Assume, the length of the foundation is 80 ft and width is 30 ft. The thickness of the foundation is taken as 9 inches. M7.5 concrete grade is used with ratio 1:4:8. (cement : sand : aggregate).

Initially, you have to determine the volume with the following formula :-

Volume = Length x Width x thickness = 80 x 30 x 0.75 (as 9 inches is equal to 0.75 feet) = 1800 cft

It is the wet volume and you have to convert it to dry volume with the following method :-

We know that in measuring unit cubic meter or cubic feet, 54% volume is increased from wet to dry in concrete. To convert from percentage to decimal, just divide by 100 i.e. 1+84/100 = 1.54

So, dry volume = 1800 x 1.54 = 2772 cft

To calculate the quantity of cement, the following formula is used :-

Cement = Ratio x Volume/Sum Of Ratio = 1 x 2772/13 = 213.23 cubic feet

As 1 bag of cement = 1.28 cubic feet