This post was lifted from Reddit

So, in my current D&D campaign, we as players were awarded a minor barony for our services to the realm. It's been an amazing source of story hooks, been used as a starting point for a spin-off campaign at lower levels, and so on. I would definitely recommend giving a quest reward like this to other DMs!

However, having a territory of land does raise some questions. Like: How much is it worth? What kind of revenue does it generate? What does it cost to defend? Granted; many DMs will choose to simply assign "close-enough" values to this, and this is of course perfectly fine.

I decided to do the math.

First off, a typical one-square-mile overland hex of farmland is composed of 640 acres.

After a few internet searches on historical farm size, I learned that an average peasant family lived on about 10 acres of land, going as low as 6 acres for very poor families, 3 acres for bachelors, and as much as 16 acres for very wealthy families. Since D&D likes round numbers in base 10, I rounded this off to poor farmers living on 5 acres of land, wealthy farmers on 15, and the average family on 10 acres.

Next, I had to figure out how much money these farms produced. Rather than look up historical productivity and prices of wheat, I simply went to the PHB. A Poor standard of living is 20 cp/day, and an Average standard of living (for Adventurers) is 1 gp/day. Since it made sense to me that a wealthy peasant might have a similar standard of living to a low-level adventurer, I simply set the Poor and Average living standards as my benchmarks and multiplied by an 'average' 5-person family to get:

A Small (5-acre) farm produces 1 gp / day, with all 5 family members living at a Poor standard of living. Picture a family with perhaps a single plow animal, using all of their land to grow food and eating most of it.

An Average (10-acre) farm produces 2.5 gp / day, probably with 2 family members (likely the adults) living at an Average standard of living and 3 family members living at a Poor standard of living (the children); or perhaps all 5 family members living on a new 5 sp/day standard of living (call it Humble.) This family may have enough land to pasture a milk-cow and a few chickens, and can grow enough extra food to have some left over for sale.

A Large (15-acre) farm produces 5 gp / day, with all 5 family members living at an Average standard of living. This family may have enough land to grow cash-crops, or to pasture multiple cows or sheep. Coincidentally, 15 acres is also a unit of land known as an oxgang, or the amount of land that a single ox can plow in one season. Since only the wealthiest of peasants owned oxen, this is another common sense check.

If you assume that there are slightly more Small farms (and thus Poor peasants) than Large farms (and Wealthy peasants), we can derive an average consumption of 2.5 gp/day/10 acres. Multiplied by 640 acres, we get a total consumption of 160 gp / day (by 64 families).

How much would this then be worth to the owner of said square mile of land? Well, historical tax rates were about 10%, so a gross income of 16 gp / day seems reasonable. Of course, the owner then pays 10% taxes to their liege lord (16 sp), and pays an equal amount in tithe (16 sp); and because this is D&D you're going to have to pay for some guards for those peasants (10 guardsmen at 1 gp / day each - roughly 3% of the total population under arms, which some google searches show was fairly typical for peacetime) and the village will probably want a mayor (2 gp / day for a Comfortable standard of living, or a nice supplement to his family income raising them from Humble to Average if he also has an average-sized farm).

At 8 sp / day left over, this comes out to a net income of roughly 300 gp / year in perpetuity. Using some financial math, we know that a perpetual income stream of K dollars per year is equal to K * (1 / i), where i is the annual interest rate. Since medieval interest rates were very high, using i = 30% implies that a square mile of farmland is worth roughly 1000 gp, which is very convenient for people who like round numbers.

This also gives us some sustainability numbers for things like castles and keeps. Since a typical castle costs 100 gp / day in upkeep, that implies that it takes about 12.5 square miles of farmland to support one person at a Noble standard of living (10 gp / day), 62.5 square miles of farmland to afford a fort or a Noble family of 5 (50 gp / day), 125 square miles for a keep, and so on.

Of course, not all land is arable - a google search of arable land in modern European countries shows that only 25% of Spain is currently being farmed, and only 33% of France. Even granting that modern agricultural techniques have reduced our reliance on farmland to a certain extent, it would not be a stretch to assume that for every 5 square miles of farmland, a medieval country might have 10 square miles of land in total, with the remaining 5 being given over to forests, rivers, mountains, cities, and other terrain features.

Thus if we assume that an average Baron maintains a Keep (on the Borderlands), a Family, a Fort, a small army (say, 400 men), and a few high-level retainers (5 people plus their families at 'wealthy' standards of living), he probably needs about 2000 square miles of land ( 1000 square miles of farmland) to do so. This is an area roughly the size of Delaware. At a value of 100,000 gp, this might be a suitable reward for a medium-level party, especially if there was buried mineral wealth or potential for trade for players to be able to invest their efforts into in order to generate future profits.

Of course, not every Barony is created equal. Territories with significant mining wealth, artisanry, aquaculture, trade, or other sources of high population-density income or food generation could be much smaller than 2000 sq. mi. Similarly, areas with low population density, or that rely on migratory food production and herding rather than agriculture might need to be much bigger to generate a similar amount of wealth. And of course, if spells like plant growth are commonly available they could create more modern standards of agriculture and allow for much more densely packed populations of people (until a BBEG steals the Holy Plot Device and prevents divine blessings from being granted, anyways...), but 'a spot of land roughly the size of Delaware' seems to be a good benchmark for what it takes to support a Castle with a mostly agrarian fiefdom in the absence of higher-level magic.

Thoughts? Comments? Problems with my assumptions? Let me know!

So there's that.