Population Growth and Lifespan

[Ares Johnson]

Lately I've been thinking about world population and one thing in particular has struck me. Is there an easy, if rough, way to figure out how a population would grow or shrink based on fertility rate and life expectancy? I've been trying to look this up, but I must not be wording my searches right.

I've seen some estimates on Earth's future population less dire than most suggesting it'll actually start shrinking within the next century and that go me wondering about similar patterns. For example, let's say people started normally living to about 120 and the fertility rate of the planet dropped below the 2.1 replacement value. How fast does the population drop?

[kch49er]

There i this http://www.metamorphosisalpha.com/ias/population.php though it doesn't account for longer lifespans. Interestingly the site links back to OA,perhapse the author will see this. I think the easiest way will be to calculate the birth rate and the mortality rate.

[Ares Johnson]

There i this http://www.metamorphosisalpha.com/ias/population.php though it doesn't account for longer lifespans. Interestingly the site links back to OA,perhapse the author will see this. I think the easiest way will be to calculate the birth rate and the mortality rate.

I've seen that before, and used it. It's figuring out what the growth rate should be I'm having trouble with. Maybe I can get a rough guess by subtracting age-related diseases and cancers from the current world death rate. That might get a similar answer to a population transitioning to a much longer life expectancy.