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In my own medieval fantasy setting, I have different countries that all have different populations. I admit that I have no idea of how many people should live in X or Y. Just using the numbers for the whole country is not really precise. I try to compare them to real medieval countries but I'm unsure of what is the best criteria to use.

Resources worth mentioning:

(they are not wrong but they have limited informations.)

Historical Statistics of the World Economy: 1-2008 AD by Angus Maddison and al.

Data from the Worldmapper by the University of Sheffield

Clarifications:

  • There is magic in the world but it's low magic. It mean people can't use it to farm. It would be a waste.
  • I'm trying to find numbers for a stable and sustainable population, not decimated, not starving or booming.This also mean that the land has been settled for quite some time.
  • About the available land: It is different everywhere and it clearly influences the population density of large area. But if we take a lot of different territories with different percentage of available land, we should have a average density. Then , we could apply a modifier depending if the land is flat or in the mountains.
  • Urbanization is another factor that will influence the density but it is not the most important.
  • Sedentary vs nomadic: I know that the population density is lower with nomadic people. There is a question about this here.

Hypothesis:

  1. The higher the temperature, the higher the population density as long as there is sufficient water to grow crops.
  2. The higher the precipitations (or the water available) the higher the population density. Past a certain threshold, precipitations stops to have an impact on the population density.

So, I was wondering if there was a way to accurately estimate the population of a country in a middle age or renaissance era ?

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  • $\begingroup$ Is there any particular reason why you need to mimic Medieval Indian metrics so closely? Because you're asking about accurately estimating the population of a country, but I assume that means the fictional country, otherwise this could be off-topic. I think I have a few things to add, I'll dig around. $\endgroup$ – mechalynx Oct 12 '14 at 10:19
  • $\begingroup$ If you want data to test out your hypothesis on there's nrcs.usda.gov/wps/portal/nrcs/detail/soils/use/worldsoils/… it's all derived so there's higher quality data elsewhere. At least you don't have to hunt for everything in the same projection though. (Note: Not too old, just for testing). $\endgroup$ – Black Oct 12 '14 at 21:04
  • $\begingroup$ India was just an example. I'm interested in India only if I can extrapolate the results to use in a fantasy world. But yea, if I want to have the demographic of Uttar Pradesh during the 11th century, I'm better to ask on History.SE. But I mostly interested in establishing a link between climate and population density. Or another variable with population density if there are any interesting studies on this. $\endgroup$ – Vincent Oct 12 '14 at 22:53
  • $\begingroup$ Possible duplicate of, but definitely related to: worldbuilding.stackexchange.com/questions/1084/… $\endgroup$ – PipperChip Oct 13 '14 at 17:33
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    $\begingroup$ Also, "countries" are a somewhat artificial concept to begin with. In a situation like you describe I believe they would be separated by natural borders, but that doesn't mean everything within those natural borders forms a single country as we think of it today. Maybe it would help you to look at this in terms of settlements which have some (likely fairly small) amount of trade with each other, and spread those out at reasonable distances in a world letting countries "just happen" from the geological features of your world, rather than designing countries first? $\endgroup$ – a CVn Oct 21 '14 at 7:24
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It really comes down to two factors: waterways and open land.

TL;DR: More waterways mean more cities and more people; more open land means more farmers and less people.

The above summary pretty much says it all, but I obviously have to expand on just where the connection lies. I think I can cite some pretty good examples to support my point, but if I've made any logical or (even worse) factual errors, I'd be obliged if anyone could point them out.


Humans like to settle where there are good natural resources nearby that can aid them. If these resources are plentiful enough, more humans will move in. The settlement becomes a village, then a town, and finally a city. What are these so-called "resources" that I've been talking about? Well, they can be a wide variety of things - open land, good sources of natural food and water, good ways for transportation, etc. One resource that combines all of these is a waterway. It could be a river, a stream, an ocean - anything you can dream up.

What are the benefits of a waterway? Well, a waterway satisfies a few of humanity's simplest needs:

  1. Food
  2. Transportation
  3. Agriculture

Food

The obvious thing that any marine environment can provide is a (fairly) good source of food. Fish, crab, lobster, eel, and a whole bunch of other delicacies. Rivers are particularly good because animals use them a lot to travel. Salmon famously use them to get upstream to spawn. Crabs may live in the shallows. And there are other animals that like to eat these aquatic animals. Otters, bears, and a whole host of other carnivores. Herbivores, too, like to come to rivers to drink. If you live in the America Northeast, just think of the venison. . .

There are, for the vegans, other options. Plants need water to live, and so if you're really out of food, you can always grab a few berries off a bush. But have your friend try them first. That could really save your life. . . Other plants, too (depending on the climate) may grow near rivers.

Transportation

Chances are, humans are going to need to go to other places outside the settlement - wars, trade, family reunions with the in-laws, etc. Waterways provide a great mode of transportation. You can't exactly use a boat in the middle of a plain, can you? If you're going downstream, you have a source of transportation that requires little effort. Upstream does require some effort (e.g. sails or rowers), but it's still an improvement over trekking miles and miles with a donkey and a cart.

