Concrete is sold by the cubic yard (a cube 3 ft on each side, which is 27 cubic feet) or in bags of premix. To order, find the volume of the slab or footing and change it to yards or bags.
For learning and planning practice. Check your local building codes and the manufacturer’s numbers before you build: the code numbers here are typical values, not the law where you live.
Try it with your own problem ↓A patio slab is 10 ft by 12 ft and 4 inches thick. How many cubic yards is that, and how many 80 lb bags?
About 1.48 cubic yards (order 1.5), or 67 bags of 80 lb premix.
Volume is length × width × height, but it only makes sense when all three are in the same unit. A cubic yard is 3 ft × 3 ft × 3 ft, so it holds 3 × 3 × 3 = 27 cubic feet. That is why you divide by 27, not by 3.
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Work it out: a 10 × 12 ft patio slab, 4 in thick, in 80 lb bagsOpens The Framing Square with this exampleOr try these
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20 × 24 × 1/3 = 160 cubic feet, and 160 ÷ 27 ≈ 5.93, so about 6 cubic yards.
Use the cylinder volume, π × r² × depth, in feet. A 10 in hole 3 ft deep: π × (5/12)² × 3 ≈ 1.64 cubic feet.
A 60 lb bag makes about 0.45 cubic feet, so 27 ÷ 0.45 = 60 bags.
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