What it works out
Work out the concrete for round piers and post holes. Enter the diameter, the depth and how many holes there are, and take off a post set in the concrete if you have one. You get the volume for one hole and for all of them, and the number of bags of the size you choose, rounded up. Waste starts at the usual figure for the trade; change it in the tool. Bag yield is printed on the bag.
The formula
- Volume
- π × (diameter ÷ 2)² × depth − post area × embedded depth
- Bags
- volume × (1 + waste) ÷ bag yield, rounded up
In words: Volume = pi times (diameter divided by 2) squared times depth, minus post area times embedded depth; bags = volume times one plus waste, divided by bag yield, rounded up
Worked example
Six round holes, 12 in across and 3 ft deep
- Diameter 1 ft
- Depth 3 ft
- Holes 6
- One hole = π × (1 ÷ 2)² × 3 = 2.356 ft³
- Six holes = 14.14 ft³
Bags = volume with waste ÷ the bag yield printed on the bag, rounded up.
Good to know
- Bag yield varies by product: use the figure printed on your bag.
- Footing size and depth must follow your local code; this only counts concrete.
- A calculator, not a building-code check: confirm local rules with your building authority.
Questions
Does it set the pier size or depth?
No. You enter the size. Check local rules for frost depth and bearing.
Can I take off a post?
Yes. Enter the post size and embedded depth and its volume is subtracted.