# What is Doubling Time and How is it Calculated?

*This article will explore the concept of doubling time and explain how one can calculate the **doubling time** for a population growing exponentially using the rule of 70.*

# What is Doubling Time?

**Doubling time** is the amount of time it takes for a given quantity to double in size or value at a constant growth rate. We can find the doubling time for a population undergoing exponential growth by using the **Rule of 70**. To do this, we divide 70 by the growth rate (r).

*Note:** growth rate (r) must be entered as a **percentage **and not a **decimal **fraction**. For example 5% must be entered as 5 instead of 0.05.*

**dt = 70/r**

For example, a population with a 2% annual growth would have a doubling time of 35 years.

**35 = 70/2**

# Key Properties of Doubling Time

- The larger the
**rate of growth**(r), the faster the doubling time. - Rate of growth varies considerably among organisms. For example, most small bodied organisms grow faster and have larger rates of population increase than larger organisms.
*Think about the difference in growth rate between bacteria and elephants.* - Most populations cannot double forever. Resistance factors like natural resource constraints and disease contribute to a leveling off in population size over time. When this happens, we say the population has reached its carrying capacity. This type of growth is also referred to as
**logistic growth**.

# Resources for Teaching Students about Doubling Time

**Double Trouble**: A secondary activity (grades 9–12) exploring the concepts of exponential growth and doubling time. Students observe and collect data on the exponential growth of yeast cultures in both a lab experiment and under a microscope, graphing their findings and comparing their results with human population growth.