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The detailed guide below is currently available in English.
How concrete volume is measured
Concrete is sold by volume, not by weight or area. Slabs and footings are simple boxes: length times width times thickness. Round columns use the circle area formula, π times the radius squared, times the height, multiplied by the number of columns. This calculator converts every dimension to feet internally and reports the result in cubic yards — the unit ready-mix suppliers quote — along with cubic meters and cubic feet.
Bags versus ready-mix trucks
Bagged concrete is practical for small jobs: an 80-lb bag mixes to roughly two-thirds of a cubic foot, with 60-lb and 40-lb bags proportionally less. Bag counts here include a 5% margin for spillage and uneven subgrade and always round up. Once a project needs more than about 30 to 40 bags, ordering ready-mix by the cubic yard is usually cheaper and far less work than mixing by hand.
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How much extra concrete should I order?
A 5–10% margin is standard practice. The bag counts here already include 5% for waste; for uneven excavation or hand-mixed pours, consider rounding your order up another bag or two.
How thick should a slab be?
Common residential practice is 4 inches for walkways, patios, and shed floors, and 5 to 6 inches for driveways or anything carrying vehicles. Local building codes govern structural slabs, so check before you pour.
How do I size post holes?
Use the "Hole, Column, or Round Footing" tab: enter the hole diameter and depth and the number of holes. The same tab works for round form tubes and cylindrical footings, and the quantity box multiplies the result for repeated holes.