Need to create a Calculated Field

Hi Folks,

I need to create a Calculated field in Opportunity(Oppty) Screen. We have product details on that page.
Like Product A is of $100.5
I get Oppty for 5 Qty for product A, so as soon as I enter the qty/save the Oppty, Estimated Revenue will automatically get calculated:

Estimated Revenue= Price for Product* Qty
So how can I achieve the same.

Many thanks!