Rule of 72 Calculator
Use the Rule of 72 to quickly estimate how long it takes for an investment to double at a given interest rate, or find the rate needed to double your money in a specific timeframe. Compare the quick estimate with the exact mathematical result.
The Rule of 72 is a simple mental math shortcut for estimating investment doubling time. Divide 72 by the annual rate of return to get the approximate number of years to double your money. At 8% return, money doubles in approximately 72/8 = 9 years. At 12%, it doubles in about 6 years.
The exact formula for doubling time is t = ln(2) / ln(1 + r), where r is the annual rate as a decimal. The Rule of 72 is remarkably accurate for rates between 2% and 15%. At 8%, the Rule of 72 says 9.0 years; the exact answer is 9.01 years. At 10%, the rule says 7.2 years; exact is 7.27 years. The approximation becomes less accurate at extreme rates.
The Rule of 72 also works in reverse: divide 72 by the number of years to find the rate needed to double. To double money in 6 years, you need approximately 72/6 = 12% annual return. The exact required rate is 12.25%, so the rule gives a very close estimate. This reverse application is useful for setting investment return targets based on your financial goals and timeline.