For an estimate of the quantity of ready mix concrete your project requires please click the Concrete Volume Calculator button, Per the National Ready Mix Concrete Association - Concrete in Practice #31 - Ordering Ready Mixed Concrete: "Order about 4% to 10% more concrete than is estimated from a volumetric calculation of the plan dimensions.  This will account for waste or spillage, over excavation, spreading of forms, loss of entrained air during placement, settlement of a wet mixture, truck mixer hold back and change in volume - hardened concrete volume is 1% to 2% less than that of the fresh concrete.  Reevaluate the needs during placement and communicate any changes to the concrete supplier.”