Allowable Blood Loss Calculator

About this calculator

Allowable blood loss (ABL) is a bedside estimate used in the operating room to decide how much blood a patient can lose before a transfusion becomes necessary. It's calculated from the patient's estimated blood volume (EBV = weight × a volume-per-kg factor, typically around 65–70 mL/kg for adult men and women, higher for infants and neonates) and the drop from the patient's starting hematocrit to the lowest hematocrit considered safe: ABL = EBV × (Hi − Hf) / Hi (or using the average of Hi and Hf in some variants).

Anesthesiologists and surgeons calculate ABL before major operations — orthopedic, cardiac, or trauma surgery — to set a transfusion trigger ahead of time rather than reacting mid-procedure. Anesthesiology teams, surgeons, and perioperative nurses use it to plan blood product availability and decide, as suction canisters and sponges accumulate blood, when the measured loss is approaching the calculated threshold.