Calculators/ Concrete/Concrete Column Calculator
Concrete Column Calculator
Concrete for round columns, piers and post holes.
Round columns, sonotube piers and post holes are all cylinders, so the volume depends on the square of the diameter. Doubling the diameter quadruples the concrete, which is why post holes swallow so much more than people expect.
How this is calculated
- Volume of one column = π × radius² × height. The radius in feet is the diameter in inches divided by 24.
- Multiply by the number of columns, then add the waste allowance.
- For post holes, use the hole diameter rather than the post size, and subtract nothing for the post itself unless it is a large structural member.
Frequently asked questions
How much concrete does a post hole need?
A 12 inch hole 4 feet deep takes about 3.1 cubic feet, which is five to six 80 lb bags. A 10 inch hole at the same depth takes closer to four bags.
How wide should a post hole be?
Three times the width of the post is the common rule, so a 4x4 post wants a 10-12 inch hole. Depth should be a third of the post height plus the frost-line requirement.
Do I need gravel at the bottom of a post hole?
A few inches of gravel helps drainage and reduces rot on wooden posts. Subtract that depth from the hole before calculating concrete.