How this calculator works
Concrete fills the ring between the hole wall and the buried post. The tool takes the cylinder of the hole, subtracts the volume of the post below grade, and multiplies by the number of posts and the waste allowance. Bags are then counted at the printed mixed yield, because a bag of dry mix does not fill its own volume once mixed. A rectangular post inside a round hole leaves more concrete at the corners than a round post of the same width, which is why the post shape and both cross-section dimensions are inputs.
Formula
Hole volume = pi x hole diameter^2 / 4 x depth; post volume = width x thickness x depth or pi x post diameter^2 / 4 x depth; concrete per post = hole volume - post volume; order = concrete per post x posts x (1 + waste); bags = ceil(order / mixed yield).
Worked example
A 300 mm hole 900 mm deep holds 0.0636 m3. A 100 mm round post buried the full depth takes 0.0071 m3, leaving 0.0565 m3 of concrete per post. Ten posts need 0.565 m3, which is 41 bags at 0.014 m3 per bag.
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Frequently asked questions
Does the post sit on gravel at the bottom of the hole?
Often yes, and then the concrete ring is shallower than the full hole. Enter the concrete depth, not the excavation depth, if you plan a gravel base. Keep the post end above the base so it is not sitting in water.
Should I use fast-setting or regular mix?
Fast-setting post mix is poured dry and needs no mixing, while regular concrete is mixed first. Both are sold in bags with different yields, so use the yield printed on the bag you buy.
How much concrete does one post hold?
That depends on the hole and the post. A 300 mm by 900 mm hole with a 100 mm post takes about 0.057 m3, which is just over four 0.014 m3 bags, so buy five. The table below shows the bag count for other hole sizes at the same depth.