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3 Practical Ways to Put the Rule of 72 to Work

3 Practical Ways to Put the Rule of 72 to Work

3 Practical Ways to Put the Rule of 72 to Work

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.
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.

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.
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.
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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