Two units of blood are 2 units of red blood cells, which is about 300 milliliters of blood. Generally, one unit of blood is the red blood cells in blood separated from 200 ml of whole blood, and one unit of blood is about 150 ml, so two units of blood is usually about 300 ml. This calculation is often used to calculate the amount of blood that needs to be transfused for the treatment of patients with iron deficiency anemia, aplastic anemia, acute leukemia, and other hematologic disorders that are complicated by anemia. In clinical practice, the degree to which a patient’s hemoglobin is supposed to rise after transfusion is calculated based on the degree of anemia and the number of units of blood to be transfused. It should be noted that before transfusion, blood type and cross-matching tests should be performed under the supervision of a doctor, and liver function tests should be evaluated to select the appropriate blood type for transfusion.