Calculating the amount of concrete you require for your project can be a very confusing exercise.
Pre-mix concrete comes in 60 lb. and 80 lb. Bags. A 60 lb. bag provides 0.45 cubic feet of cured concrete and an 80 lb. bag provides 0.6 cubic feet of cured concrete.
If you have mixed concrete delivered, it is sold by the yard (which is a cubic yard = 27 cubic feet).
Square or Rectangle Slab
Following are the formulas to calculate how much concrete you will require for a square or rectangular shaped pour: