The rule of 70 and the rule of 72 give rough estimates of the number of years it would take for a certain variable to double. When using the rule of 70, the number 70 is used in the calculation. Likewise, when using the rule of 72, the number 72 is used in the calculation.
The Rule of 70
The rule of 70 is used to determine the number of years it takes for a variable to double by dividing the number 70 by the variable's growth rate. The rule of 70 is generally used to determine how long it would take for an investment to double given the annual rate of return.
For example, assume an investor invests $10,000 at a 10% fixed annual interest rate. He wants to estimate the number of years it would take for his investment to grow to $20,000. He uses the rule of 70 and determines it would take approximately seven (70/10) years for his investment to double.
The Rule of 72
The rule of 72 is a simple method to determine the amount of time an investment would take to double, given a fixed annual interest rate. To use the rule of 72, divide 72 by the annual rate of return.
For example, assume an investor invests $20,000 at a 10% fixed annual interest rate. He wants to estimate the number of years it would take for his investment to double. Instead of using the rule of 70, he uses the rule of 72 and determines it would take approximately 7.2 (72/10) years for his investment to double.

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