Im writing a story where the main character lost a lot of blood and i was wondering how much blood would be needed for a blood transfusion.

How much loss?: Did the character lose the blood through profuse bleeding or a slow leak? If profuse sudden blood loss occurred, loss of about a liter of blood would warrant transfusion, but not necessarily of blood, it could be saline or plasma. If the hemoglobin falls below 7 gm/dL, then red cell transfusion would be warranted. For that much drop in hemoglobin, he would have to lose about half his blood.