How concrete volume is calculated
For a rectangular slab, footing or wall, multiply length × width × depth, using consistent units. Convert cubic feet to cubic yards by dividing by 27. For a circular slab or cylindrical column, multiply π × (diameter ÷ 2)² × height or thickness. Then multiply by the number of identical shapes.
Worked example: 20 × 10 ft slab, 4 in thick
Convert the thickness to feet: 4 ÷ 12 = 0.3333 ft. Volume is 20 × 10 × (4 ÷ 12) = 66.67 cubic feet. Divide by 27 = 2.47 cubic yards before extra. With a 10% allowance, the calculation gives 2.72 cubic yards after rounding to two decimals.
Ready-mix versus bagged concrete
Bag counts are rounded upward separately for 40, 60 and 80 lb bag sizes. Starting yields (0.30, 0.45 and 0.60 cubic feet per bag) are approximate and editable; confirm actual output on the manufacturer packaging. A ready-mix order requires checking supplier minimums, truck access and delivery policies.
Limits and practical checks
Compaction, excavation variance, formwork and site conditions may change actual use. Material volume does not establish reinforcing requirements, soil bearing suitability, footing depth or structural compliance. Consult a qualified professional and local code where necessary.
What if I have several equal footings?
Use the count field to multiply each footing's volume. For different footing sizes, calculate them separately and add the volumes.
Does this tell me what slab thickness is safe?
No. Enter a thickness determined for your project; this calculator estimates material only.