A prospective client first question is an estimate to know the numbers of blocks and number of bags of cement required to complete his building project.
But for this article I will write on calculating the numbers of bags of cement for a simple project of 2 bedroom bungalow. Using the plan shown below.
For this plan the total wall both internal and external is 72.5m all (check for the calculations http://buildingcontractorsecrets.com/2010/09/how-to-calculate-numbers-of-blocks-from-a-building-plan/ )
Taking the thickness of block as 150mm or 6” the foundation base is taken as 150mm(or 0.15m x 3)=450mm or 0.45m or 18”.
Now for any concrete you want to calculate, first find out the volume.
For the foundation of the plan above
450mm(breadth) x 72.5m (entire length) x 150mm(foundation base thickness)=4.894m3
Now the volume of a bag of cement is given as 0.035m3 but lets use 0.03m3 to cater for shortages.
There are two major mixture ratio for concrete 1:2:4 or 1:3:6, there are others, but we are going to base our calculations on these two. 1:2:4 means one portion of cement is mixed with an equivalent volume or ratio of two of same portion of volume of cement in sharpsand and four of same portion of volume of cement in granite. That is, if I have a cement of 1m3, then I will need to mix it with 2m3 of sharpsand and 4m3 of granite stones. However one bag of cement is calculated to be approximately two headpans of sand or granite, therefore for one bag of cement in ratio 1:2:4, you will add four headpans of sharpsand, and eight headpans of granite, this is the standard , though experience has shown that materials properties varies, hence there is always little variations in quantities used on site.
Now, for the plan above, the numbers of bags of cement needed in the concrete is simply calculated by dividing the total volume of cement in concrete by a volume of one bag which is taken as 0.03.
For total volume of cement in ratio 1:2:4, add the numbers together,1+2+4=7
Divide the volume of concrete by 7 to get the volume of cement in concrete.
i.e 4.894/7=0.7m3 for volume of cement.
Remember volume of one bag of cement is 0.03, then divide 0.7m3 by 0.03
=23 bags of cement
Also if the ratio is in 1:3:6, simply add 1+3+6=10
Divide volume of concrete by 10 to get the required volume of cement in concrete
i.e 4.894/10=0.4894 for volume of cement
Don’t forget we are taking the volume of one bag of cement as 0.03,
Then divide 0.4894m3 by 0.03m3
=16 bags of cement (this is most applicable on good and fairly stable soil.