference or enter the volume printed on your bag.
How the estimate is calculated
Concrete depth = hole depth − bottom gravel − top gap. The round hole area is π × diameter² ÷ 4. Subtract width² for a square post, or π × post diameter² ÷ 4 for a round one, then multiply by concrete depth. Divide cubic inches by 1,728 to obtain cubic feet. Bags per post remain fractional; only the pooled total rounds up.
Example: one round 4 in post in a 10 in diameter, 24 in deep hole, with no gravel and a 3.5 in top gap, leaves 20.5 in of concrete depth. It takes about 0.7827 cu ft, shown as 0.8 cu ft. At 0.375 cu ft per bag that is about 2.1 bags per post, so the whole-bag purchase is 3 bags.
Package yield is different from bag weight
| Mix reference | Yield used | Source status |
|---|---|---|
| 50 lb fast-setting | 0.375 cu ft per bag | Quikrete Fast-Setting search summary; confirm packaging |
| 80 lb Concrete Mix 1101 | 0.60 cu ft per bag | Quikrete 1101 data sheet, revised October 2022 |
| Top gap and bottom gravel | 3 in and 6 in defaults | Editable planning choices within manufacturer search-summary guidance |
The fast-setting summary describes about two bags for a round-post example. That is an approximate usage figure; this worksheet rounds the calculated 2.09 bags upward for a whole-bag purchase. It does not silently round down to match the description. The 3–4 in top gap and roughly 6 in bottom gravel are planning guidance from a search summary, not universal footing requirements.
Setting and bracing the posts
Fast-setting and regular concrete have different mixing and placement instructions. Follow the selected bag for water, placement, curing, and when loads may be applied. Keep posts braced and aligned while the mix sets. This calculator supplies no setting-time promise and does not select the foundation depth; use the post-depth tool and confirm local requirements before digging.
Common mistakes
- Subtracting bottom gravel from a hole depth that already excluded it.
- Entering nominal lumber dimensions instead of the actual post width.
- Rounding each post’s bag count up before multiplying, which can overstate the pooled purchase.
Sources
Quikrete Fast-Setting Concrete Mix was checked only as a search summary. The Quikrete Concrete Mix 1101 PDF extract, revised October 2022, confirms the 80 lb yield of approximately 0.60 cu ft. Package information takes priority over these estimating references.
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Frequently asked questions
Why does the example buy three bags when it uses about two?
Calculated use is about 2.09 bags, which is greater than two full bags. The purchase count rounds up to avoid reporting an incomplete whole-bag quantity.
Can I calculate several different hole sizes together?
Run each group separately. The entered post count repeats one identical set of dimensions, so mixed holes need separate estimates.
Is the bottom gravel included in the concrete volume?
No. Its depth is subtracted from the full hole depth before the volume around the post is calculated.
Which size belongs in the square-post field?
Use the actual side width, such as 3.5 in for a nominal 4×4. The tool also checks that the square diagonal fits the hole.
Does this tell me when to hang the gate?
No. Follow your concrete manufacturer’s loading and curing instructions and keep the assembly braced as directed.