A horseback method for estimating the doubling effect of compound interest.

The 72 rule or rule of 72 is a shortcut way of comparing different interest rates.
It is a way to determine how long a given interest rate takes to double your money. It
is a substitute for using a math equation for doing the calculation. Because it is a
shortcut it is not as accurate as the math method, especially for larger interest rates.

How It Works

If you have an interest rate that you know that your money is going to compound at, then you
can simply divide that interest rate into the number 72 to get the number of years required to
double your money.

72 / interest rate = years to double money

You can also do it the opposite way. If you want to double your money is a certain number of
years, you can divide the years into the number 72 to get the interest rate needed to reach
your goal.

72 / years to double money = interest rate

Why The 72 Rule Is Important

Find out how fast your savings will grow. If your savings are invested at 5% annual
interest, then you will have double money in 72/5=14.2 years.

Calculate the depreciation of your savings with inflation. If the inflation rate
is 4%, then the buying power of your savings will halve in 72/4=18 years.

If you combine the above 2 examples you can see how fast your true buying power will
increase. 5%-4%=1%; Your buying power would double in 72/1=72 years.

You want to double your money in 8 years. What interest rate do you need to accomplish
your goal? 72/8=9% You will probably need a riskier approach to achieve this return.

The same rule of 72 can apply to loans as well. If you have a 25% interest rate loan
that you don't make payments to, then the amount owed will double in about 3 years.(The rule of 72
gives 2.95 years and the interest rate formula gives 3.1 years)

Summary

The 72 rule is a quick and easy way of figuring out how fast money will grow with interest.
This is the power of earning interest on interest. Albert Einstein is quoted as saying that
compound interest is the 8th wonder of the world.