Story points/sprints mapped to duration for use in gannt chart


I am really struggling to make this work in my project.

I have a calculation formula which turns the sprint estimates into a number.

E.g 1 = 14, 2 =28, 3 = 42 etc

I am trying to get the duration column to pick this up nd hence give me the value in days which then correctly moves the gaunt chart if a start date is entered. I can't seem to make it work. Does anyone have an answer of how to do this?