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The Magic Number is 72

Updated: 11/9/2022
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9y ago

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One of the first things I learned when I started to look into the field of Personal Finance was the rule of 72. It is a useful tool and one that can help you amaze and astound your friends. Say for example you had a sum of money and you wanted to double it in a given number of years. For example you want to set aside a certain sum for your daughter’s wedding in 10 years and you’d like to double the initial amount in that period of time. Simply divide the sum you’re setting aside for the occasion into 72, the result will be the interest rate you’ll have to earn to accomplish this goal. For example if you were to set aside $5,000 and hope to double it in 10 years, you’d have to earn 7.2% interest on it in order to see this dream become a reality. (72 / 10 = 7.2). The rule of 72 also works in reverse. Assume you had put that $5,000 in the highest yielding money market you could find. In this fund it was earning 4% interest. How long would it take you to double this sum? You can find out by dividing 72 by the interest rate. 72 / 4 = 18. At that rate of interest it’ll take 18 years to double your money. So, if your daughter was hoping to have double the fun at her wedding, she’d better postpone another eight years. From whence does this magical number come? The Rule of 72 is derived from natural logarithms. (Technically, it should be called the rule of 69.3, but that’s not nearly as easily divisible than 72. The difference between the two will cause negligible variances unless you’re using huge numbers. If you are using huge numbers, I suggest using a financial calculator or spreadsheet program to do your calculations.) The origin of the natural logarithm isn’t important. What is important is that the rule of 72 gives a quick and easy way to perform back of the envelope calculations regarding doubling your money. Of course there’s much much more to the topic of personal financial planning but the rule of 72 can be a fun way to enter this sometimes daunting world by allowing you to do some quick calculations, or at least be the star of your next party.

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9y ago
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