How Many Bags of Concrete Do You Need?
Calculate cubic yards of concrete needed for slabs, footings, columns, and steps. See bags needed by size.
Concrete Volume Formula
Volume = Length × Width × Depth (all in feet, convert inches to feet by dividing by 12). Then divide by 27 to convert cubic feet to cubic yards. A standard 4-inch thick, 10×20 foot slab: 20 × 10 × 0.333 = 66.7 cubic feet ÷ 27 = 2.47 cubic yards. Always add 10% for waste — uneven ground, spillage, and form variations consume more than expected.
Bags vs Ready-Mix
Bags (60-80 lb): Best for small projects under 1 cubic yard. An 80-lb bag yields about 0.6 cubic feet. You will need roughly 45 bags per cubic yard at ~$6 each ($270/yd). Ready-mix truck: Best for projects over 1 cubic yard. Costs $125-175 per cubic yard delivered with a typical minimum order of 1 yard. A 10-yard load for a driveway costs $1,250-1,750. Ready-mix is stronger, more consistent, and dramatically less labor than mixing bags by hand.
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