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

You can lay your own concrete slab by using ready-made concrete or mixing your own concrete from a bag. You will need to know the depth of the slab to convert the square footage into cubic feet for bag concrete. You will need to convert cubic feet into cubic yards if you will use ready-made concrete. You also need to account for waste when calculating the amount of concrete needed.


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.


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.


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.