How it Works for Your Investments

The Rule of 72 is a simple yet powerful formula—a quick mental math shortcut that lets you estimate how long it will take to double your money at a given rate of return. It provides a quick snapshot of your financial growth, helping you make smarter decisions and move closer to your Rich Life.

The Formula 

The formula for the Rule of 72 is incredibly simple: Divide 72 by your expected rate of return to estimate how many years it will take for your investment to double.

Unlike other financial formulas that require calculators or spreadsheets, the Rule of 72 offers a quick and reliable way to estimate compound growth, making it easier to make informed financial decisions. It’s simple but powerful when it comes to understanding the impact of different investment choices.

Financial experts have used this formula for decades, as it delivers surprisingly accurate results for most investment return rates between 4% and 12%.

If you’re looking for other quick and easy rules to help you stay on top of your finances and build wealth that can unlock your Rich Life, watch this video on the 10 Money Rules to Build Life-changing Wealth. 

How to Use the Rule of 72

The basic calculation

To apply the Rule of 72, divide the number 72 by your expected annual return rate (in numeric value), which refers to the percentage gain (or loss) your investment generates over a year:

72 ÷ return rate = years to double investment

The result will be the number of years it will take for that investment to double, assuming the same rate of return continues to apply.

For example, if your investment earns an 8% annual return, it will double in approximately nine years (72 ÷ 8 = 9). Increase the return to 12%, and your money doubles in just six years (72 ÷ 12 = 6). 

The Rule of 72 works with any percentage. For instance, for a 7.2% return, the calculation would be 72 ÷ 7.2 = 10 years to double your investment.

This quick calculation helps you compare different investment options such as stocks, bonds, retirement funds, and savings accounts, making it easier to visualize potential returns. 

Real-world examples 

Let’s explore how the Rule of 72 applies to various investment scenarios:

• High-yield savings accounts (2%): A savings account earning 2% interest would take 36 years to double your money (72 ÷ 2 = 36). Hence, these accounts are best for growing emergency funds rather than long-term wealth building. 
• Stock market (10%): With the stock market’s historical average return of 10%, your investment could double in 7.2 years (72 ÷ 10 = 7.2). This demonstrates the power of long-term stock investing in growing wealth over time.
• Credit card debt (18%): If you’re paying 18% interest on credit card debt, your balance doubles against you in just 4 years (72 ÷ 18 = 4). This shows how high-interest debt can quickly spiral out of control, making debt repayment a top priority.
• Real estate (6%): A typical real estate investment with a 6% return would double your money in 12 years (72 ÷ 6 = 12). This figure does not account for potential rental income or property appreciation, which makes it a feasible investment option for those with solid capital looking for steady, long-term growth.

These examples illustrate how different return rates impact your money’s growth—and why understanding them can help you make smarter financial decisions.

Rule of 72 in action with my podcast guests

On my podcast, Money for Couples, I spoke with LaKiesha and James, who at ages 38 and 45 had zero savings or investments. With retirement approaching and no financial safety net for their children, they knew they needed to take action.

Using the Rule of 72, if they invested aggressively and achieved an average 7% return, their money would double approximately every 10.3 years (72 ÷ 7 = 10.3).

For James, at 45, this means he would see two doubling periods before reaching 65. Meanwhile, at 38, Lakiesha would have the potential for nearly three doubling periods, giving her more time to grow her wealth.

This simple calculation provides a clear visualization of how your investments can grow—and why it’s crucial to start investing as early as possible to take advantage of compounding growth. 

Quick mental math for financial decision-making

The Rule of 72 helps you quickly assess whether an investment aligns with your financial goals and time horizon. For example, if you’re looking to double your money in five years, you’d require an annual return of approximately 14.4% (72 ÷ 5 = 14.4%).

This rule is also helpful when comparing different investment options side by side to evaluate which ones align best with your goals. If one investment offers 6% returns while another offers 9%, you can instantly see that the difference means doubling your money in 12 years versus eight years.

The rule also applies to inflation. At 3% inflation, the purchasing power of your money halves in 24 years (72 ÷ 3 = 24), emphasizing the importance of investments that outpace the rate of inflation.

The Rule of 72 in Action

Here’s how the Rule of 72 acts as a powerful tool in various financial scenarios: 

Doubling $10,000 at various interest rates

Let’s take $10,000 as a hypothetical base investment amount and explore its growth with various interest rates. How long does it take to double this amount with the Rule of 72?

