18/06/2026
The rule below can give you a grasp on the effects of Compound Interest and inflation, and a few other things. If you haven't seen it before, it can give you another reason to build your own business, or to see why you might be struggling.
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The Rule of 72 is a simple formula to estimate how many years it will take for an investment to double at a fixed annual rate of return.
The Rule of 72 works by dividing the number 72 by the expected annual rate of return. For example, if your investment earns 8% per year, 72 ÷ 8 = 9 years, meaning your money will approximately double in 9 years. If the return is 6%, it would take 12 years (72 ÷ 6 = 12) to double your investment. This method is most accurate for moderate rates of return, typically between 4% and 15%
How It Works
The Rule of 72 is based on the concept of compound interest, where interest is earned on both the initial principal and the accumulated interest. For instance, if $1,000 grows at 7% annually, after one year it becomes $1,070, and the next year, interest is calculated on $1,070, not just the original $1,000. This compounding effect accelerates growth over time, and the Rule of 72 provides a quick mental estimate of the doubling period.
Practical Applications
1. Investment Planning: It helps investors visualize how different rates of return affect the time needed to double their money, aiding in long-term financial planning.
2. Impact of Fees: You can estimate how long it will take for fees to reduce your investment by half. For example, a 2% annual fee would halve your principal in 36 years (72 ÷ 2 = 36) if the investment doesn’t grow.
3. Inflation Effects: The Rule of 72 can also estimate how inflation erodes purchasing power. At 3% inflation, it would take 24 years for $1 to be worth $0.50 in real terms (72 ÷ 3 = 24)
4. Comparing Investments: It allows quick comparisons between different investment options. An 8% return doubles money in 9 years, while a 4% return takes 18 years, showing the impact of higher returns.
Limitations
The Rule of 72 is a shortcut, not a precise calculation. Its accuracy decreases at very high or very low interest rates, and it assumes a constant rate of return over time. For more precise calculations, especially with daily or continuous compounding, other formulas like the Rule of 69 or logarithmic calculations may be used.
In summary, the Rule of 72 is a quick, practical tool for estimating investment growth, understanding the effects of compounding, and planning for long-term financial goals. It is widely used because it simplifies complex calculations into an easy-to-remember formula.
