this was an estimation of course since the amount a blood that an adult human can be variable. blood counts like a hemoglobin or hematocrit can determine to some degree the level of anemia. by giving blood. one unit at a time. and watching the response to those tests a medical provider can estimate blood loss based on how much blood it takes to return those number to normal again. just like adding oil to a car and checking the dipstick.
how do doctors determine hom much blood you've lost. family member bleeding internally told he only has 4 1/2 pints of blood left. he rcvd a transfusion of 5 pints of blood.