Concrete Calculator
Yards for the truck, bags for the mixer — with the waste factor already in, because the last half-yard is the expensive one.
Bag yields are the standard printed yields: a 60 lb bag makes about 0.45 cubic feet, an 80 lb bag about 0.60. Yards are exact math; the truck is not — order with your supplier's minimum and short-load fees in mind.
How the math works
Volume first, in cubic feet. A slab is length × width × thickness (thickness in inches ÷ 12). A footing is length × width × depth. A round hole is π × radius² × depth. Cubic yards are cubic feet ÷ 27. The waste factor multiplies the volume before anything is rounded — spillage, uneven grade, and over-excavation are real, and they come out of your pour, not the supplier’s.
Bag counts use the yields printed on the bags: an 80 lb bag of pre-mix makes about 0.60 cubic feet, a 60 lb bag about 0.45.
A worked example
A 10 ft × 10 ft slab at 4 inches thick: 100 sq ft × ⅓ ft = 33.33 cubic feet = 1.23 cubic yards exact. With the standard 10% waste factor: 1.36 cubic yards to order — or, mixing it yourself, 62 bags at 80 lb or 82 bags at 60 lb. Run those numbers above; the calculator agrees, and now you know why.
Questions from the field
How many bags of concrete do I need? Divide your volume in cubic feet (waste included) by the bag yield: 0.60 for an 80 lb bag, 0.45 for a 60 lb bag — and always round up. A 10 × 10 ft slab at 4 inches takes 62 of the 80 lb bags.
How many cubic yards is a 20 × 20 slab? At 4 inches: 400 × ⅓ = 133.3 cubic feet ÷ 27 = 4.94 yards — call it 5.4 with 10% waste. At 6 inches it is 7.4 yards exact.
Should I order a truck or mix bags? Past roughly a yard, a ready-mix truck usually wins on labor alone — but short-load fees change the math on small pours. The calculator gives you both numbers so the comparison takes ten seconds.
What waste factor should I use? 10% is the standard planning number for flatwork on decent grade. Rocky or uneven subgrade, long wheelbarrow runs, and first-time crews argue for 15%.
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