How much is enough? Is there ever enough?
A thirst for blood is arguably THE defining trait of a vampire. They drink it to exist and helpful humans are readily available; what varies from story to story is the actual need.
The first consideration is what the blood is for. In a modern twist, vampires may be portrayed as biological, needing blood due to an inability to manufacturer their own or requiring some essential element that only living blood contains. In such cases, the vampire may be susceptible to blood diseases or the effects of substances such as drugs or alcohol. For the more traditional “mystical” type, “the blood is the life,” allowing the vampire to literally take the life force of the living into themselves to empower an animated corpse.
In either case, how much is enough? How long does it last? How often must the vampire feed?
On average, a human adult may have up to ten pints of blood. A single pint can be safely donated, but lose three or more and the body’s ability to survive is compromised. If a particular vampire requires this and has only a single donor, someone’s going to die.
A vampire in control of themselves might consume more blood than necessary if they can, essentially storing it up to go longer between feedings, but what’s the no-frenzy minimum? A 200-pound living human arguably needs 100 ounces of water a day or around 6 pints. Assuming a vampire requires only as much blood per day as a human needs water, they might kill one or two people every day or feed sparsely enough to let three people live – and that’s at a minimum.
Using these guestimated numbers, a so-called conscientious vampire might not have to kill, but one would certainly need a stable of no less than nine adult donors to exist for three days without killing in addition to allowing time for donors to eat and rest before another bleeding. Those numbers increase significantly assuming a vampire takes damage or uses up blood to heal, so add a few more just in case. With such small numbers, it doesn’t make much sense for a local vampire lord setting down roots in a community to kill off the locals unless they fully intend to move along when there’s nothing left – just a thought.
Whether you’re reading vampire stories or writing them, consider the numbers. Do they all add up? If not, someone could have done more research or there may be room to improve your detail.
Don’t let your vampires go thirsty.