5 LITERS. An average person have around 4.5 to 5.7 liters of blood in the circulatory system.The amount of blood in the human BODY is generally equivalent to 7 percent of body weight.Babies : Babies born full-term have about 75 MILLILITERS (ML) of blood per kilogram of their body weight.Children : The average 80-pound child will have about 2,650 mL of blood in their body, or 0.7 gallons.Adults : The average adult weighing 150 to 180 pounds should have about 1.2 to 1.5 gallons of blood in their body, which is about 4,500 to 5,700 mL.