Rule Of 72

see variables

How long (n) does it take to double your money at a specified interest rate (i)?
What interest rate (i) would double your money over a specified number of years (n)?

Theoretically,

(1)
\begin{align} {(1+i)}^{n} = 2 \: \Rightarrow \: i = {2}^{1/n } - 1 \: \Rightarrow \: n = \frac{ {log}_{e} (2) }{{log}_{e} (1+i)} \end{align}

If you would rather avoid using natural logarithms, the "rule of 72" provides a simple approximation.

(2)
\begin{align} n = \frac{72 }{100i} \end{align}

and

(3)
\begin{align} i = \frac{72}{100n} \end{align}