Banking » Investing » What Is The Rule Of 72?
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What Is The Rule Of 72?

Discover the financial magic trick: "Understanding the Rule of 72" reveals the secrets to accelerating your money's growth.
Author: Baruch Mann (Silvermann)
Baruch Mann (Silvermann)

Writer, Contributor

Experience

Baruch Silvermann is a financial expert, experienced analyst, and founder of The Smart Investor.  Silvermann has contributed to Yahoo Finance and cited as an authoritative source in financial outlets like Forbes, Business Insider, CNBC Select, CNET, Bankrate, Fox Business, The Street, and more.
Interest Rates Last Update: June 3, 2024
The banking product interest rates, including savings, CDs, and money market, are accurate as of this date.
Author: Baruch Mann (Silvermann)
Baruch Mann (Silvermann)

Writer, Contributor

Experience

Baruch Silvermann is a financial expert, experienced analyst, and founder of The Smart Investor.  Silvermann has contributed to Yahoo Finance and cited as an authoritative source in financial outlets like Forbes, Business Insider, CNBC Select, CNET, Bankrate, Fox Business, The Street, and more.
Interest Rates Last Update: June 3, 2024

The banking product interest rates, including savings, CDs, and money market, are accurate as of this date.

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Table Of Content

The Rule of 72 is a straightforward concept that offers a quick and easy way to estimate the time it takes for an investment to double in value, based on its annual rate of return.

What's the Rule of 72?

The Rule of 72 is a simple trick to estimate how long it will take for an investment to double in value. It works like this: you divide 72 by the annual rate of return you expect to earn on your investment. The result is approximately the number of years it will take for your investment to double.

For example, let's say you have an investment with an annual return rate of 8%. By applying the Rule of 72, you divide 72 by 8, which equals 9. So, it would take roughly 9 years for your investment to double at this rate.

This rule is handy because it provides a quick way to gauge the potential growth of your investments without needing complex calculations or financial formulas.

However, it's important to note that the Rule of 72 is an approximation and may not be perfectly accurate, especially for higher or fluctuating interest rates.

How Does The Rule Of 72 Work: Examples

Here are a few more examples to illustrate different scenarios using the Rule of 72:

  • Conservative Investment: Let's say you have a conservative investment with an annual return of 4%. Using the Rule of 72, you divide 72 by 4, which equals 18. So, it would take approximately 18 years for your investment to double at this rate.

  • Moderate Growth: Now, consider an investment with a moderate growth rate of 6% per year. Applying the Rule of 72, you divide 72 by 6, giving you 12. Thus, it would take around 12 years for your investment to double.

  • Aggressive Growth: For an investment with an aggressive growth rate, such as 10% annually, using the Rule of 72 yields 7.2 years (72 divided by 10). This means your investment would double in about 7.2 years with this higher rate of return.

  • Impact of Fees: Suppose you're considering an investment with a 3% annual fee deducted from your returns. With an actual return rate of 7%, your effective growth rate is reduced to 4%. Using the Rule of 72, 72 divided by 4 gives 18 years, showing how fees can significantly slow down the doubling of your investment.

These examples demonstrate how different rates of return can affect the time it takes for your investment to double and highlight the importance of understanding both potential growth and associated costs when making investment decisions.

How Different Interest Rates Change the Rule of 72?

Different interest rates directly impact the Rule of 72, influencing the time it takes for an investment to double.  Higher interest rates accelerate growth, while lower rates slow it down.

For instance, with a 6% annual interest rate, an investment would double in approximately 12 years (72 divided by 6).  Conversely, at a 3% interest rate, it would take about 24 years to double.

This relationship illustrates the exponential effect of compound interest: higher rates compound faster, leading to quicker doubling, while lower rates require more time to achieve the same result.

How Does Inflation Change the Rule of 72?

Inflation affects how fast your money grows over time. When we use the Rule of 72 to estimate how long it takes for an investment to double, we're assuming the investment grows at a certain rate every year.

