Good utility.

And to some more advaced, you may calculate it yourselves with any standard calculator:

x - turns needed to pop. increase
T - target population
C - current population
g - growth rate as a decimal number
ln - natural logarithm
^ - power (mat.)

C * ( 1 + g )^x = T
( 1 + g )^x = T - C
x * ln ( 1 + g ) = ln ( T - C )
therefore:

x = ln ( T - C ) / ln ( 1 + g )

Enjoy :)