Calculation/columns

Hello, wondering if anyone got a solution for my spreadsheet. Tried to google, but i can't seem to find anything and i really don't know what to search for.

For example: I want every cell in the column B to multiply with the cells from column D, and make the answer show up in column F.

B4 * D4 ---> F4

I do know how to make this manually on every cell in column-F, but i would like to have one single line of code that makes the spreadsheet understand that the B-column should always multiply with column-D.

Hope the text isn't to hard to understand, would appreciate all the help i can get. Thanks in advance.