Concrete quantity or volume is calculated by using 3 dimensions of element.
From drawing you should be able to understand, what is column, beam and slab etc.
So open drawing look at the element, understand its length, width / breadth and depth / height
Once you know its 3 dimension, you can calculate the concrete quantity of that element.
Example: a column of 1m width and 1m length of height 3 meter will require concrete quantity of
1 x 1 x 3 = 3 cubic meter.