• Conservative investments at 4% returns: Your $10,000 doubles to $20,000 in 18 years, then grows to $40,000 in 36 years, and $80,000 in 54 years.
• Moderate portfolios with 8% returns: Your $10,000 becomes $20,000 in nine years, then $40,000 in 18 years, and $80,000 in 27 years—growing twice as fast as a 4% return.
• Aggressive growth portfolio with 12% return: Your $10,000 doubles in six years, grows to $40,000 in 12 years, and $80,000 in 18 years. At this rate, after 36 years, your original $10,000 could grow to over $320,000.

This illustrates how compound growth can significantly increase your wealth over time; even with a small initial investment, you can achieve substantial financial growth in the long run. 

Comparing common investment vehicles

Using the Rule of 72, here’s how various investment types grow:

• Index funds (8-10% historical returns): Doubling your money every seven to nine years, index funds are a strong choice for long-term, hands-off wealth building.
• Corporate bonds (5% yield): This will take approximately 14.4 years to double your investment, offering more stability but slower growth compared to stocks.
• Real estate investment trusts (REITs) (7% average returns): Double your investment in about 10.3 years, providing diversification beyond stocks.
• Treasury bills (2% yield): These require 36 years to double, which shows that relying solely on ultra-safe investments is not as effective for building wealth. 

For a more detailed calculation of your investment potential, you can use my Investment Calculator. 

The dramatic difference between 4% and 10% returns

When it comes to investing, a small difference in return rates can lead to a massive gap in long-term wealth. 

Let’s put this into perspective: Over 40 years, a $10,000 investment at 4% grows to about $48,000, while the same amount at 10% skyrockets to approximately $452,000—a staggering $404,000 difference from just a 6% higher annual return.

This also highlights why minimizing fees is crucial. For example, an index fund with 0.1% fees versus an actively managed fund with 1.5% fees could mean adjusting the earnings from 9.9% to 8.5%, significantly extending the time it takes to double your money.

Compound Interest: The Eighth Wonder of the World

Since we’re discussing investments and compound growth, let’s take a closer look at compound interest—one of the most powerful tools for reaching your financial goals. Here’s how it works and why it can make a massive difference over time.

How doubling doesn’t stop at the first cycle

The true magic of compound interest becomes more apparent in the later doubling cycles, when your money grows by larger and larger absolute amounts even though the percentage remains constant.

While the first doubling of $10,000 adds $10,000 to your wealth, the fourth doubling adds $80,000, and the seventh doubling adds $640,000. This acceleration explains why people who start investing even small amounts in their 20s often end up with more money than those who start with larger amounts in their 40s.

If you’re excited to take action towards investing, here’s a quick and easy guide on investment for beginners.

Visualizing multiple doubling periods

Most people easily grasp the concept of linear growth—for example, saving $5,000 per year for 10 years adds up to $50,000. However, exponential growth, driven by compound interest, works wonders in the same amount of time.

Instead of just adding a fixed amount each year, your investments grow on top of previous gains, leading to massive long-term results.

Take this example: 

If your money doubles every seven years, a $10,000 investment can grow far beyond your expectations. After the first doubling, it becomes $20,000. By the third doubling, it’s $80,000. But the real magic happens further down the line—by the tenth doubling, your $10,000 has skyrocketed past $10 million.

This illustrates why starting early and staying invested matters. The longer you allow your money to compound, the more powerful each doubling period becomes, transforming even the most modest investments into substantial wealth over time.

Why Einstein called compound interest “the most powerful force in the universe”

Albert Einstein famously called compound interest the “eighth wonder of the world,” highlighting its ability to turn small, consistent gains into extraordinary results over time. 

His attributed quote about compound interest—“He who understands it, earns it; he who doesn’t, pays it”—serves as a powerful reminder that compounding is a double-edged sword. When you invest, compound interest accelerates your wealth. But when you owe money, especially high-interest debt like credit card debt, it can rapidly spiral out of control.

The Rule of 72 captures this power in a simple, intuitive formula, helping you visualize just how quickly money can grow—or how quickly debts can double—based on the rate of return.

