Find out how many cubic yards of concrete you need for a slab, footing or column — with bag counts and an optional cost estimate.
For a slab or footing, multiply length (ft) by width (ft) by thickness (in) divided by 12 to get cubic feet. Divide by 27 to convert to cubic yards — the unit ready-mix trucks use. Always add 5–10% for waste and variations in depth.
For a circular column, the volume is pi times the radius squared times the height. Pre-mix bags state a yield on the label; common values are 0.6 cu ft for an 80 lb bag, 0.45 for a 60 lb bag, and 0.3 for a 40 lb bag.
One cubic yard (27 cubic feet) of concrete covers 81 square feet at 4 inches thick, or 54 square feet at 6 inches thick. Use the calculator to get the exact volume for your dimensions.
Ready-mix trucks are more economical for large pours (roughly 1 cu yd or more). Pre-mix bags are better for small repairs or footings where a truck cannot access the site.
Formwork is never perfect and ground can be uneven, so the actual volume poured is usually 5–10% more than the calculated figure. It is much cheaper to order a little extra than to need a second delivery.
This calculator tells you how much concrete you need — in cubic yards, cubic feet, and cubic metres — for a rectangular slab, footing, or circular column. Use it before ordering ready-mix or buying pre-mix bags so you have the right quantity on hand without over-purchasing.
The headline figure is the adjusted concrete volume in cubic yards, which is the unit ready-mix suppliers quote. Below that, the result card lists the same volume in cubic feet and cubic metres, plus the number of pre-mix bags you would need instead. For slabs, a horizontal bar comparison shows how the cubic-yard volume changes across three common thicknesses — 3 in, 4 in, and 6 in — for your entered length and width, so you can quickly see the cost impact of going thicker.
A driveway slab 12 ft long by 20 ft wide, 4 inches thick, using 80 lb bags and no waste allowance.
The slab is 80 cubic feet (about 2.96 cubic yards, 2.27 cubic metres). You would need 134 bags of 80 lb pre-mix concrete.
Ready-mix trucks are priced per cubic yard, so suppliers and contractors use that unit by default. One cubic yard equals 27 cubic feet. The calculator shows all three volume units so you can communicate with both suppliers and contractors.
Most professionals add 5–10% for waste on slabs to account for spillage, uneven sub-grades, and form overfill. For footings and columns the figure is closer to 5%. Enter your waste percentage in the waste field and the calculator adjusts every output automatically.
The numbers refer to the dry weight of the bag. An 80 lb bag yields roughly 0.6 cubic feet of mixed concrete, a 60 lb bag yields about 0.45 cubic feet, and a 40 lb bag yields about 0.3 cubic feet. Larger bags need fewer opens but are heavier to handle.
The input fields use feet and inches — the standard for pre-mix concrete in the US and many construction markets. The result is shown in cubic metres alongside cubic yards and cubic feet, so you can use the metric figure for reference or ordering in countries that quote by the cubic metre.
For a slab or footing the volume in cubic feet is length (ft) × width (ft) × thickness (in) ÷ 12. For a circular column the volume is π × (diameter in inches ÷ 24)² × height (ft), which converts the diameter to a radius in feet before applying the cylinder formula. Both raw volumes are multiplied by (1 + waste % ÷ 100) to get the adjusted volume. Cubic yards equals cubic feet ÷ 27; cubic metres equals cubic feet × 0.0283168. Bag count is the adjusted cubic-foot volume divided by the bag yield, rounded up to the nearest whole bag. These formulas follow the methodology published by Concrete Network and CalculatorSoup.
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