On The Double

How long does a quantity take to double?

Our World in Data and news articles refer to the doubling time of COVID-19 confirmed cases and deaths.

This article looks at what doubling times are, and how to interpret this figure.

Doubling Times

A doubling time is the time it takes for a quantity to double in size, assuming constant growth. Researchers often calculate this figure for population growth, whether cells, animals, or humans.

To use an example, imagine there were 1,000 rabbits on a island. Their population increases by 20% every year.

How long does it take for the number of rabbits on the island to double?

After one year, there are 1,200 rabbits. We then increase that number by 20%, to get 1,440 in the second year. By the third year, that number of rabbits is 1,728. After four years, that number is 2,074. I rounded that final figure, as our rabbit population should always be a whole number.

It takes around four years for the number of rabbits to double. To be more precise, the doubling time for our rabbit population is about 3.8 years.

Cases and deaths

The Our World in Data page says the doubling time for COVID-19 confirmed deaths in Cameroon is five days. What does this mean?

On April 24th, the number of confirmed deaths was 49. Five days earlier, on April 19th, that number was 21…