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Circle, not a rectangle
Round Concrete Calculator
Use this for a circular pad, or for cardboard tubes that stand in as simple piers. The result is cubic yards. That figure is a quantity estimate, not a concrete order.
Your estimate
Assumptions
A round pad uses π × radius² × thickness. Diameter is in feet. Thickness is in inches and is divided by 12. A 10 ft diameter pad that is 4 inches thick is about 26.18 cubic feet, which is 0.97 cubic yards.
A cardboard tube uses π × radius² × height. Tube diameter is in inches, so radius in feet is the diameter divided by 24. Height is in feet. Multiply by the number of tubes. The calculator treats each tube as a solid cylinder. It does not subtract a bell bottom, a cage, or a void you leave empty.
Empty fields and values of zero or less are rejected. This page does not size a pier for frost, load, or soil. Check the diameter and embedment your project actually needs.