Formula of Comound Interest will help you. Here is the formula :
So : savings you placed
Sf : savings you get after years
r : rate
n : years
Sf = So x (1+r)^n
Here we try to get "n" so that Sf = 2So.
"n" is a math power, if you want to get it, you should use Logarithm and transform the equation :
Sf - So = (1+r)^n
Then, you isolate "n" and by doing that, every other equation's terms will be expressed in Logarythm :
n = (log sf - log so) / (log 1 + r)
Example : you have 10$, you want 20$ with 7.2% interest (Any other amount will match) :
n = (log 20 - log 10) / (log 1.072)
n = 0.301029 / 0.030195
n = 9.9696
You will need 9.9696 years. So (0.9696*365)/1 = 9 years, 11 months and 19 days.
