What is the minimum amount of bleeding which may require blood transfusion?

A person may not have significant symptoms as long as the amount of bleeding is up to 15% of the circulating blood owing to the compensatory effect of the living body. When the amount of bleeding is 20% to 30%, it’s impact is significant and blood transfusion is required. It is said that hemorrhagic shock occurs in no time, when more than 30% of circulating blood is lost.