Scientists estimate the volume of blood in a human body to be approximately 7 percent of body weight. An average adult body with a weight of 150 to 180 pounds will contain approximately 4.7 to 5.5 liters (1.2 to 1.5 gallons) of blood. An average child with a body weight of 80 pounds will have approximately half the amount of blood as an adult. Blood carries out many critical functions in the body.