Rule of 72

The Rule of 72 is a quick mental calculation shortcut that estimates how long it takes for an investment to double in value — by dividing 72 by the annual rate of return. Doubling Time (years) ≈ 72 ÷ Annual Return Rate (%). For example, an investment earning 12% per annum doubles approximately every 6 years (72 ÷ 12 = 6), while one earning 8% takes approximately 9 years to double. The Rule of 72 works because 72 is close to 69.3 (the mathematically precise answer using natural logarithms) and divides evenly by many common interest rates — making it particularly convenient for mental arithmetic. For Indian investors, the Rule of 72 is a powerful intuitive tool for comparing different investment options — a Nifty 50 index fund historical CAGR of approximately 12% doubles every 6 years, while an FD at 7% takes over 10 years to double, illustrating why long-term equity investing creates substantially more wealth than fixed income for patient investors. The rule also works in reverse — for inflation: at 6% inflation, the purchasing power of money halves in approximately 12 years (72 ÷ 6 = 12), reinforcing the urgency of investing in assets that outpace inflation. AMFI's investor education materials and financial advisors across India frequently use the Rule of 72 to make the abstract power of compounding tangible and immediately understandable for first-time investors.