Calculating slab area. Patio slabs and driveways are often irregular in shape. For example, a driveway may be wider at the garage end and narrower at the street. The easy way to figure the area is to measure the width at both ends and then divide by two. Then multiply the result by the length of the driveway. For example, assume:  
The length of the driveway is 30 feet  
The driveway width at the apron end is 22 feet  
The driveway width at the street tapers to 8 feet  
Add width at the two ends: 20' + 8' = 28'  
Then divide by two: 28 feet divided by 2 is 14 feet  
Then multiply by the average width by the length: 14 feet times 30 feet is 420 square feet.  
A cubic yard of concrete will cover the following area, assuming no waste:  
At 2" thick, 1 cubic yard covers 162 square feet  
At 3" thick, 1 cubic yard covers 108 square feet  
At 4" thick, 1 cubic yard covers 81 square feet  
At 5" thick, 1 cubic yard covers 64.8 square feet  
At 6" thick, 1 cubic yard covers 54 square feet  
At 7" thick, 1 cubic yard covers 46.2 square feet  
At 8" thick, 1 cubic yard covers 40.5 square feet.  
If the job requires 420 square feet of 4" concrete, divide 420 by 81 to determine that 5.18 cubic yards are needed. It’s good estimating practice to allow 5 percent for waste and overexcavation (removing more soil than needed to maintain slab or footing depth). Add 5 percent to find the order quantity of 5.44 cubic yards. 


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