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# Rule of 72

Modified on 2010/08/23 12:35 by Grigs Categorized as Uncategorized
Investment Banks
NPV vs Payback Method
Internal Rate of Return Method
Weighted Average Cost of Capital (WACC)
Effective Rate of Interest Calculation

# Rule of 72 Definition¶

The rule of 72 is an approximation tool used to determine the amount of time it will take for money to double on the earnings of compound interest.

## Rule of 72 Explained¶

The rule of 72 is essentially an estimation for determining the amount of years or the doubling time of an investment. This is done by taking the interest available on the investment and dividing it by 72. Rule of 72 investing is usually fairly accurate, it is more accurate with lower interest rates than it is for higher ones. The rule of 72 is normally used solely for compound interest situations and is not a very good indicator if the investment earns a simple interest at the end of the investment term. The rule of 72 is most useful if an investor cannot perform an exponential function and simply needs to do simple math for an estimate of an investment.

## Rule of 72 Formula¶

The rule of 72 formula is as follows:

Doubling Time (# years) = 72/Interest Rate

## Rule of 72 Example¶

What is the doubling time for an investment with a compound interest rate of 8%? A person using the rule of 72 equation would find the doubling time equal to 9 years. This was calculated by taking 72 and dividing it by 8. By performing this the investor can tell that it will take approximately 9 years to double the principal. It is fairly accurate as the exponential function yields an actual doubling time of 9.006 years.