Rivers and oceans easily bolster trade. There's a reason that the term "port city" is so ubiquitous. Back in the Middle Ages (and today), port cities were a dime (or shilling, rupee, guinea, yen, etc.) a dozen. Some of the bigger ones include London, Liverpool, Rotterdam, etc. More trade means a healthier economy, and more available resources.

Agriculture

Yep, waterways can help agriculture near cities. Even if land isn't directly near the river/ocean/whatever, canals can be built to help with irrigation. A farming economy an exist near an urban area, drawing people even closer to cities. This fell apart with the rise of suburbs, but the Middle Ages saw many serfs and peasants working and living near large population centers.


Open land, however, also draws people. Sure, Mesopotamia was the poster child for settlements by the water, but it wouldn't have succeeded without agriculture - which resulted from a lot of open land. Plains are helpful, as are valleys - which are often created by glaciers, which eventually melt to become rivers. Wherever it is, open land draws people. There are primary uses for it:

  1. Agriculture
  2. Grazing and raising livestock

Agriculture

Okay, back to farming. It's hard to grow corn in the Himalayas. Just think about that sentence for a while. Crops are incredibly important to a civilization, and so humans will also settle where there is room to grow food. The Three Sisters (corn, squash, and beans) were important to the indigenous people of North America. They could be grown in a variety of regions, from rocky New England to the sunny Midwest. Sure, they needed a certain climate to thrive in, but open land was another factor. Can you grow corn in the Rocky Mountains or in the eastern woodlands? I didn't think so. So open land definitely draws people.

Livestock

I hope that satisfied all the vegans out there, because they aren't going to like this next bit. The other good thing about open land is that animals like it, too. The tribes of the North American plains used it to their advantage by hunting buffalo. Later on, cattle ranchers drove out the tribes and used the land to raise cattle. Both groups were drawn by the allure of open land and the possibilities it held. Why waste your time staring at corn kernels when you can just go out and kill a buffalo?


How does this relate to you question?

I went off on quite a tangent there, and I did it to try to show how important waterways and open land are to civilizations. I emphasized them because the rest of my answer depends upon those two factors, and those two factors alone.

Calculating population

Here's the bit you have to wake up for. I'll start out by counting rural farmers and ranchers. In medieval Europe, many peasants worked as serfs, working on a lord's land. In fact, a large portion of the population lived in rural communities:

The High Middle Ages saw an expansion of population. The estimated population of Europe grew from 35 to 80 million between 1000 and 1347, although the exact causes remain unclear: improved agricultural techniques, the decline of slaveholding, a more clement climate and the lack of invasion have all been suggested. As much as 90 per cent of the European population remained rural peasants.

This suggests about 8 million people living in "suburban" areas (i.e. small towns and villages) and cities.

Let's say a lord owns $a$ acres of land. On each acre he might have $80$ serfs working on it. So to calculate the population of a region, you would simply do $$a \text { acres} \times \frac{80 \text { serfs}}{\text { acre}}$$ Let's assume that all the open land in a region is used for this type of agriculture (which is likely the case). So you can simply use the above formula to calculate the population.

What about cities? There's no easy formula for this; you'll just have to do estimation. You'll have more cities if

  1. You country has a long coastline (or any coastline at all)
  2. Your country has a lot of waterways

So western China might not have a lot of cities, while eastern China will.

I'd estimate perhaps 1.5 cities per major river, and 10 cities per length of the east coast of the United States (2,000 miles, give or take. From here,

The dimensions of the Western European cities were too small. Usually, their population numbers from 1000 up to 3-5 thousand people. Even in XIV-XV century, the cities with 20-30 thousand inhabitants were considered large. Only a few very large cities have a population of more than 80-100 thousand (Paris, Milan, Venice, Florence, Cordoba, Seville).

A region like Western Europe could, perhaps, have a half-dozen of these large cities, with perhaps 20 others of 5,000 people or more. Let's estimate a population of roughly 800,000 people in European cities during the High Middle Ages - 1% of Europe's population.


Summary

Contrary to what I had originally hypothesized, cities were not a huge part of the population of Europe; they held perhaps only a few percent of the population. Most people were rural farmers, living in densities of roughly 80 people per acre. If you know what fraction of your country is either arable land or land that can be made arable by magical means, you can figure out what the rural population is. From there, you can either use the rule of them that 90% of people were peasants, or simply sprinkle in a half-dozen 80-100,000-person cities per continent, with maybe 20 or so at 5,000 or more.

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  • $\begingroup$ how do I know how many serfs are working on each acre? $\endgroup$ – Vincent Oct 22 '14 at 19:32
  • $\begingroup$ about the cities, they are usually located along rivers but having more rivers does not give more cities. Or maybe more cities but they are smaller. $\endgroup$ – Vincent Oct 22 '14 at 19:34
  • $\begingroup$ @Vincent I would think that a river would draw people in, but I can see your logic. Rest assured, I'm working on an edit. $\endgroup$ – HDE 226868 Oct 22 '14 at 21:17
  • $\begingroup$ @Vincent I have some better numbers. You were right about cities not being a huge factor in population. $\endgroup$ – HDE 226868 Oct 22 '14 at 22:16
  • $\begingroup$ @HDE226868 I don't believe an acre is the same as a square mile. $\endgroup$ – trichoplax Oct 26 '14 at 16:10
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There are many variables involved when one seriously tries to model the cause and effect of populations, and there is no answer that can simply be expressed as something like:

population = function of (climate, food-creation-technologies[x], medicine, land available, cultural effects[y], previous-population[age][gender][fertility][z]...)

