## Rule of 72

The time required for a given Principal to double (assuming Conversion Period) for Compound Interest is given by solving

 (1)

or
 (2)

where Ln is the Natural Logarithm. This function can be approximated by the so-called rule of 72'':
 (3)

The above plots show the actual doubling time (left plot) and difference between actual and time calculated using the rule of 72 (right plot) as a function of the interest rate .