Okay, say I have a castle with a single vampire living in it. They need to drink the blood of 5 humans per day to survive, but they can substitute with animal blood, or by taking small amounts from humans so they won’t die, but will need to suck more humans if they need to do so. There’s a nearby town, with a population growth rate of 10% per year, ignoring deaths caused by the vampire. If the vampire needs a population that’s stable, assuming they kill five people a day, then they’d need to kill 1826.25 people per year.

That means that the town population will need to be at least 18263 people for the vampire to sustain themselves indefinitely. Let’s put that number at exactly 19000. How long can the vampire go without dying, in this model?

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    $\begingroup$ About as it long as it takes the government running a town/small city to decide they have the capacity to ends the killing spree of a murderer with 1,827 victims annually. $\endgroup$ May 18, 2021 at 21:07
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    $\begingroup$ I'm a bit confused - you seem to have answered your own question. Given a population of 19,000, the vampire can live forever. Of course, this ignores things like age distribution (if the vampire is always taking the 5 oldest you're golden). $\endgroup$
    – JBH
    May 18, 2021 at 21:07
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    $\begingroup$ The concise answer to this question is Green, with Blue polka dots. $\endgroup$
    – PcMan
    May 18, 2021 at 21:09
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    $\begingroup$ p.s. A town with 10% growth rate per year? Where is this town? I want to be their viagra provider, he must be making millions! $\endgroup$
    – PcMan
    May 18, 2021 at 21:10
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    $\begingroup$ Don't they have a dungeon? They always have a castle with a dungeon. Visiting carnivals, snake-oil salesmen, tradespeople, the nearest prison, hospitals, tax-collectors. The vampire needs to expand their horizons or get caught and burned on the village green before even a few months have passed. $\endgroup$ May 18, 2021 at 22:19

6 Answers 6


infinitely, even at a 0% growth rate!

5 humans' worth of blood is 25 liters. 25 liters are 50 blood donations.

A typical blood donation is half a liter. Let's assume one donation per 12 weeks to be on the entirely safe side.

50 blood donors per day times 12 weeks a 7 days = 4200 people could maintain the vampire indefinitely.

So we don't need to get every citizen to donate blood every 12 weeks: assume only a third of the people can donate due to age (too young and too old are the other 2/3) and we have a 150% of the people we actually need to feed our vampire forever available! not enough to get him a wife, but every one of them only needs to donate half a liter of blood once every 126 days (about 4 months). That's a tiny price for the protection of an immortal!

The town doesn't even need to grow but can stagnate - as long as it always has at least 4200 viable adult donors.

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    $\begingroup$ After 50y of donations, the people of this region probably know the vampire fairly well (Discworld's town of Don'tgonearthe and the local castle), and there's a decent chance they're some kind of legal arbiter after 200y, especially if they have "abilities". They've also had plenty of time to practice music (the pipe organ?), carpentry, painting, weaving, metalwork, masonry... so they possibly can open a school for polymaths, and be well-regarded (sought after) over the entire area. $\endgroup$
    – Anon
    May 20, 2021 at 0:14


As long as your vampire doesn't mind eating old people. The people who matter is the adults, who are the breeding stock and the young who are the next generation of breeding stock. The elderly are past breeding age thus getting eaten won't affect future population growth.

Alternately they can also eat males as a man could have multiple wives so not as many men are required.

It's no different to what a farmer does. You have your breeding females and stud males and older stock and excess males get culled.

  • $\begingroup$ Yeah, the vampire could get meals from euthanizing the terminally ill, and helping the suicidal end their lives. Or, they could suck fresh corpses, and drink unused blood donations. $\endgroup$ May 20, 2021 at 4:18

Frame challenge: vampires can fast almost indefinitely

It is worth noting that vampires are famously reclusive and can sleep for years if left undisturbed. Vampires in older stories are often introduced to the audience while emerging from one of these deep sleeps.

