Broad Brushstrokes
Here's some really rough numbers you can use to quickly create demographics with the feel of mediaeval England (~1300ish). The objections that could be raised are almost infinite but they should be close enough for horseshoes or hand-grenades. Obviously, the idea is to start with "model" numbers and then adjust specifics to suit a campaign, or an area within a campaign.
Acres of farmland = population x 1⅔
Urban population = population / 10
Non-farming population = population / 4
Clerical population = population / 100
Number of adults (16+) = population / 3
Number of single adult homes = number of adults / 10.
Number of couple homes = number of adults x 0.45.
Forest for fuel = number of couple homes x 6 acres. (If you start with an area of land, this means that the number of people supported will be acres/2.567)
Largest city = population / 100 (or urban population / 10)
nth largest city = largest city / n
Each acre of farmland produces 12 bushels of "grain", of which 3 (25%) support non-farming population somewhere.
20bu of grain support one person for a year.
Working backwards, each acre of land requires 0.6 head of population (man, woman, and child) or 0.45 head of rural population if you prefer, meaning that each head of rural population is producing ~26⅔ bushels per year. Obviously, in the specific case of adult males, much more than this can be worked and none of these values will hold when dealing with small numbers of population.
Large Self-Contained Model Village
Population 300, 225 in farmer families, 75 non-farmers.
75 farming adults: 8 single dwellings, 34 couples with 150 children (4.4 each).
25 non-farming adults: 3 single, 11 couples with 50 children (4.5 each).
500ac of farmland, 270ac of woodland available for fuel. So just over 1.2 sq-mile, plus space for water, waste areas, and meadow. This easily fits into a classic Judges' Guild 5-mile hex which is 20 sq-miles or 13856 acres.
Village produces 6000bu of grain per year. If non-farmers were removed, 1,500bu could be sold per year, granting a per adult income of 20 times the value of a bushel of grain (so, if 1bu=1sp, that's 1gp per year, for example).
In reality, unless there were special reasons otherwise, the majority of the non-farmers would not be in the village, they would be in towns, or serving the local lord, living in monasteries, in the army, etc.
There would be some non-farmers in a village this size, however. Widows and widowers and the "village idiot", millars, bakers, coopers, carpenters, teamsters and maybe even a full-time aletender, as well as a scattering of odd-ball tinkers and similar passers-through. Brewing would be done privately and about half the homes would be doing some and the brewing itself was traditionally the responsibility of the wife (very few single people brewed) who would be called by the female form of "brewer" - "brewster".
There are possible scenarios where the bulk of the listed non-farmers could actually be "on-site" as it were. For example, a village near a salt mine could have a substantial industry of families who extract, pack, transport and sell the salt and who pay the farming families for their food out of their income.
Alternative food sources, such as fishing, are something I'll look at in another post.
Large Model Country
Population: 4,000,000
Adults: 1m
Couple homes: 450,000
Acres of farmland: 6⅔ million acres (10417 sq miles).
Acres of forest: 2.7m (4219sq mi).
Farming population: 3m
Non farming: 1m
Clergy: 40,000
Urban population: 400,000
Largest City population: 40,000, consuming 2192bu (54t) of grain per day.
Next largest cities: 20,000; 13,300 ; 10,000 ; 8000 etc.
This represents about 465 6-mile hexes, or 19 30-mile hexes assuming 100% human land usage. Waste and marginal land will probably double or triple that.
Of the adult population, 13,333 are capable of level advancement. The "natural" level for the clergy would be 400, but if people were able to spell cast, then I imagine that they would be accepted in preference to anyone else. Given the ease of becoming a cleric then roughly 8170 would be eligible (assuming NPCs are 3d6 in order) and perhaps we can knock that down to 5,000 due to alignment requirements so that ⅛th of the clerical population would be spellcasters.
That would leave 8,333 adults capable of class advancement, some of whom may be clerics of other deities than the mainstream Church's.
Of the original 13,333 adults 7774 would qualify as fighters; 8,170 as magic users, clerics, or thieves; 853 as assassins; 383 as druids; 50 as illusionists; 50 as monks; 27 as bards; 21 as rangers; and just 13 as paladins. Obviously, many would qualify for more than one class.
Population Density
If you plod around the web looking for information on mediaeval demographics and economics to use in your fantasy roleplaying game (and, honestly, who doesn't?) you'll see figures for the population density of mediaeval England of around 120/mi² whereas the figures here give a much higher density of about 380/mi².
If ye be a foreigner you might not be aware that there has always been a large difference in population density across England (and still moreso over the British Isles generally), with the north and southwest being much less heavily populated than the south and east. This difference was enhanced by the events of the Norman Conquest (the "harrying of the North"), boosted by Scottish raiders, and remain with us today.
So, at the height of the middle ages, England did indeed have a population density overall of 120/m², but in places like Northumberland and Cumbria it was much lower and in places like Suffolk and Cambridgeshire it was a lot higher, much higher than the figures given here which are likewise based on "average" farmland; better and worse land will exist and rural densities of 500/mi² are perfectly plausible if you're just looking at 6-mile hexes rather than whole countries.
So, don't worry too much about high population densities even before adding in fantasy elements that might allow it to go even higher.
Children
Lot of children in those villages, aren't there? It's hard to get good estimates on what is a realistic number but my feeling is that there would be more than is usually expected. The families generated by the numbers above are a "snapshot" and the kids will be all sorts of ages and many of them - maybe 1 in 4, certainly 1 in 5 - will not live long enough to even start helping around the farm. Consequently, the average age of all people in the village will be skewed heavily towards the low end.
However, you might want to divide population by 2½ to find the number of adults.
Grain
I've abstracted grain crops into bushels of 34.83% wheat, 42.7% barley, and 22.47% oats giving ~60500 kcal per bushel.
The question of barley is a vexed one as the calorific value of this crop is largely tied up with the details of ale/beer making, with Bruce Campbell claiming that 70% of the value was wasted in the 1300's. This value seems to me to be very unlikely indeed and certainly by the 1600's it was down to ~25%. Given the size of the barley crop, the difference is very significant in terms of the land's ability to support population.
In particular, Campbell assumes that only one wort was produced in the brewing. I suspect that no brewer ever only used one wort, at least this side of the bronze age. Very early brewing was done in large troughs and it would be the most natural thing in the world to try to get more value out of the trough before having to go to the bother of clearing it out. But solid evidence from before 1400 is very thin.
Animals
Animals are abstracted away at this level. Their calorific input into the population is ultimately derived from either feed, which is included in the grain figures, or pasture, which is not.
Productivity and The Plague
Productivity per
person seems to have increased after the Black Death. There's a few reasons for this. For one, the survivors could pick and choose the best land to farm, abandoning the poorer land and thus increasing the actual yield per acre. For another, survivors were often able to drive much harder bargains with their employers/landlords and so able to gain more from their efforts, thus encouraging them to work harder. Probably another aspect was the opportunity to re-jig the way any given farm was run, with fewer people to object and complain about changes. Technology did not change but, at least for a while, the survivors had more cash to invest in replacements for old tools and so on.
However, the jump does also speak of a degree of under-employment in the pre-plague period and so these figures could well be altered in favour of lower numbers of workers and higher numbers of "off-farm" people without invoking any fantasy elements.
