Estimate plasma and blood volumes from weight and hematocrit.
Total blood volume ≈ 70 mL/kg (men) or 65 mL/kg (women). Plasma volume = TBV × (1 − Hct). Red cell volume = TBV × Hct. Plasma makes up ~55% of blood volume. These estimates guide fluid resuscitation, transfusion planning, and drug dosing calculations. Obesity may reduce the mL/kg estimate.