The “Rules of 72, 78 and 115”


By Dr. David Edward Marcinko MBA


Use the rule of 72 to calculate how long it would take an investment to double. The rule of 72 is that an investment that earns 10 percent interest will double in 7.2 years. Use this as a starting point for calculating various interest rates and lengths of time, by dividing the number 72 by your interest rate.

For instance, if you are investing at a more conservative rate of 5 percent, you’d divide 72 by 5 for a total of 15 years (rounded up) for your money to double.



RULE 115

To figure out how long it would take your money to triple, use 115 instead of 72. So at an interest rate of 3 percent, it would take 38 years (115/3), for your initial amount to triple.


Comprehensive Financial Planning Strategies for Doctors and Advisors: Best Practices from Leading Consultants and Certified Medical Planners™

%d bloggers like this: