How we calculate
Volume
Every calculator converts your inputs to meters internally, applies the shape's formula (length × width × thickness for slabs/footings/walls, π × r² × height for round columns, and similar for stairs and curb & gutter), then converts the result to cubic yards, cubic feet, and cubic meters for display. Nothing is rounded until the final display step.
Waste / overage
The "Recommended order" figure applies your chosen waste percentage (10% by default) on top of the exact geometric volume. Concrete work almost always uses more than the pure math suggests — uneven subgrade, form leakage, and spillage all add up — so ordering the exact number is a common way to come up short mid-pour.
Bag counts
Bag yields are typical figures for common pre-mixed bag sizes (40/60/80 lb in the US; 20/25/40 kg metric) and are rounded up to the next whole bag. Actual yield varies by brand and mix design — check the yield printed on your specific bag for a precise count on a large order.
What this isn't
This tool estimates concrete volume for ordering purposes. It does not size reinforcement, check structural adequacy, or replace a stamped engineering drawing where one is required. For footings, retaining walls, and load-bearing structures, confirm dimensions and reinforcement with local code or an engineer.