You could use the rule of 70 to estimate a country's gross domestic product (GDP) growth by dividing 70 by the expected GDP growth rate. The economic growth rate could be used to determine the amount of years it would take a country's GDP to double.

The rule of 70 is used to estimate the number of years it would take for a certain variable to double. Divide 70 by the variable's growth rate to estimate the number of years it takes for the variable to double.

The economic growth rate could be used in the calculation using the rule of 72 to estimate a country's GDP growth. The economic growth rate is calculated by subtracting the GDP of year 1 from the GDP of year 2 and dividing the resulting value by the GDP of year 1.

For example, assume you want to compare the number of years it would take the U.S. GDP to double to the amount of years it would take China's GDP to double. The United States had a GDP of $15 billion for the current year and a GDP of $14.5 billion for the previous year. The economic growth rate is 3.45% (($15 billion - $14.5 billion)/($14.5 billion)).

On the other hand, assume China had a GDP of $10 billion for the current year and $8 billion for the previous year. China's economic growth rate is 25% (($10 billion - $8 billion)/$8 billion).

It would take approximately 20.29 (70/3.45) years for the U.S. GDP to double. On the other hand, it would take 2.8 years (70/25) for China's GDP to double.