Follow the two inputs into the result
First the tool multiplies its calculated post count by the hole volume per post that you enter. It then divides that total volume by the yield per bag that you enter:
total concrete volume = calculated posts × volume per post
bag count = ceil(total concrete volume ÷ entered yield per bag)
The form asks for both values and does not assume a hole size or product yield. Read the yield from the bag you plan to use; the examples below use hypothetical numbers only.
A small change across a cutoff
For an arithmetic example, keep the calculator's default 100 ft run and 8 ft post spacing, with no gates. The current handler returns 13 straight-run sections and 14 posts. Let the entered yield be 0.5 ft³ per bag. These figures define a software example; they are not a recommended hole size or a real product specification.
| Volume per post | Total for 14 posts | Total ÷ 0.5 ft³ yield | Whole bags returned |
|---|---|---|---|
| 0.25 ft³ | 3.5 ft³ | 7 | 7 |
| 0.2501 ft³ | 3.5014 ft³ | 7.0028 | 8 |
Only the hypothetical per-post volume moved by 0.0001 ft³, but the quotient moved above 7. The whole-bag rule therefore changes the displayed count from 7 to 8. The calculator is showing a rounded quantity, not claiming that the underlying volume jumped by a full bag.
Post count is upstream of concrete volume
Concrete volume also depends on the post count. To isolate that effect, compare two hypothetical inputs with the same 100 ft net run, 8 ft spacing, 0.25 ft³ per post, and 0.5 ft³ entered yield:
- With no gate, the current formula gives 14 posts. Entered hole volume totals 3.5 ft³, which returns 7 bags.
- With one 4 ft gate opening, enter 104 ft total length so the net run remains 100 ft. The current formula still has 13 straight-run sections and 14 baseline posts, then adds two gate posts for 16 posts. The entered volume totals 4 ft³, which returns 8 bags.
This illustrates the dependency in this calculator's formula. It does not decide whether the assumed opening, post arrangement, hole volume or yield is right for a real layout.
Check the inputs when the count changes
- Confirm the post count and gate inputs first; they determine how many per-post volumes are multiplied.
- Use the hole volume you intend to plan with, in cubic feet per post.
- Enter the chosen bag's printed yield in cubic feet per bag.
- Compare the displayed total volume with the bag count; one is a volume and the other is rounded to whole bags.
For the base formula, read how the calculator estimates concrete bags. For the upstream quantity, see how it counts posts.
Method and source
The post, concrete-volume and bag-count outputs were checked against this site's current form and calculation code. The cutoff examples are hypothetical input cases calculated with that handler. No bag size, excavation dimension, product, price or installation requirement was assumed.