Say cement sand ratio is 1:6
calculate the quantity of mortar in 1m2 of brickwork, say
it is 0.01912.
just devide by the ratio considering 60% for wastage, say
0.01912/1*0.6=0.01147 volume of cement in the mortar.
Divide the total volume of cement by 0.0345 which s the
volume of 1 bag, say 0.01147/0.0345=0.33257/m2*number of
m2, say 100m2=34bags.