Sometimes just a dribble of fake blood is enough. But if you're putting on a play or movie with wholesale slaughter, you might need gallons of the stuff. Most fake blood recipes use ingredients that, when multiplied to the quantity of gallons, would become quite expensive. Knowing that huge amounts of blood on stage or screen probably won't be subjected to great amounts of scrutiny, we can skimp a little on the ingredients and come up with a recipe for gallons of fake blood that is economical yet convincing.
Mix the red gelatin dessert packets together with about three times as much water as called for on the box. Do your mixing in the 5-gallon bucket, and keep track of how many gelatin dessert packets you use.
Add 4 drops of red food coloring and 1 drop of green food coloring to the mix for every gelatin dessert packet used.
Stir thoroughly and add two tablespoons of flour to thicken the mixture. You can add more flour just before using, if you want a thicker consistency, but don't store it with extra flour as it may thicken too much if left to sit.