Concrete Calculator
Calculator
Bags needed by bag size
About this calculator
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.
How to read your results
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. A horizontal bar comparison shows how many pre-mix bags you would need at each bag size for your volume, with your selected size highlighted, so you can weigh fewer-but-heavier bags against more-but-lighter ones.
How it's calculated
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 standard concrete volumetrics from the Portland Cement Association (PCA) and the National Ready Mixed Concrete Association (NRMCA).
Worked example
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.
Frequently asked questions
Why is concrete measured in cubic yards rather than cubic feet?
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.
How much waste should I add?
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.
What is the difference between a 40 lb, 60 lb, and 80 lb bag?
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.
Can this calculator handle metric dimensions?
Yes. Switch the Units toggle to Metric to enter dimensions in metres and centimetres and choose kg pre-mix bags (20, 25 or 40 kg). Results are shown in cubic metres alongside cubic yards and cubic feet, and you can switch back to Imperial (feet, inches and lb bags) at any time.
Sources
Reviewed by the YouCalc Team · Last reviewed
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