There are a few staples of the horror movie genre that are essentially requirements today. You'll need characters to kill, some sort of unstoppable force to kill them, and gallons of fake blood to cover the bodies. Somebody has taken on the task of attempting to calculate just how much fake blood has been used by horror movies over the years and has actually come up with a number. In this case, that number is 619,920 liters of fake blood.
The number comes from Kynan Eng, who has some experience with research as well as dealing with big data, so we can assume at least a basic level of competency at doing the math. As Eng explains in his piece for Slate, he did not sit down and watch ever horror movie ever made, such a task would be impossible. The only way to come up with a number is to estimate and make assumptions based on available data. Here's a brief summary of how the numbers broke down.
First off, everything is based on the existence of 17,712 horror movies, which is the current number of listings that Amazon has on DVD. We're assuming that all horror movies have been released on DVD and that they are all available from Amazon. Then he makes the assumption that each horror movie scene that requires blood uses five liters per scene, based on the fact that the stuff can be purchased in a standard five-liter jug, fitting since that's about the average amount of blood in an adult human body, give or take. Then, he's using a standard average of five death scenes per movie with two scenes that require blood as set dressing. Seven scenes, at five liters per scene, multiplied by 17, 712 films, gets us to our magic number of 619,920 liters.
There are, of course, a few issues with this particular breakdown of the numbers. Kynan Eng himself admits that his use of Amazon's list alone has the potential to exclude many foreign movies that may not be available through that particular store. There's also the assumption that every horror movie has been put on DVD. There have almost certainly been some films that have been lost to time and never published that way, though Eng feels those are being taken into account in some way. The amount of fake blood used in older movies was much less than it is now, so those additions would either be too minor, or their being accounted for with the current averages. Also, we're assuming that Amazon classifies Psycho as a horror movie and they famously didn't use fake blood at all, they famously used chocolate syrup.
This number certainly isn't exactly right, coming up with that number would be impossible. However, we can't really think of another way to run the numbers that could possibly get us a number we'd feel is more accurate. This is probably as close as we'd ever get. If you've got a better idea, put your math in the comments and see where you come out. Is the real number a lot more? Let us know what you think.