The Rule of 72 for Different Financial Goals

Retirement planning

If you’re mapping out your retirement goals, here’s how you can utilize the Rule of 72: 

• Growing your retirement fund: If you need $1 million for retirement but currently have $250,000, you need to double your money twice. At an 8% return, this would take approximately 18 years (9 years × 2). 
• Understanding why early investing matters: Doubling your money six times turns $10,000 into $640,000. This means that a 25-year-old investing just $10,000 at an 8% return could have over half a million by age 65, even without adding more funds.
• Planning withdrawals in retirement: When you retire, you can use the Rule of 72 in reverse to determine a safe withdrawal rate. If you want your savings to last 24 years, dividing 72 by 24 suggests a sustainable 3% annual withdrawal rate to avoid running out of money.

If you’re looking to calculate how much you need to retire, use this simple retirement calculator to help you identify your goals so you can plan and take action toward them. 

College savings

Planning for your child’s education? The Rule of 72 helps you estimate how your savings will grow over time. 

Spoiler: The earlier you start, the less you’ll need to save. 

• For new parents: If you start saving when your child is a newborn, you have approximately 18 years until college. At an 8% return, your money will double roughly every nine years (72 ÷ 8 = 9). That means $10,000 invested today could grow to $40,000 by the time they need it—without making extra contributions.

• For parents of older kids: If your child is already 10, you only have about one doubling period left before college. This means $10,000 invested now would grow to just $20,000, requiring you to save more upfront to reach the same goal.

By understanding these doubling periods, you can make smarter, more realistic savings decisions. The earlier you start, the more you allow compound growth to work in your favor, reducing the amount you need to contribute out of pocket.

Emergency funds

While emergency funds prioritize liquidity and safety over growth, the Rule of 72 highlights the long-term cost of keeping excessive amounts in low-yield accounts.

For example, a high-yield savings account with a 2% return doubles your money every 36 years. However, with inflation averaging at 3% annually, the purchasing power of that money halves every 24 years—meaning your savings may not keep up with rising costs over time.

This is why I always recommend balancing safety with smarter allocation to ensure your money retains its value.

Rule of 72 Variations and Refinements

Here are some variations of the Rule of 72 formula, used to calculate returns in less-common scenarios.

Rule of 69.3 (for continuous compounding)

For investments that compound continuously (i.e., when interest is calculated and added constantly rather than at discrete intervals), the more precise formula uses 69.3 instead of 72:

While financial professionals may use this for sophisticated investment models and precise projections, the Rule of 72 remains the preferred tool for everyday use. Its simplicity makes mental calculations quick and easy, and for most practical interest rates, the difference in accuracy is negligible.

Rule of 70 (for more precise calculations)

For lower return rates (typically below 8%), some financial textbooks suggest using 70 instead of 72 for a slightly more accurate estimate.

The Rule of 70 is particularly useful for estimating the effects of inflation, as inflation rates usually fall within the 1–5% range. This small adjustment provides a more precise projection in such cases.

However, in everyday finance, the difference between using 72, 70, or 69.3 is minimal. The Rule of 72 remains the most popular because it allows for easier mental calculations, thanks to its many convenient divisors (2, 3, 4, 6, 8, 9, 12, etc.).

Limitations of the Rule of 72

While the Rule of 72 is a useful shortcut for estimating how long it takes to double an investment, it does have some limitations.

Lower accuracy at very high or very low rates

The Rule of 72 is most accurate for interest rates between 5% and 15%, as its precision decreases outside this range.

• For rates above 20% or below 1%: The estimate can be off by a year or more.
• For extremely high rates (50% and more): The rule tends to overestimate the doubling time.
• For very low rates (under 1%): The Rule of 72 tends to underestimate the doubling time.

Fortunately, these discrepancies rarely impact everyday personal finance decisions, as most long-term investments fall within the range where the Rule of 72 provides a reliable estimate.

Assumption of constant returns over time

The Rule of 72 assumes your investment will earn the same percentage return year after year, which rarely happens in real-world investing due to natural market volatility. 

While the stock market has historically returned an average of around 10% annually, individual years can see increases or decreases of up to 30%, creating a much more unpredictable scenario that the rule doesn’t account for.

Despite this limitation, the Rule of 72 remains useful because volatility tends to average out over long periods of time, making the simplified calculation a reasonable approximation for long-term planning.

When more complex calculations are needed

While the Rule of 72 is a handy shortcut, certain financial scenarios require more precise methods:

• Retirement planning for withdrawals: Tools like Monte Carlo simulations provide more accurate projections by factoring in market volatility and withdrawal rates.
• Investments with irregular cash flows: Internal Rate of Return (IRR) calculations offer better insights than simple doubling-time estimates.
• Tax-advantaged accounts: Since taxes can significantly impact growth, after-tax return calculations should be considered alongside the Rule of 72.

