Answer:
The Rule of 72 is a quick, useful formula that is popularly used to estimate the number of years required to double the invested money at a given annual rate of return. Alternatively, it can compute the annual rate of compounded return from an investment given how many years it will take to double the investment.
While calculators and spreadsheet programs like Microsoft Excel have functions to accurately calculate the precise time required to double the invested money, the Rule of 72 comes in handy for mental calculations to quickly gauge an approximate value. For this reason, the Rule of 72 is often taught to beginning investors as it is easy to comprehend and calculate. The Security and Exchange Commission also cites the Rule of 72 in grade-level financial literacy resources.
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KEY TAKEAWAYS
The Rule of 72 is a simplified formula that calculates how long it'll take for an investment to double in value, based on its rate of return.
The Rule of 72 applies to compounded interest rates and is reasonably accurate for interest rates that fall in the range of 6% and 10%.
The Rule of 72 can be applied to anything that increases exponentially, such as GDP or inflation; it can also indicate the long-term effect of annual fees on an investment's growth.
This estimation tool can also be used to estimate the rate of return needed for an investment to double given an investment period.
For different situations, it's often better to use the Rule of 69, Rule of 70, or Rule of 73.
Rule of 72
The Formula for the Rule of 72
The Rule of 72 can be leveraged in two different ways to determine an expected doubling period or required rate of return.
Years To Double: 72 / Expected Rate of Return
To calculate the time period an investment will double, divide the integer 72 by the expected rate of return. The formula relies on a single average rate over the life of the investment. The findings hold true for fractional results, as all decimals represent an additional portion of a year.
Expected Rate of Return: 72 / Years To Double
To calculate the expected rate of interest, divide the integer 72 by the number of years required to double your investment. The number of years does not need to be a whole number; the formula can handle fractions or portions of a year. In addition, the resulting expected rate of return assumes compounding interest at that rate over the entire holding period of an investment.
The Rule of 72 applies to cases of compound interest, not simple interest. Simple interest is determined by multiplying the daily interest rate by the principal amount and by the number of days that elapse between payments. Compound interest is calculated on both the initial principal and the accumulated interest of previous periods of a deposit.
How to Use the Rule of 72
The Rule of 72 could apply to anything that grows at a compounded rate, such as population, macroeconomic numbers, charges, or loans. If the gross domestic product (GDP) grows at 4% annually, the economy will be expected to double in 72 / 4% = 18 years.
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