How Many Men Would You Need to Kill to Forge a Sword from the Iron in their Blood?

The average man has 4 grams of iron in his blood.

According to Wikipedia, the average British longsword was between 1.1 and 1.8 kg. We'll use 1.45, the median value.

Also according to Wikipedia, the carbon content of steel is anywhere between .002% and 2.1%. Averaged, the carbon percentage of steel is 1.051%, though I doubt the percentage was anything approaching consistent (if anyone has better numbers for that please share). So 1.45kg - (1.45kg * 1.051%) = 1.4347605kg of iron in the average longsword.

At .004kg of iron in the average man, and assuming complete iron extraction from each corpse, forging a sword from blood-iron would have taken 358.69, or 359 dead men (far fewer than I expected, frankly). 

TL;DR: at 359 humans, it's one damn expensive sword to make. Original text