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Understanding the rule of 72 to double your investment

In the world of finance, time is a crucial factor that can greatly impact the growth of your investments. One powerful tool that offers a quick and approximate estimation of an investment’s doubling time is the Rule of 72. This simple and widely used financial concept allows investors to gain valuable insights into their investments’ potential growth based on a fixed annual growth rate or interest rate. By harnessing the Rule of 72, individuals can make more informed decisions about their financial future and gain a better understanding of how to maximize the returns on their investments. In this article, we delve into the mechanics of the Rule of 72 and explore five ways in which it can be a valuable asset in your personal investment planning.

The Rule of 72

The Rule of 72 is a simple and widely used financial concept used to estimate the time it takes for an investment or debt to double in value or size. It provides a quick and approximate way to calculate the number of years required for the value of investment to double based on a fixed annual growth rate or interest rate.

Formula for the Rule of 72

Number of years to double investment  =  72/Annual compounding interest rate

In this formula, the “Annual compounding interest rate” is the percentage rate at which your investment or debt grows or compounds annually. This could represent the interest rate on an investment or the annual percentage rate (APR) on a debt.

By dividing 72 by the annual compounding interest rate, you can estimate how long it will take for the initial value to double. Again, it’s important to note that the Rule of 72 is an approximation and may not be as accurate with interest rates or growth rates that fluctuate significantly over time.

How to use the Rule of 72 to double your investment

Using the Rule of 72 for personal investment planning can be a helpful tool to estimate how long it might take for your investments to double based on a given annual compounding interest rate. Here’s how you can incorporate it into your investment planning:

  1. Identify your investment’s expected annual return: Determine the average annual return you anticipate for your investment. This could be based on historical data, financial projections, or the advice of a financial advisor.
  1. Apply the Rule of 72: Divide 72 by the annual compounding interest rate to estimate the number of years it will take for your investment to double.

Let’s illustrate this with an example:

Suppose you plan to invest in a long-term investment with an expected average annual return of 8%.

Number of years to double =  728

Number of years to double ≈ 9 yrs

According to the Rule of 72, it would take approximately 9 years for your investment to double at an average annual return of 8%.

Importance of the Rule of 72 in making investment decisions

  • Quick Estimations: The Rule of 72 provides a fast way to estimate the potential growth of an investment without the need for complex calculations. It helps you get a rough idea of how long it might take for your money to double based on a given interest rate.
  • Ease in Comparing Investment Options: When considering different investment opportunities, the Rule of 72 allows you to compare potential growth rates quickly. If you have two investment options with different interest rates, the Rule of 72 can help you assess which one has the potential to double your money faster.
  • Setting Investment Goals: By using the Rule of 72, you can set realistic investment goals and understand the timeframes required to achieve them. It gives you a sense of how long you might need to hold onto an investment to reach a specific financial milestone.
  • Understanding the Power of Compounding: The Rule of 72 shows how even a modest annual return can significantly grow your investment over time, motivating you to start investing early to take advantage of compounding effects.

Different Investment tools and the time they take to double investments 

  1. Kenya Financial Market Illustration
Investment ToolYield (Range)Time to Double Investment (Rule of 72)
Savings Account3% – 7%10.3 – 24 years
Fixed Deposit6% – 10%7.2 – 12 years
Treasury Bills12% – 12.5%5.7 – 6 years
Treasury Bonds14% – 15.5%4.6 – 5.1 years
Stocks 7% – 10%7.2 – 10.2  years
Real Estate /Real Estate Investment Trusts6% – 12%6 – 12 years
Unit Trusts / Mutual Funds8% – 15%4.8 – 9 years
Corporate Bonds10% – 13%5.5 – 7.2 years
  1. USA Market Illustration 
Investment ToolYield (Range)Time to Double Investment (Rule of 72)
Savings Account0.5% – 2%36 – 144 years
Certificate of Deposit (CD)1.5% – 3%24 – 48 years
Treasury Bills2.5 –  5.35%13.4 – 28.8 years
Government Bonds3.5% – 4.75%15.1 – 20.5 years
Corporate Bonds3% – 6%12 – 24 years
Stocks (Equities)7% – 12%6 – 10 years
Real Estate Investment Trusts (REITs)4% – 8%9 – 18 years
Mutual Funds5% – 10%7.2 – 14.4 years


In conclusion, the Rule of 72 stands as a powerful tool for investors seeking to comprehend the dynamics of compounding and the potential growth of their investments over time. By providing a quick and accessible estimation of doubling time, this rule allows individuals to set realistic investment goals, compare different investment options, and grasp the impact of compounding on their financial future. However, it is crucial to acknowledge that the Rule of 72 is a simplified approximation and has its limitations. For comprehensive investment planning, consulting with a financial advisor and considering other relevant factors like taxes, fees, and inflation remains essential. Armed with the knowledge of the Rule of 72 and complemented by professional guidance, investors can navigate their financial journey with greater confidence, making informed decisions to secure a more prosperous future.

photo source: freepik

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