Human blood is made up of plasma, red blood cells, white blood cells, and platelets. The density of blood is only slightly more than pure water, and it makes up about 7% of a human's body weight. Therefore, the actual blood volume will depend on the size of the person, their age, their sex, their state of hydration, the volume of white blood cells and red blood cells in their blood, and their overall state of health.
An average adult who weighs between 150 and 180 pounds will contain approximately 1.2 to 1.5 gallons of blood in their body. A child weighing about 80 pounds will contain about half the amount of blood as an adult.