How Much Concrete Do I Need for a 20x40 Slab?

Depth

• Before calculating how many cubic yards of concrete you need for a slab 20-by-40 feet, you need to add the depth to the equation. The standard depth of a slab is 4 inches or 0.33 feet. You may need to check with your local building inspector to find out what the specific depth of the slab must be in your area.

Waste

• Before finalizing the cubic yards of concrete you will need to complete the slab, you need to add 5 percent for waste. Although no one wants to spill concrete or over excavate, it does happen. By adding 5 percent to the total cubic yards you need, you will not need to worry about being short.

Ready-Made Concrete

• Calculate the amount of concrete needed by multiplying the length by the width by the depth to find the cubic feet. There are 27 cubic feet in a cubic yard. Divide the cubic feet by 27. The total cubic yards of concrete needed is 9.88. Multiply 9.88 cubic yards by 5 percent for waste. The total amount of concrete needed is 10.37 cubic yards from a concrete company.

Bags

• Bag concrete comes in 60- or 80-lbs. bags. When using bag concrete, you use cubic feet to determine how much concrete you need. One 60-lbs. bag of concrete will make 0.45 cubic foot of concrete. An 80 lb. bag of mix will make 0.6 cubic feet. A 20-by-40-feet-by-4-inch concrete slab is 267 cubic feet, plus 5 percent for waste is 280 cubic feet. Divide the cubic feet by the cubic feet of the size bag you will use. If you are using a 60-lb. bag, you divide 280 cubic feet by 0.45. You would need 623 bags of concrete.