So, your vampire always has the option of withdrawing from the world if his predations threaten the viability of his food supply. That, and lying low to avoid detection by vampire hunters, are two of the biggest reasons vampires go quiet for long stretches.

Few other monsters have the ability to "hibernate" for generations, and I think this ability is a key driver of why vampires are generally considered arch-villains with something like true political power in the monster world: a werewolf has no choice but to announce his existence to the locals every full moon, but a vampire can always hunker down for as long as it takes for passions to quiet down and memories to fade, only to emerge rested, refreshed, and a little wiser about handling prey -- while his latest antagonists are now old, decrepit, and likely discredited by society after years of fruitless vampire hunts.

I think none of this is relevant to the modern reinterpretation of the vampire, which is essentially an emo superhero whose goals usually don't contemplate much beyond the next semester of classes. (And that's not a knock against Twilight -- or not just -- it was true even in the '80s.)

The classic vampire is a sophisticated predator that is characteristically hard to detect, harder to corner, and hardest to truly eradicate, because the classic vampire is essentially a normal human who has been forced by their circumstances to dedicate their entire existence to relentless serial murder. You can trust they will use all their guile and cunning to avoid detection and continue killing.

Consider that every vampire starts as an innocent human who is attacked by a monster. The consequences of that attack are hard to truly imagine. Spoiler: you can't just bandage yourself up and get a rabies shot.

  1. As a vampire, your soul is forever damned, so you face eternal suffering when you expire, which is the greatest possible incentive there could be to avoid death. (By contrast, a human has a chance at paradise.) This is your life now.
  2. At the same time, avoiding death has become far more complicated for you:
    • You have a new basic need that society won't tolerate: fresh blood pumped by a dying victim's still-beating heart. There is no app for that.
    • You've also taken on a bunch of fatal new allergies: sunlight, holy water & crosses, garlic, and those are just the pedestrian things that can kill you instantly. Avoiding these things -- which most people love -- constantly threatens to reveal your nature.
  3. If anyone ever gets wind of your existence, you can expect them to form a mob to hunt you down and punch your ticket to Hell. People who knew your victims may dedicate their lives to murdering you. And it is the official policy of the Church -- one of the most powerful forces on Earth -- to exterminate you.

Being turned into a vampire is the single worst thing that can happen to anyone, short of personally earning the wrath of almighty God. As Cicero famously said:

Lorem ipsum dolor sit amet There is no one who loves pain itself, who seeks after it and wants to have it, simply because it is pain...

With that kind of motivation, you can expect every vampire that survives its first year to be an infinitely desperate, and thus formidable, opponent.


Depending on what is a vampire in your story.

For example traditional vampire was considered undead, turning into the vampire after death. Such vampire needed not the blood, but a lifeforce "youthness" that was supposedly transferred through blood to keep not-living essentially forever. Such vampire needed young victims so blood donations, elders, people dead or dying and animal blood will either not work at all or just barely.

Also, assuming we are talking about middle ages, a city with a population of 19000 would be one of the biggest cities around - a metropolis like f.e. Paris. Also 10% yearly growth rate would be unheard of. Typical growth rate of medieval cities oscillated between 0-1%. 10% growth would mean that around 50% females in proper age give birth each year while infant and child mortality is very low (modern low) - at least 40% of population would be children and 10% elders, assuming vampire is not eating all elders. This mean that females in proper age (15-35 max in medieval society) would consist of no more than 20% of the total city population. Some would die giving birth (1-2%+ chance for each pregnancy on average and much more in fact for the first and each after fifth or so in typical medieval setting without proper healthcare). Some would be infertile or will have infertile husbands, others still would not have children for cultural reasons (f.e. nuns).

BDW if it is such a huge city, I do not see why they would not just go and terminate your vampire. It do not have army to protect it's castle so it wold need really awesome magical powers to stop 8000 strong angry mob...

