# How many gallons of human blood are there?

For this we need to know how many people there are and the average amount of blood in each one. Unfortunately, I will be assuming that all humans are adult sized with an adult amount of blood, which I’m fairly sure is not the case, so the estimated value will be higher than the actual value by a fairly substantial amount.
Now then. As of January 3rd, 2014, the date which this was written, the Worldometers world population clock [1] put the total world population at about 7.204 billion people.
As for blood, hypertextbook [2] cites several scientific studies which have found that the amount of blood in the average adult is right around 5 liters (or about 5 quarts). This means that the average person has about 1.25 gallons of blood.
By multiplying the two results we find that the total amount of human blood is right around 9 billion gallons.
Using data from a 2010 Pew poll of world religions [3] we find that there are 2.8 billion gallons of Christian blood, 2.1 billion gallons of Muslim blood, 1.4 gallons of Hindu blood and only 18 million gallons of Jewish blood. (These numbers don’t add up to the total because of other religions and the 1.5 gallons of unaffiliated blood)