Rule of 72! - Math trick to calculate time for money to double in Mind

Have you always wanted to be able to do compound interest problems in your head? Perhaps not... but it's a very useful skill to have because it gives you a lightning fast benchmark to determine how good (or not so good) a potential investment is likely to be.

The rule says that to find the number of years required to double your money at a given interest rate, you just divide the interest rate into 72. For example, if you want to know how long it will take to double your money at eight percent interest, divide 8 into 72 and get 9 years.

Years to double = 72 / Interest Rate

This formula is useful for financial estimates and understanding the nature of compound interest. Examples:

• At 6% interest, your money takes 72/6 or 12 years to double.
• To double your money in 10 years, get an interest rate of 72/10 or 7.2%.
• If your country’s GDP grows at 3% a year, the economy doubles in 72/3 or 24 years.
• If your growth slips to 2%, it will double in 36 years. If growth increases to 4%, the economy doubles in 18 years. Given the speed at which technology develops, shaving years off your growth time could be very important.

You can also use the rule of 72 for expenses like inflation or interest:

• If inflation rates go from 2% to 3%, your money will lose half its value in 24 years instead of 36.
• If college tuition increases at 5% per year (which is faster than inflation), tuition costs will double in 72/5 or about 14.4 years. If you pay 15% interest on your credit cards, the amount you owe will double in only 72/15 or 4.8 years!

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