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 socalled ``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 .
See also Compound Interest, Interest
References
Avanzini, J. F. Rapid DebtReduction Strategies. Fort Worth, TX: HIS Pub., 1990.
© 19969 Eric W. Weisstein
19990525