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.