What Is the Rule of 72?
The Rule of 72 is a popular mental math shortcut used in finance to quickly estimate how many years it will take for an investment to double in value, assuming a fixed annual rate of compound interest. Instead of pulling out a financial calculator or using complex logarithm formulas, you simply divide the number 72 by your expected annual interest rate.
How the Rule of 72 Formula Works
The formula is incredibly straightforward:
Years to Double = 72 ÷ Annual Interest Rate
If you expect an 8% return from an equity mutual fund, 72 ÷ 8 = 9. It will take roughly 9 years for your money to double. If you have ₹5,00,000 invested, you can expect it to reach ₹10,00,000 in 9 years without adding any additional funds.
When the Rule of 72 Is Useful
The Rule of 72 is most useful for rapid portfolio assessments, comparing different investment assets, and understanding the impact of inflation. For instance, if inflation is running at 6%, you can use the Rule of 72 to deduce that the purchasing power of your money will be cut in half in 12 years (72 ÷ 6).
Rule of 72 vs Rule of 114
While the Rule of 72 tells you how long it takes to double your money, the Rule of 114 tells you how long it takes to triple it.
For example, at a 12% interest rate:
- Rule of 72: 72 ÷ 12 = 6 years to double.
- Rule of 114: 114 ÷ 12 = 9.5 years to triple.
These shortcuts allow investors to quickly map out financial timelines without complex spreadsheets.