Okay so:
For the sake of this calculation we'll be basing vampires off of human physiology, being as generally, vampires come from humans. It doesn't really take into account magical means or anything of that sort, so take this all with a grain of salt.
Vampires have a lot of cool abilities generally, like heightened strength, speed, shapeshifting, etc. etc. I would imagine that this would be somewhat strenuous to do, so I decided to base vampiric caloric intake on an athlete, who needs anywhere from 2k-10k calories a day.
based on the research I did (sorry I wrote this all out like 2 years ago and I don't think I saved the links. If I find them I'll edit this post/put in the comments) ~1 liter of human blood is about 700 calories. Based on that, a vampire would need at least 3 liters of blood a day to meet their caloric requirements.
The average human stomach can hold ~1-1.5 liters of food/liquid. That could be pushed to 3-5 liters, however at that point things can rupture. That being said, there are some cases where people have had more, with one person having upwards of 12 liters of liquid in their stomach.
While I'm not accounting for magical means since that can vary a lot, I imagine that during the turning process, since a lot of other physical changes happen, one could presume that vampires would increase their stomach capacity. Since they're predominantly if not wholly getting by on blood, having a digestive tract that will better support those needs would make sense. Looking at the natural world, animals such as ticks and mosquitoes swell significantly after they've fed on blood.
All of that being said, perhaps they are able to pace it out throughout the night. It feels more like a case-by-case basis tbh. (Thinking of a few pop culture examples, Dracula seemed to not take a whole lot of blood at one time based on the book. At the end of the first season (I still need to read the book sorryyy) of Interview with a Vampire, Louis, Lestat, and Claudia drink a lot of blood. However they're a bit messier and can't drink it after the person dies, so who knows how much they managed to get really. Meanwhile Astarion in Baldur's Gate 3 drinks a whole bear one night, which safe to say has more than 5 liters of blood.)
So, what does this mean for humans? The average person has ~5 liters of blood in their body, give or take depending on height, weight, etc.
- At 14% (.7 liters) of blood loss, most people will be fine, but may feel weak, lightheaded, or dizzy. Most people's vitals would not be affected
- At 15% (.75 liters) and 30% (1.5 liters) blood loss, vitals will show an affect, but this likely would not result in death for most people
- At 30% (1.5 liters) blood volume lost, a person is in a bit of trouble. Without fluid replenishment and/or a blood transfusion, this amount of blood loss can lead to death.
- At 40% (2 liters) or more blood volume lost, without immediate and aggressive life-saving measures, this will usually lead to death.
To reiterate, a vampire likely will need at least 2000 calories a day, if they are not overly active. The more active they are, the more blood they'll likely need. One liter of human blood is ~700 calories. Therefore, a vampire will need approximately 3 liters of human blood a night in order to sustain themselves. At 2 liters of blood loss, most humans will need serious intervention or it will result in death.
Generally, it likely is in the vampire's best interest not to kill too many people they feed on. More missing and dead people means more questions, and more questions might lead back to them. Being that an angry mob isn't ideal for most vampires, they should try and feed from ~2-3 people a night in order to meet their caloric needs while keeping their food source alive. Vampires should also try and vary who they feed from, since it takes people 6-8 weeks to replenish their red blood cells.
Sources: (will update if I find more of my old sources!)
