## Bill Mitchell

HowMoneyWorks Educator

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## 3 Practical Ways to Put the Rule of 72 to Work

February 15, 2021

### The Rule of 72 is useful for all kinds of financial estimates and understanding the nature of compound interest.

Here are a few examples of how the Rule of 72 can be utilized in the real world to get an estimate about how money will compound in various situations.

Example 1 - Estimating the Growth of an Inheritance. <br> You inherit \$100,000 at the age of 29. What interest rate must you earn for it to become \$1 million by the time you turn 65? You’ve got 36 years for your money to grow to \$1 million, so it will take 3.25 doubles to grow \$100,000 to \$1 million dollars. Dividing 36 years by 3.25 doubles equals 11. Your money must double every 11 years. Knowing that, now you can run the formula to find your interest rate: 72 ÷ 11 = 6.54. <p> There you go. You need a financial vehicle that can offer no less than a 6.5% rate of return to hit your goal.

Example 2 - Estimating the Growth of an Economy. <br> Let’s say you want to approximate the growth rate of your country’s Gross Domestic Product (GDP). If your GDP is growing at 3% a year you can use the Rule of 72 formula: 72 ÷ 3 = 24. Therefore, in approximately 24 years, your nation’s GDP will double. Unless of course it changes. Were it to slip to 2% growth, how many years would the economy take to double? 72 ÷ 2 = 36 years. Should growth increase to 4%, GDP doubles in only 18 years (72 ÷ 4 = 18).

Example 3 - Estimating Inflation, Tuition, & Interest. <br> If the inflation rate moves from 2% to 3%, the time it will take for your money to lose half its value decreases from 36 to 24 years. If college tuition increase at 5% per year, costs will double in 14.4 years (72 ÷ 5 = 14.4). If you pay 15% interest on your credit cards, the amount you owe will double in only 4.8 years (72 ÷ 15 = 4.8)!

– Tom Mathews & Andy Horner

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