Free Concrete Calculator — All Cities

Concrete calculation trips up homeowners because it uses volume (cubic feet), not area (square feet). You need to multiply length times width times depth — and the depth must be in feet, not inches. A 4-inch slab is 0.33 feet deep, not 4. Getting this conversion wrong is the most common reason people buy 3x too much or too little concrete.

An 80 lb bag of Quikrete yields exactly 0.6 cubic feet of mixed concrete. A 60 lb bag yields 0.45 cubic feet. For a standard 4-inch patio slab, you need about 1.25 bags (80 lb) per square foot of surface area. The 10% waste factor covers spillage, uneven subgrade, and form overruns.

Climate has a bigger impact on concrete than on any other renovation material. Temperature during pouring and curing determines whether your slab reaches design strength or cracks prematurely. Most concrete fails not because of the mix, but because it was poured in the wrong conditions — too hot (sets before you can finish it), too cold (freezes before it cures), or dried too fast (surface crazing and weak top layer).