Aggregate table columns

Suppose if my fact table has 2 columns like cola and colb. And I have an aggregate table with colc and the value of colc is cola*colb from fct.
Is it better to do the calculations in fct and store and then move to aggregate or shall I this transformation while moving to aggregate table rather than storing it in fct table?

The decision to create an aggregate is a matter of wither it will produce a significant performance enhancement over just using the atomic facts. With that decision made, do the calculation as part of the transformation into the aggregate. There is no need to store the value in the original fact, as there is no technical reason to do so and it would hamper performance of the original fact table by making it larger (assuming the transformation is as simple as you have expressed).