Fencing & decking
Fence Concrete Calculator
Works out concrete for post holes, where the post's own volume is the part people forget.
Result
The post displaces concrete, which is why the post size is subtracted. Fast-setting post mix is poured dry into the hole with water added on top and does not follow the same yield, so check the bag if you are using it.
Get a Free Business AuditHow the calculation works
Concrete per hole is the hole volume less the volume the post occupies, and the post displacement is not trivial on a large post in a modest hole. Total that across the holes and divide by the yield of a bag to get the bag count. Bag yield is the figure worth checking rather than assuming, since it differs by bag weight, and working from a bags-per-post rule instead of the actual hole size is where orders come out wrong. Round up generously, because running short mid-pour on a run of holes is worse than a spare bag.
What moves the number
- Hole diameter, which dominates the volume since it is squared.
- Depth, set by the frost line and the post height.
- Post size, because a large post displaces a meaningful share of a small hole.
- Bag yield, which varies by bag weight and should come from the bag rather than memory.
Where this one goes wrong
Hole diameter matters far more than depth because volume goes with the square of the radius, so a hole dug a little wide across a whole fence adds up quickly. Digging tidy holes of a consistent diameter is both cheaper and stronger than wide irregular ones. Buy more than the calculation says: holes are never exactly uniform, some will be oversized where the auger wandered or the soil collapsed, and stopping a run of posts to fetch concrete means the earlier ones set without their neighbours plumbed against them. Check whether local practice or your soil calls for gravel at the base rather than concrete straight onto soil.
Before you quote
- Use the actual bag yield printed on the bag rather than a remembered figure.
- Buy spare bags, since holes are never perfectly uniform.
- Keep hole diameters consistent, as width drives the volume.
Questions
About this calculator
How many bags of concrete per fence post?
It depends entirely on the hole size, which is why working from volume is better than a per-post rule. Calculate the hole volume, subtract the post, and divide by the yield printed on the bag. A wider or deeper hole changes the answer substantially.
Does the post volume really matter?
On a large post in a modest hole, yes. The post displaces concrete that you would otherwise be buying, and across twenty holes the difference is several bags. Subtracting it is the difference between an accurate order and a generous one.
Why does hole width matter more than depth?
Because volume grows with the square of the radius but only linearly with depth. A hole dug a couple of inches wider than intended uses disproportionately more concrete, and across a full fence that is a large overspend. Consistent, tidy holes are cheaper as well as stronger.
The formula
concrete per post = hole volume - post volume; bags = total / 0.45 cubic feet
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How to use this calculator
- Number of posts. Enter your own figure; the page starts at 20.
- Hole diameter (in) (in). Enter your own figure; the page starts at 10 in.
- Hole depth (ft) (ft). Enter your own figure; the page starts at 2.5 ft.
- Post size (in square) (in). Enter your own figure; the page starts at 4 in.
- Cost per 60 lb bag ($). Enter your own figure; the page starts at $6.2.
The result updates as you type, so you can try a figure, see what it does to 60 lb bags total, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Number of posts | 20 |
| Hole diameter (in) | 10 in |
| Hole depth (ft) | 2.5 ft |
| Post size (in square) | 4 in |
| Cost per 60 lb bag | $6.2 |
What comes out
| Result | Value |
|---|---|
| 60 lb bags total main result | 49 |
| Bags per post | 2.4 |
| Concrete volume | 21.7 |
| Estimated cost | $303.80 |
About 2.4 bags per post, 49 bags for all 20, roughly $303.80.
How sensitive the answer is
Holding everything else steady and changing only cost per 60 lb bag, this is what happens to 60 lb bags total. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per 60 lb bag ($) | 60 lb bags total |
|---|---|
| $3.1 | 49 |
| $4.65 | 49 |
| $6.2 example above | 49 |
| $9.3 | 49 |
| $12.4 | 49 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Number of posts
- A number, between 0 and 999, starting at 20.
- Hole diameter (in) (in)
- Measured in in, between 0 and 36, starting at 10 in.
- Hole depth (ft) (ft)
- Measured in ft, between 0 and 10, starting at 2.5 ft.
- Post size (in square) (in)
- Measured in in, between 0 and 12, starting at 4 in.
- Cost per 60 lb bag ($)
- A money amount, starting at $6.2.
What the results tell you
| Result | What it is |
|---|---|
| 60 lb bags total | The headline result. A number. |
| Bags per post | A decimal figure. |
| Concrete volume | A decimal figure. |
| Estimated cost | A money figure. |
Keep going
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