It depends on the era, but in the ancient (first) era, it is 10*current population.
So to get from 1 to 2 pop, it is 1*10 = 10, while from 15 to 16 it is 15*10 = 150.
In later eras this multiplier is reduced to 9,8 and finally 6 (so cities of the same size grow faster in later eras, but they are typically larger, so the actual growth may be slower). You can get this information from
this awesome chart.
I believe that aqueduct reduces the required about by 1/2. I'm not sure how the hanging gardens works exactly, nor how it stacks with the aqueduct.