I'm sure there's a complex mathematical formula somewhere, but use the following table until someone coughs it up:
f1.4
-- f1.6
-- f1.8
f2.0
f2.8
-- f3.2
-- f3.5
f4
Each figure on the left column is a full f-stop, while the indicated figures on the right at the universally accepted (for convenience) 1/3 stops.
So f1.8 is 1 and 2/3 stop faster than f3.2, hence you should use that amount a lengthier shutter speed, or 3s.