Years to Double = 72 ÷ Interest Rate
For other multiples: Years = (72 × ln(Multiple)) ÷ (ln(2) × Rate)
The Rule of 72 is most accurate for rates between 6% and 10%.
The Rule of 72 is a simple, powerful mental math shortcut used to estimate the number of years required to double an investment at a fixed annual rate of return. By dividing 72 by the expected annual return percentage, investors can quickly approximate how long their money will take to grow twofold. This rule has been used by financial professionals and everyday investors for centuries as a quick estimation tool.
The Rule of 72 works because of the mathematical properties of compound interest. While not perfectly accurate (especially at very high or very low interest rates), it provides remarkably close estimates for rates between 6% and 10%, making it invaluable for quick financial planning and comparing investment options without needing a calculator.
Using the Rule of 72 is straightforward: simply divide 72 by your expected annual rate of return. For example, if you expect an 8% annual return on your investment, divide 72 by 8 to get 9 years. This means your investment should approximately double in 9 years at that rate.
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While the Rule of 72 is the most common, there are variations for different accuracy levels and use cases:
Rule of 69.3
More mathematically accurate for continuous compounding. Use for academic or precise calculations.
Rule of 70
Often used for lower interest rates (2-5%). Easier to divide mentally for some numbers.
Rule of 114 (Tripling)
Divide 114 by the interest rate to find years to triple your investment.
Disclaimer: Rule of 72 calculations are estimates and assume a fixed annual return. Actual investment growth may vary due to market fluctuations, inflation, fees, and taxes. This calculator is for educational purposes only. Consult a qualified financial advisor for personalized investment guidance.