  • $\begingroup$ I think your "zero" result, which appears to flow from a quibble over word choice, is preventing this otherwise good answer from getting more upvotes. I suspect that by "living," OP means something like "remain active and independent." It's not very satisfying to let the dead/undead distinction short-circuit the meat of the question. And I think it adds nothing to your interesting population math (which I bet is exactly the kind of info OP was hoping to find). $\endgroup$
    – Tom
    May 20, 2021 at 23:13
  • $\begingroup$ Thanks for suggestion, edited the answer $\endgroup$
    – Archelaos
    May 21, 2021 at 9:33

Not very long at all I'm afraid

You've tagged biology - so I'm going to answer in terms of biology.

Although your vampire can drink blood - is this really enough to sustain a life form? In particular an animal - an organism that eats, metabolises, reproduces and moves.

There are various limits placed on our own lifespan that unfortunately we cannot escape from. These can be 'bent' with technology, medicine and supplements, but in the end time will claim us. These factors include:

  • Cell division: All biological cells must divide (or at least transform) and have a life cycle of their own. The Hayflick Limit is a statistical limit based on reproduced chromosomes not being complete every cycle. This is both a negative and a positive: Negative in that eventually a cell cannot reproduce (hence we age, our skin wrinkles and brain cells die) but positive in that cells are replaced regularly, preventing damaged cells from continuing. Your vampire, if he does not have a Hayflick limit, will have a body riddled with errors, defunct cells, and possible cancers and tumours. This limit varies, but for us is generally around 80 to 100 years.
  • Evolution: How did your vampire evolve? All life is based on evolutionary forces, ie. incremental changes that yield benefits over many generations. Death is actually a mechanism of renewal for genes over a period of time. Most life is therefore self-limiting, an immortal species will stop evolving and will be replaced/eaten/superceded by one that does and thus is not likely to have occurred in the first place. There are small exceptions, but all macro-organisms are subject to the same evolutionary pressures. That's why you don't see animals generally age beyond 200 years - any longer and generations are just too long to compete with 'neighbouring' organisms over time.
  • Nutrients: Not withstanding the ready supply of blood, unfortunately blood itself only has a limited number of nutrients. There are major deficiencies in your vampires diet. Although he may have some protein and perhaps salt, he would sorely be lacking in carbohydrates and sugars, fibre and various vitamins, including Vitamin C. Vitamin C is an absolutely essential nutrient in various enzymes and without it, your Vampire will succumb to Scurvy within 3 months, a terrible disease. We are all told 'Eat your vegetables' when young, good advice for us, and unfortunately fatally unreachable for your vampire.
  • Life: There are various factors that will reduce our lifespan too outside of health. 'Being hit by a bus' is a saying we use to describe inevitable events outside our control. Cancer, heart disease, infection all have the ability to fatally affect us, or impact our health to allow other fatal impacts. Even if your vampire lies all day every day in his coffin, he is surrounded by bacterial and viral phages causing infection, and impacted by radiation causing cancer or immune disorders. This is the main limiter of human lifespans and why life expectancy can be from 21 years of age (middle ages) to 80 years (modern age), regardless of the above limits.

So even if your blood supply is secured according to math, your vampire with these biological limits wouldn't last long, at best case 200 years, worst case a few months.

  • $\begingroup$ The vampire has a healing factor that keeps them at peak condition as long as they have enough nutrition, and is unable to get cancer, as their immune system immediately destroys defective cells. And the vampire can survive solely on blood, and is strong enough for an lifespan that lasts centuries. $\endgroup$ May 20, 2021 at 4:22
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    $\begingroup$ Medieval live expectancy was NOT 20 years. It was average lifespan, dragged down by massive infant and child mortality. $\endgroup$
    – Archelaos
    May 20, 2021 at 6:11

10% growth rate per year is incredibly high... every seven years the population is doubled... in 50 years at a 10% growth rate 100 people become 11'739 plus one pregnant woman.

the world will eventually die out if the growth rate is never ending, so your vampire will die because there are too many people


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