2 units of blood is usually about 260 ml. 1 unit of blood refers to the red blood cells separated from 200 ml of whole blood, which accounts for about 40% to 50% of whole blood; therefore, 1 unit of blood is between 80 and 100 ml. With the addition of other separated substances such as anticoagulants, calcium ions, and so on, the volume of 1 unit of blood is about 130 ml; 2 units of blood is generally around 260 ml. When a patient is transfused with 1 unit of blood, the patient’s hemoglobin is generally elevated to some degree. Therefore, according to the degree of anemia of the patient and the number of units of blood transfused, the degree of elevation of hemoglobin of the patient after blood transfusion can be roughly estimated. It is a common method to determine the effect of blood transfusion in patients. Before transfusion, it is necessary to carry out cross-matching, blood type test, evaluation of liver function, infection program and other tests under the guidance of professional doctors. And select the appropriate blood type for transfusion.