There rule of 72 it is a tool as simple as it is effective, often at the center of conversations about finance and investments. It doesn’t require a degree in economics or a sophisticated calculator: just a few seconds are enough to estimate how long it will take for an investment to double its value.
How does the rule of 72 work?
The formula is incredibly simple: divide 72 by the annual interest rate of your investment. The result corresponds to the years needed to double the initial capital. For example, with an interest rate of 6%, just calculate 72 ÷ 6 = 12 years. In this case, your investment will take 12 years to double.
This rule is not reserved for bankers or financial advisors: it is useful for anyone who wants to understand the growth of their savings quickly and intuitively.
Choosing between two investments? The rule of 72 helps you
Let’s say you’re choosing between two savings accounts. One offers a 2% annual return, while the other guarantees 4%. With the rule of 72, you can understand that:
The difference is evident.
If we consider higher returns, the rule becomes even more interesting. For example, a stock market index fund with an average return of 8% annually will double your investment in just 9 years (72 ÷ 8 = 9). In this way, the power of compound interest is no longer an abstract concept: it becomes tangible.
Other uses: inflation and debt
The Rule of 72 isn’t limited to investing. It can also be applied to inflation or interest rates on debt.
Why does the rule of 72 work?
The rule of 72 is based on the mathematics of exponential growth. While it is not perfect—it becomes less accurate at very low or very high interest rates—it is surprisingly reliable for rates between 4% and 15%.
For rates below 4%, the is more precise rule of 70while for rates higher than 10%-12%, you can opt for the rule of 74. The origin of this rule dates back to 15th century merchants, who noticed the power of compound growth long before financial calculators existed.
Mathematically, the exact calculation for doubling involves logarithms, complex tools used to solve exponential growth problems. The rule of 72, however, simplifies everything, offering a practical approximation that works in most real-world situations.
An ally for quick and intelligent decisions
The appeal of the Rule of 72 lies in its simplicity. It allows you to make fast and immediate calculations without the need for advanced tools. While it’s not perfect, it’s an extremely useful tool for evaluating savings, investments, the impact of inflation, and the costs of debt.
If you want to better understand the world of personal finance, this rule will become a precious ally for making informed and conscious decisions.
