How much blood does a vampire need?
Assuming that a vampire expends as much energy as a human of similar size how much blood would they need to consume a day?
My naive estimate is 2500 calories a day, there are (apparently) about 9 calories per ounce of blood, thus they need about 7 litres a day, but I don't know enough about biology and nutrition to know if there are other issues at play (like vampires needing more or less energy then humans?, is it easier to digest blood? etc).
The operational definition of vampire for this question is a creature that used to be a human, but subsists entirely on blood. As such the internal structure is more or less similar to humans but it can be assumed that whatever changed them into a vampire optimized the digestive system for blood consumption (and muscles for strength and speed in case that's relevant).
So in general what I'm curious about is:
How much blood would a vampire need a day to have enough energy to move around?
What effect would it have on the victim (is possible/practical to withdraw non-lethal amounts from several people, or would they need to kill several times a day to stay mobile)?
This post was sourced from https://worldbuilding.stackexchange.com/q/17087. It is licensed under CC BY-SA 3.0.
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It depends on other things, like metabolism and digestion.
If you convert the blood into proteins and sugars like we do, then it isn't very efficient, however you could use blood from other sources like cattle.
But classic vampires don't feed off anything but humans.
So maybe it's not just the blood.
Sympathetic magic has always been a big thing; get a couple hairs, a toenail, some blood... If you get into that kind of power then you could be getting life/energy from the victim through the sympathetic link, instead of just digesting the blood. Otherwise there are a lot of other fluids that would be more energy dense than blood.
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