However, inflation eats into that growth. Let's say your investment earns 6% interest each year, but inflation is 3%. In reality, your investment only grows by 3% after adjusting for inflation.

So, if you use the Rule of 72 with the adjusted rate of 3%, it will take longer for your investment to double. Instead of doubling in 12 years like it would without considering inflation, it might take closer to 24 years.

This happens because your money's purchasing power is reduced by inflation, making it take longer to reach double its original value. 

Rule of 72 Pros & Cons

Here are the pros and cons of using the Rule of 72:

Pros
Cons
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The Rule of 72 is straightforward and easy to use, making it accessible to a wide range of investors without requiring complex calculations or financial expertise.

It provides a rapid estimate of how long it will take for an investment to double, allowing investors to gauge potential growth without extensive analysis.

It helps investors grasp the power of compound interest intuitively, demonstrating how small changes in interest rates can significantly impact investment growth over time.

The Rule of 72 offers only a rough estimate and may not be precise, especially for interest rates that are significantly above or below average or when considering compounding frequency.

It doesn't consider factors like compounding frequency, inflation, or fluctuations in interest rates, leading to potentially misleading estimates in certain situations.

For investments with irregular cash flows, variable interest rates, or non-linear growth patterns, the Rule of 72 may not provide adequate insight, requiring more sophisticated financial analysis tools.

Common Mistakes People Make With The Rule of 72

Common mistakes people make with the Rule of 72 include:

  • Misapplying the Rule: Some individuals mistakenly apply the Rule of 72 to investments or scenarios where it doesn't accurately reflect the growth rate. For example, using it for investments with irregular cash flows or variable interest rates can lead to inaccurate estimates.

  • Ignoring Compounding Frequency: The Rule of 72 assumes annual compounding, but if an investment compounds more frequently (such as quarterly or monthly), the Rule of 72 will overestimate the time required for doubling.

  • Forgetting to Adjust for Inflation: Failing to account for inflation can lead to overly optimistic growth estimates. Adjusting the interest rate for inflation is crucial for obtaining a more accurate projection.

  • Using It as a Sole Tool: While the Rule of 72 is convenient for quick estimations, relying solely on it without considering other factors like fees, taxes, or market conditions can lead to flawed financial planning.

By being aware of these pitfalls, individuals can use the Rule of 72 more effectively and avoid potential misinterpretations or inaccuracies in their financial planning.

Estimating Growth: Alternative Financial Tools

When it comes to estimating investment growth, various financial tools are available, each with its own approach and level of precision. Understanding how these tools work can help investors make informed decisions about their financial strategies.

The compound interest formula calculates the future value of an investment by considering factors such as the initial principal, the annual interest rate, the number of compounding periods per year, and the time period.

This method provides a more precise estimate than the Rule of 72, especially for investments with variable interest rates or compounding frequencies.

Present and future value calculations determine the current or future value of an investment based on parameters like the initial investment amount, the interest rate, and the time period.

These calculations offer precise estimations, accounting for variables such as inflation, periodic contributions, and varying interest rates over time, albeit at the cost of complexity and requiring more detailed mathematical formulas.

FAQs

The Rule of 72 is used to determine approximately how long it will take for an investment to double in value based on its annual rate of return.

The Rule of 72 is commonly attributed to Albert Einstein, although its exact origin is uncertain.

Inflation affects the accuracy of the Rule of 72 by diminishing the purchasing power of money over time, potentially lengthening the time it takes for an investment to double in real terms.

Compound interest enhances the effectiveness of the Rule of 72 by magnifying the growth of an investment over time, especially when the interest is compounded more frequently than annually.

Picture of Baruch Mann (Silvermann)

Baruch Mann (Silvermann)

Baruch Silvermann is a financial expert, experienced analyst, and founder of The Smart Investor.  Silvermann has contributed to Yahoo Finance and cited as an authoritative source in financial outlets like Forbes, Business Insider, CNBC Select, CNET, Bankrate, Fox Business, The Street, and more.
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