Blood of most vertebrates contains hemoglobin, which contains iron. I've imagined a world where someone once had a seemingly stupid idea of using blood as a source of iron.

Now, the world is analogous to Earth pre-firearms Middle Ages (but with magic and dragons) and iron ore availability is the same too, i.e. quite enough. However, it turned out the blood iron has magic properties, which the usual metals most unfortunately lack. Actually, before this invention no one was able to enchant any mundane weapon or piece of armor, and battle magic as a whole is nonexistent. The mage-smiths who create this weapons are called "bloodforgers".

For now, the blood iron is mostly used to create swords and daggers. Its properties depend on source creature; each case needs unique spells to unlock the hidden properties. I can imagine three most important cases:

  1. Dragon blood. The dragon must be slain in honest heroic battle. The sword will be fiery and will easily hack through thickest armor. Even a frail teenager can easily split a full-plated knight with its horse in two with it, and they will be instantly roasted to "well done".
  2. Blood of innocents. Innocents should better be humans or unicorns, but puppies will suffice for weaker effect. They must be brutally killed, no mere bloodletting will do. The blade is no better than usual to cut through armor and bones. But when it touches flesh, it bites into it on itself, leaving deep wounds, and the bleeding is very hard or (if the victims were innocent enough) to stop. Favored by assassins.
  3. Wielder's blood. A dedicated warrior "donates" their blood repeatedly to create a weapon which is literally and extension of the arm. It feels the owner's intention and strikes with supernatural precision, it can even bend and become thinner to pass through the smallest holes in armor, and then it aims to the most critical organ within its reach, so it would never miss the heart or major artery by couple of inches.

The bloodforgers are trying to unlock any special properties in blood from such abundant sources as fallen warriors left on battlefields and slaughtered farm animals, but without success for now.

(Skip until here if you're not into fluff)

And now the hard part. I need to calculate how much blood is needed for a sword. No magic here, iron atoms won't pop out from nowhere.

This needs the following values:

  1. Mass fraction of iron in hemoglobin
  2. Hemoglobin content in blood (mass of hemoglobin per volume of blood)
  3. Total volume of blood in the body, or, for case 3, the amount of blood which can be donated safely per year.

Humans have the same physiology as in real world. Smallest dragons are of full-grown T. rex's size, largest - of Argentinosaurus's. Hemoglobin count in dragons is similar to large land mammals, or, more precisely, giraffes. Unicorns are just horses with a horn.

So, how many dragons or innocents do I need to slay, or can I donate enough blood in my lifetime?

  • $\begingroup$ @0celouvsky While the world is fantasy, the question itself concerns only real-world biochemistry, and I want the answer to be backed by sources. You may scroll to the bottom of the question to get straight to hard-science part (beginning with "And now the hard part"). $\endgroup$
    – Neith
    Apr 22, 2017 at 5:09
  • $\begingroup$ I'm almost sure I've seen a question about extracting iron from blood already. Sadly, can't find it now. $\endgroup$
    – Mołot
    Apr 22, 2017 at 5:48
  • 3
    $\begingroup$ That's a lot of words just to ask for a simple number that you can get via Google. $\endgroup$
    – JDługosz
    Apr 22, 2017 at 5:56
  • $\begingroup$ Welcome to Worldbuilding! And, good show in reading the tag description and flagging for hard-science as you posted. $\endgroup$
    – JDługosz
    Apr 22, 2017 at 6:00
  • 3
    $\begingroup$ I'm voting to close this question because it asks for hard-science but does not provide enough information to be answered as it described in hard-science. $\endgroup$
    – MolbOrg
    Apr 22, 2017 at 12:08

2 Answers 2


This information is a mere Google away. The top result from that mere Google tells us that blood contains around 0.5g iron per litre of blood.

Another Google tells us an adult human contains around 5 litres of blood, though obviously it depends on the size of the person. And lastly there is no clear consensus on the amount of blood you can lose per day, but a 400ml blood donation can be made every 2 or 3 months. For several years I gave blood four times a year without any obvious consequences.

According to Wikipedia, a longsword weighs 1.1kg to 1.8kg. Let's take 1.5kg as a nice middle value - assuming that you can forge the sword without wasting any iron, you're looking at 3,000 litres of blood, or all of the blood from 600 average human adults. If we're using 400 mL donations, you'd need 7,500.

  • $\begingroup$ so, how much blood he needs finally?? 100 liters? 500? $\endgroup$ Apr 22, 2017 at 5:53
  • 1
    $\begingroup$ @maxkoryukov 3000 liters, actually, it says so in the answer if you'd actually read it $\endgroup$
    – rappatic
    Apr 22, 2017 at 6:15
  • 2
    $\begingroup$ @pagie_ to be fair Ben edited that in after Max commented. $\endgroup$ Apr 22, 2017 at 6:23
  • 1
    $\begingroup$ Now for answer to be complete only estimates for blood volume in T. rex and Argentinosaurus are needed. $\endgroup$
    – Neith
    Apr 22, 2017 at 6:39
  • $\begingroup$ @pagie_, actually yes, the answer was edited (worldbuilding.stackexchange.com/revisions/78885/2) and now it is much clearer (at least for me). Now I know, that a community with more than 7500 members can forge a sword in day even being closed in crystal clean room. $\endgroup$ Apr 22, 2017 at 11:48

2788 liters of human blood for 1.5 kg of iron

  • 4
    $\begingroup$ To be more specific: this is a hard-science question, so you can't just pluck a number out of thin air. You need evidence to back up your answer. How did you come to this conclusion? You need to edit that into your answer; at the moment, it's going to get deleted. $\endgroup$
    – F1Krazy
    May 1, 2019 at 20:59

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