How many yards of concrete is a 12x12 slab?
A 12' × 12' slab 4" thick holds 12 × 12 × 4/12 = 48 cubic feet, which is 1.78 cubic yards (48 ÷ 27). With 10% added for waste it is 1.96 cubic yards, or 88 bags of 80 lb concrete. At 6" thick the same slab takes 2.93 cubic yards with waste, or 132 bags. The slab table has the common sizes from 8' × 8' to 24' × 24' at both thicknesses.
How many bags of concrete for a 10x10 slab?
At 4" thick, a 10' × 10' slab is 33.33 cubic feet. With 10% waste that is 36.67 cubic feet, 1.36 cubic yards, or 62 bags of 80 lb concrete (61.11 exactly, rounded up to whole bags). With no allowance for waste it is 56 bags. Open the app’s concrete tool and this slab is already filled in.
The 10% waste allowance
Construction Calculator’s concrete tool starts at 10% waste, and every slab figure in the chart includes it. For a different allowance, multiply the no-waste volume by 1 plus your percentage, then divide by the bag’s yield (0.60 cubic feet for an 80 lb bag).
Bag sizes and yields
The bag counts use the yields printed on QUIKRETE’s data sheet for its Concrete Mix (No. 1101): about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb and 0.30 for 40 lb. Other mixes and brands yield differently; the yield is printed on the bag, so check it against the chart before you buy.
How the bag counts are worked out
Construction Calculator 1.1.1 works the volume as length × width × depth × (1 + waste) in exact fractions, shows cubic yards to 2 decimal places, and rounds bags (cubic feet ÷ the bag’s yield) up to whole bags, with the exact figure in brackets when it is not whole.