Challenges in predicting actual investment performance

No one can perfectly predict future returns, making any Rule of 72 calculation inherently speculative rather than an accurate guarantee. 

Needless to say, the rule also fails to account for external factors like changing tax laws, inflation fluctuations, or major economic shifts, all of which can impact investment performance. Ultimately, your risk tolerance and investment behavior will also play a significant role in determining your actual returns. 

Using the Rule of 72 to Evaluate Investments

The Rule of 72 provides a clear picture of how your investment decisions today can shape your financial future.

Comparing different investment opportunities

By using the Rule of 72 before investing, you can assess your options more accurately and understand the impact of different return rates.

For example, comparing a 5% CD with an 8% stock portfolio shows a stark difference—your money doubles in 14.4 years with the CD, while it takes only nine years with stocks. This highlights the opportunity cost of playing it safe with lower-return investments over long periods.

The rule is also useful when evaluating whether higher fees are justified. If Fund A charges 0.5% in fees and Fund B charges 1.5%, the 1% difference in fees means Fund A could double your money about 1.4 years faster—a small change that compounds significantly over time.

Setting realistic expectations for returns

The Rule of 72 serves as a reality check, helping to counter excessive optimism or pessimism about investment performance.

If someone promises that your investment will grow four times bigger in five years, you can use the Rule of 72 to check if it’s realistic. Since quadrupling means doubling twice, you divide 72 by 2.5 (the time needed for each doubling). This gives an annual return of about 29%, which is unusually high and a major red flag for most legitimate investments.

For retirement planning, using conservative estimates (such as 6–7% for a diversified portfolio instead of the historical 10%) provides a buffer for market fluctuations while still offering a realistic projection of your investment’s potential.

The time value of money in practical terms

The Rule of 72 turns the abstract concept of the “time value of money” into a clear, practical tool. This understanding helps justify the importance of investing early rather than waiting, regardless of your starting capital:

• Investing $5,000 at an 8% return at age 25 allows for six doubling periods by age 67, growing the investment to $320,000.

The rule also highlights opportunity costs, demonstrating how money spent today could have grown if invested instead:

• A $30,000 car purchase at age 30 could mean nearly $960,000 less in retirement savings (assuming 8% returns and five doubling periods).

Combining the Rule of 72 with Regular Contributions

By incorporating regular contributions alongside the Rule of 72, you can further optimize your wealth-building by leveraging both time and compounding:

How additional investments accelerate growth

While the Rule of 72 applies to lump-sum investments, regularly adding to your portfolio enhances growth through dollar-cost averaging and compounding.

By making consistent, regular contributions, you create a layered effect in which both old and new money will grow and compound simultaneously. With this approach, even a very modest monthly contribution can dramatically accelerate your wealth-building progress. 

Dollar-cost averaging with doubling in mind

Dollar-cost averaging—the practice of investing a fixed amount at regular intervals regardless of market conditions—helps reduce the impact of market volatility, a factor the Rule of 72 doesn’t account for.

This strategy complements the Rule of 72 by keeping your returns closer to long-term averages. By automatically buying more shares when prices are low and fewer when prices are high, you maximize growth potential while mitigating short-term market fluctuations.

Calculating your path to specific financial targets

To reach a specific financial goal, you can also work backward using the Rule of 72 to estimate how much you need to invest today.

For example, if you need $1 million in 30 years, and you expect 8% returns (doubling every nine years), your money will double approximately three times. This means you need to invest about $125,000 now ($1M ÷ 2^3).

If you don’t have the required starting amount, you can calculate the necessary regular contributions to bridge the gap between what you have and what you need to stay on track.

How to Incorporate the Rule Of 72 into Your Financial Planning

When you have a clear understanding of the Rule of 72, you can use it as a practical decision-making tool to assess the long-term impact of your financial choices, from saving and investing to spending:

• Create a personal “doubling chart” to visualize how your current investments will grow over multiple doubling periods, reinforcing the power of compounding.
• Evaluate investment opportunities with a key question: How does this affect my doubling time? This simple yet powerful perspective helps cut through marketing hype and investment trends, keeping the focus on long-term wealth-building.
• Stay motivated during market downturns by remembering that temporary losses have minimal impact on long-term doubling cycles, especially if you continue making regular contributions.

By using the Rule of 72 as a guiding principle, you can make smarter financial decisions, stay focused on long-term growth, and maximize the power of compounding by investing early.