Rule of 70 Doubling Time

What is this formula?

The Rule of 70 estimates the number of years required for an economic quantity to double when growing at a constant annual percentage rate.

It is a widely used approximation in economics, finance, demographics, and population studies.

When to use it

Use this formula when estimating how long GDP, population, investments, or other growing quantities will take to double.

Example

Annual GDP growth rate:

g = 3.5%

Formula:

Td = 70 / g

Substitution:

Td = 70 / 3.5

Td = 20 years

Result:

At a constant growth rate of 3.5% per year, GDP will approximately double in 20 years.

Applications

- Economic growth analysis

- Population projections

- Investment planning

- Long-term forecasting

- Economic education

Note

The Rule of 70 is an approximation derived from exponential growth mathematics. It is most accurate for moderate growth rates, typically between 1% and 10% per year. For higher growth rates or greater precision, the exact doubling time formula based on natural logarithms should be used.