This is based on the idea of a society where vampires are known and subsist off of 'willingly' given human blood. An average adult human needs approximately 2000 calories a day to maintain their weight, and so would a vampire. There are 900 calories per liter of blood, and about 5 liters of blood in the human body. A vampire would essentially need 2 - 2.5 liters a blood a day...or approximately 4 - 5 pints of blood. However, if donating platelets is healthier/more efficient for humans than donating whole blood then I may change the vampire's nutritional needs to be focused on needing a certain number of platelets rather than calories per liter of blood.

I know that these are the standard donation practices according the redcrossblood.org, but I can't find anything specific about how much blood you can lose every day and still be healthy.

You must wait at least eight weeks (56 days) between donations of whole blood and 16 weeks (112 days) between Power Red donations. Platelet apheresis donors may give every 7 days up to 24 times per year.

So, how much whole blood/platelets would a single human be able to give every day, consistently, without dying/becoming unhealthy?

Since the time is less for those donating platelets, I'm assuming humans can donate platelets more often than whole blood while still remaining relatively healthy. If so how much?

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    $\begingroup$ @DonQualm The bases of the world is that all humans are obligated to donate based on their income status, like a 'blood tax.' The benefits they get are debatable, just as our RL taxes are. $\endgroup$ – Teanna Schmaeh Jun 5 '19 at 17:03
  • $\begingroup$ Is this a male or female human? Depending on sex, they have more blood sometimes (males more than females according to what I have read). $\endgroup$ – A Writer Jun 5 '19 at 17:36
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    $\begingroup$ Worth noting (and unaddressed in the current answers) - even at acceptable blood donation levels, consistent donation exhausts the body's ferritin stores, potentially inducing anemia. This is why frequent donors are often requested to take a break longer than the standard eight weeks every so often. $\endgroup$ – jdunlop Jun 6 '19 at 0:11

On Blood Donation:

Okay, so according to the link below a human can lose 14% of their blood without being affected. After that, it starts to affect them.


A person has roughly 4.5 to 5.5 liters of blood.


(going off of 5 liters) So 14% of human blood equates to roughly .7 liter of blood. Which is about three cups of blood.

So, to keep a vampire alive off of the stats you have it would have to have the blood donations of roughly three humans to feed it.

Though, I am not positive if that is possible with the same humans day after day. So a Human Blood Donation Rotation (as bad as that sounds) may be what happens. Could be useful if there's really bad overpopulation (of the human race).

Though this is what I could find on blood replacement:

Your body will replace the blood volume (plasma) within 48 hours. It will take four to eight weeks for your body to completely replace the red blood cells you donated. The average adult has eight to 12 pints of blood.


On Platelet Donation:

As said earlier, donors can donate every 7 days

But Math says

One Platelet concentration can have 3033-5555 platelets. But roughly four times this amount can be derived from this original concentration. Which means 12132 to 22220 platelets can be taken from one unit donation.

One apheresis donor can give roughly 4 units of platelet concentrations.

So four units would mean roughly 48528 to 88880 platelets per donation (on a normal red cross blood donation).

So I would say divide that number by seven which produces roughly: 6932 to 12697 a day (min. to the max.). So a human could probably donate 6932 to 12697 platelets a day.

https://www.bloodworksnw.org/donate/platelets http://www.donateblood.com.au/sites/default/files/PLATELET_DONATION_Sept2016.pdf https://reference.medscape.com/drug/platelets-999506 (and my math from numbers within)

  • $\begingroup$ Ok, or it could all be stored and they are harvested from every three to seven days. It takes 72 hours for the platelets to regenerate. $\endgroup$ – A Writer Jun 5 '19 at 18:46
  • $\begingroup$ Um, okay... In what way is the calorific value of 12697 platelets sufficient to match the OP's needs, you've given us interesting info, but not answered the actual question. $\endgroup$ – A Rogue Ant. Jun 5 '19 at 19:52
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    $\begingroup$ No, the question is how much can a human give without dying... $\endgroup$ – A Writer Jun 6 '19 at 12:17

Okay. Let's take a look at the facts. Bone marrow (in a healthy adult) produces about 200 billion blood cells per day. (Yes, technically blood cells form things other than blood marrow, but it's not significant enough that we need to care for an estimate like this.) That number is what normal human produces to the point that they have a stable amount of blood within her body. For reference, when a human donates blood, typically a pint, that's 2.4 trillion. So it would take a body 12 days to produce that much. (56 days, hah.) Except, like I said, your body needs that 200 billion blood cells, because those only last a few months before dying. (So, uh, wait those 56 days.)

The real question, than, is how much extra blood that bone marrow could produce. And here's where I'm guessing. Given the proper stimulates (by which I mean biological ones, aka hormones), it's not hard to say that bone marrow can work at 200% capacity, meaning that you could give up a pint once every two weeks.

Of course, that's the amount you could do while surviving. The humans involved would probably suffer from some ill effects, like chronic headaches, dizziness, weakness, inability to perform labor for long amounts of time, shortened lifespans, increased risk of anything heart-related, and death. (Yes, death. You're giving up a pint of blood every two weeks, it's not going to safe.) 4/5 pints per day means that in 14 days you need about 65-70 pints of blood. So each vampire needs a livestock of around 80 or so odd humans, to be on the safe side.


There are 850 calories per liter of blood for women and 920 calories per liter of blood for men. Assuming the vampires are of average human height and a healthy weight (5'9" and 126-169 lb) they would need 2,266 to 2,552 calories per day to maintain their weight. That is 2.67-3 of female blood or 2.46-2.77 liters of male blood for a male vampire. A female vampire of average height and a healthy weight (about 5'4" and 109-145 lb) they would need 1,794 to 2,033 calories per day, which is 2.11-2.39 liters of female blood or 1.95-2.21 liters of male blood. There are about 4.5-5.5 liters of blood in a human body. A donor can only donate 1 pint at a time, or 0.47 liters. 4.15-6.38 people would need to donate their blood every day for every vampire.


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