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)

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    $\begingroup$ 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. $\endgroup$
    – user67090
    Aug 24 '19 at 10:15
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    $\begingroup$ 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. $\endgroup$
    – user
    Aug 24 '19 at 10:53
  • $\begingroup$ This should be possible to model just mathematically given certain start values birth and death rates. $\endgroup$ Aug 24 '19 at 11:52
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    $\begingroup$ 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. $\endgroup$ Aug 24 '19 at 12:08
  • $\begingroup$ 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. $\endgroup$
    – AlexP
    Aug 24 '19 at 14:35

Whilst I appreciate it's not exactly what you had in mind, I think the Demographic Transition Model will go a long way in helping you work out what you need relative to your own starting population. Graph of the Demographic Transition Model, a popular measure of a country's development in the real world. This is a measure of development, and reflects population growth as an area begins to put more into things like healthcare and become a civilised society. Easily avoidable deaths like infected cuts become a thing of the past and families grow in size as a developed society can now support them through organised crops, water supply, and so on. It's worth noting that the population shrinkage in stage five (called natural decline) is a result of changing attitudes to sex and people just living longer, not a societal disaster.

In terms of predicting population growth based on birth and death rates, you may as well roll the dice. Disease, your world's development and even cultural taboos like contraceptives will completely throw off your numbers. However, this does give you the freedom to define your own numbers based on your world's individual circumstances.


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