There are about 6 × 109 people in the world. Thus, the total volume of blood is
V = 5 L/person × 6 × 109 people = 3 × 1010 L
There are 1000 L in a cubic meter, so that V = 3 × 107 m3 . Now let’s see how large a volume this is.
If we want to get biblical, we can compare it to the volume of blood shed at the battle of Armageddon as mentioned in the book of Revelation: “They were trampled in the winepress outside the city, and blood ﬂowed out of the press, rising as high as the horses’ bridles for a distance of 1,600 stadia” Rev. 14:20 (NIV). A horse’s bridle is about 2 m high. At almost 200 m per stadion, 1600 stadia is 300 km. Now we just need the width. That much liquid will spread out a lot, especially when ﬂowing 300 km. Let’s use a width of 3 km. Thus, the volume of blood predicted to ﬂow at Armageddon is
VArmageddon = 2 m × 3 × 105 m × 3 × 103 m = 2 × 109 m3
That is about 15 times more blood than humans currently have. We guess we just need more people.
So, Armaggedon is predicted to happen when the population will reach a minimum of
PArmageddon = 2 × 1011 L / 5 L = 4 × 1010
However, according to the moderate UN projection, this will never happen, because the population is expected to level off at about 1 × 1010 around 2050.
On the other hand, considering the high estimate (an increase of about 4 billion people every 50 years), we’ll have to wait for Armageddon for only another 412 years :)