# Graphs for understanding population dynamics - where to find?

I'm trying to figure out a world in my story (eg. whether dialects of a language would be mutually intelligible, how often would designer babies be born compared to 'usual' babies, usual family composition given lack of stigma towards single parents / same-sex couples) and the answers often come back related to time and population / population dynamics.

Essentially, I'd love it if there was a website where I could input stuff like...birth rate, death rate, initial population, average age at death etc and get some graphs so I could understand, for example, how many babies need to be born per year in order to sustain my population (initially I'm aiming for a population of 50 000 to 100 000, with a lifespan of 150 to 200 years, but I'd like to fiddle around with the parameters to see what I get).

Is there such a website you could recommend? (or Excel Sheet etc)

• Most national census bureaus publish an aggregated and anonymized version of their census data online as well as certain statistical analyses, so you can easily get some data to fiddle around with. At the same time, the correlation between births, deaths, average life span, family structures and so on is a very complex one and involves hundreds, if not thousands, of factors that are not part of that data and whose effect on the statistics is unknown (e.g. influence of health care cost or taxes on sugared foods). So you will not be able to find reliable predictions for what you need. – user67090 Aug 24 at 10:15
• Regarding birth rates to sustain a population, you might want to look at What is a reasonable amount of population growth for 900 years? Full disclosure: The highest voted answer is my own. – a CVn Aug 24 at 10:53
• This should be possible to model just mathematically given certain start values birth and death rates. – World Peace Aug 24 at 11:52
• By "sustain my population" do you mean that it remains the same over a long period of time? If so, all you need is BirthRate = DeathRate = Population/LifeExpectancy. In your example, 100000/200 = 500. Your society would need to average 500 births and deaths each year. – Ray Butterworth Aug 24 at 12:08
• If your population has N people and the average lifespan is Y years, then, on the average and assuming that no people emigrate out, N / Y people will die each year. If less than N / Y new people come in (from births or from immigration) then the population will shrink; if more then N / Y new people come in, then the population will expand. – AlexP Aug 24 at 14:35