Thanks so much.

So What I did was, I created the new 7 day calendar first. Then, without changing my Project calendar, (keeping the standard 5 day calendar), I added the column "Duration1" and modified that column with your formula. Now I am showing that column (I called it "Calendar days duration" and I am displaying that column near my regular "Duration" column, which is still based on 5 day calendar.

So this way, except showing that new column to the user, I am not causing any modifications in any calculations to my project, and, still I am keeping my regular 5 day calendar right? So I am showing the new column just for information. By the way I am not able to enter data to this new column. I can only change it if I change my regular duration column, and my new column displays the corresponding number of calendar days. Is there a way to edit this column directly also?

Also now I wanted to name my old duration column as "workday duration" but i wasnt able to change it. how can i do it?