Explain the Rule 72?

The Rule of 72 is a straightforward mathematical principle that offers a quick and easy way to estimate how long it will take for an investment to double in value, given a fixed annual rate of return. It simplifies the concept of compounding interest, making it easier for investors to make informed decisions about their investments. Here’s how it works and some practical examples to demonstrate its utility.

What is the Rule of 72?

The Rule of 72 states that by dividing 72 by the annual rate of return, you can approximate the number of years it will take for your initial investment to double. This rule is remarkably accurate for interest rates that fall within typical investment scenarios.

How to Use the Rule of 72

To use the Rule of 72, divide the number 72 by your expected annual rate of return. The result is the approximate number of years it will take for your investment to double.

Examples Using the Rule of 72

Let’s apply the Rule of 72 to several scenarios with a portfolio amount of $100,000 to see how it works with different rates of return:

  1. 5% Rate of Return
    • Calculation: 72 / 5 = 14.4 years
    • In this case, it would take approximately 14.4 years for a $100,000 investment to grow to $200,000 at an annual return of 5%.
  2. 10% Rate of Return
    • Calculation: 72 / 10 = 7.2 years
    • At a 10% rate of return, the same $100,000 would double in about 7.2 years.
  3. Investing Over 20 Years
    • Suppose you want to see how much your $100,000 will grow in 20 years at different rates of return:
      • At 5% per year: After 20 years, the Rule of 72 suggests your money will double twice (every 14.4 years). Your investment grows to around $400,000.
      • At 8% per year: The rule estimates doubling every 9 years. In 20 years, your investment will have doubled more than twice, resulting in approximately $450,000.

Accuracy and Limitations

The Rule of 72 is a quick tool and works best within a range of interest rates from 6% to 10%. It becomes less accurate at very low or very high rates. It assumes a constant rate of return, ignoring potential market fluctuations and the impact of taxes and fees on investment returns.

Practical Applications

The Rule of 72 is particularly useful for financial planning and setting goals. It helps investors understand the impact of different rates of return on their investments and can aid in decision-making about where to place funds based on financial goals and timelines.


The Rule of 72 is a handy and practical financial tool, providing a simple way to estimate how quickly an investment can grow given a fixed rate of return. Whether you are planning for retirement, saving for a major purchase, or just exploring investment options, understanding how to apply this rule can greatly enhance your financial strategy and decision-making process.

For detailed and personalized investment advice, especially when dealing with larger or more complex portfolios, it’s advisable to consult with a Fee-Only financial adviser. They can provide tailored advice that accounts for your personal financial situation, risk tolerance, and investment objective.

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