Exponential growth is of the form . Yes, , the initial size. What is depends upon the rate of growth. In one time unit (hour, day, year, depending on the units for t), y will have grown from to for a rate of . In fact, between year "n" and year "n+1", y will have grown from to again giving a rate of growth of .

Note that, since , that can also be written as where is that rate of growth.