Not only is accurate population simulation very complex, but the population level in any area is caused by the actual history and its many details, starting with what the population was before the time you are interested in, what they actually do, etc. For example, looking at medieval England, there were great swings in population related to periodic famines and plagues.

There are however some interesting immediate limits such as how much food, water, and shelter is available, and how predictable that is. So, especially for a fictional world, I would say it does make a lot of sense to try various views on what the limits might be on the population, especially in terms of where they get their food, and how much that can be based on the sources and labor available.

I think a very reasonable and time-saving approach would be to go over to History.SE and ask about population demographics in a region/time you feel is similar to your setting. You might want to study what the contributing factors are, and adjust if your world is different.

On the other hand, if you are making a simulation, then of course you will be interested in more of the actual cause and effect of the various details, rather than the resulting population and a description of conditions.

Both simulation effects and historical populations are estimates. Historical estimates often have a wide range, and change as new theories or historians come and go.

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A possible answer:

Using the Köppen classification of climates, I did try to see if I could set a specific population density for each climate and I think I managed to get some numbers.

The data for each area needed to be evaluated. For example: Jiangsu, Shangdong and Uttar Pradesh all have very high densities. This is mainly because they are very flat and almost all the space is used to grow food. That gives them a 20% or 30% bonus compared to other less fortunate regions with the same climate. Having a lot of data helps to figure where the marginal values are.

My main sources are mentioned in the question and some of my other statistics include numbers form the game Victoria 2 of Paradox. The game studio made researches about the era and they try to be as accurate as possible. It's not foolproof but it's better than nothing. Using Madison numbers I see that the population was multiplied by 2 or 3 between 1500 and 1836.

Factors other than the climate to take in consideration:

  • These values are the average and suppose a good deal of fertile land but also some areas unsuitable for farming. If the area is hilly, reduce the population density but increase it if it's mostly flat.

  • These values suppose that the country has enjoyed several decades of stability to allow the population to reach a certain level. The density is relatively high but sustainable.

  • Climates that have a dry summer will have lower population density because their crops have less water to grow.

  • The population from the Victorian era is 2 to 3 times higher than the population at the end of the medieval era.

  • High urbanization increase the population density. Maybe by 20%.

  • Non sedentary lifestyle usually mean a lower population density. Most temperate and humid climates are probably inhabited by sedentary people but nomadism is very common in arid, semi-arid climates and some cold climates. There, the possibilities are more limited for agriculture and the densities are low even with farming. The actual densities are between 10 or 100 times lower, I don't know exactly.

  • Trading: if a country is wealthy enough, he can import the food form elsewhere.

The results are classified by density of population per km2 in a decreasing order:

30 to 40 : BWh but the density fall around 0,2 without water

  • My main source of information was Egypt. Around 1500, the population was about 4 million people on a area of 1 000 000 km2. Since the population only live on 6% of the land, the real density is around 66 people per km2. But it's flat and very urbanized, so I lowered the values. This climate is the hottest and therefore can give a very good farming output with sufficient water.

30 to 35 : Cwa, Cfa, and BSh if water is available but 5 without water.

  • These areas include mostly regions of central and eastern China, but also Japan and some in Europe such as Montenegro. They pretty much all have high densities and are well developed and they tend to be pretty flat too. These are subtropical climates with almost no winters. Some areas can grow crops almost all year long.

20 to 25 : Cwb, Cfb, Dwa, Dfa

  • These two bunch of climates don't have much in common. Cb climates are well documented since it's the most common European climate. Thus, I just had to figure what the average was. Belgium and Italy have higher density because they are more urbanized.

15 to 20: Am, Af, Aw

  • It is pretty much an estimation of the average. It is usually lower than that but never higher.

10 to 15: Csa, Csb, Dwb, Dfb, Dsa, Dsb

  • Cs and Db had a lot of information and I managed to find information on Turkey regarding the Dsd climate. This is the average.

5 to 10: Cwc, Cfc

  • I just have the numbers for Cfc but I extrapolated the results for Cwc.

4 to 6: BSk

  • The cold steppes are usually pretty dry. Farming is possible but most of the population will be nomadic. if the population is only nomadic, divide the population by 10 or more.

0,5 to 1: Dwc, Dfc, Dsc

  • These areas are not well suited for agriculture. The hottest parts might be acceptable for farming but the population is scattered.

0,25 to 0,5: BWk

  • The cold desert is very dry and not suitable for farming except is some rare areas because rivers are also rare.

0,01: Dwd, Dfd, Dsd and Ef (tundra)

  • Even nomads find that this is a harsh climate. Still some might live here.

0: Ef (ice cap)

  • Nobody can live here because it's always frozen.
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