Rule of 72

Here's a simple rule of thumb to quickly calculate compounding interest in your mind. It is called the rule of 72.

To calculate the annual compounding interest required to double your money in a fixed number of years, or conversely, given an annual interest rate, to calculate the number of years it would take to double your money, divide it my 72.

For example, if the rate of interest is, say 9%, it would take 72/9 = 8 years approx. to double your money. Similarly, if you want to double your money in, say 6 years, you'd need an annual
compounding interest rate of 72/6 = 12%.

This rule gives good results for upto 20 years or 20% rate, and very good results at typical compounding rate between 6% to 10%.

The same rule can also be applied to find out the number of years for the value of money to get halved at a certain inflation rate. For example, for an inflation rate 6% it would take 72/6 = 12 years for the value of your money to get halved. This means the money which gets you a certain commodity now would only buy half of that after 12 yrs. In other words, you'd need double of this money to buy the same thing after 12 yrs.