Blood Volume Calculator (Nadler Equation)
Estimate total blood volume with Nadler’s equations from height, weight and sex — plus plasma and red cell volume from haematocrit, allowable blood loss and the 70 mL/kg rule of thumb.
How it works
Men: BV (L) = 0.3669 × height³ (m) + 0.03219 × weight (kg) + 0.6041. Women: 0.3561 × height³ + 0.03308 × weight + 0.1833. Allowable loss = BV × (Hct − Hct min) ÷ Hct
Nadler and colleagues measured blood volume with isotope dilution in 1962 and fitted equations to height and weight that are still the standard for dosing apheresis, planning surgery and calculating transfusion needs. Height enters as a cube because blood volume scales with body size, and the weight term is small — so an obese patient has far less blood per kilogram than the 70 mL/kg rule suggests, which matters when estimating haemorrhage. Multiplying by haematocrit splits the volume into red cells and plasma, and the Gross formula estimates how much blood can be lost, with isovolaemic replacement, before the haematocrit hits a chosen floor. Children have higher volumes per kilogram (about 80 mL/kg in infants, 85–90 in neonates).
Worked example
A 175 cm, 80 kg man: 0.3669 × 5.36 + 0.03219 × 80 + 0.6041 = 5.15 L (64 mL/kg). At haematocrit 42% he could lose about 1,470 mL before reaching 30%.
Frequently asked questions
Why is the per-kilogram figure below 70?
The 70 mL/kg rule was derived from lean adults; fat is poorly vascularised, so heavier patients have less blood per kilogram. Nadler accounts for this through the height term.
Is there a version for children?
Nadler was derived in adults. For children use 80 mL/kg (infants), 75 mL/kg (children) or the Gilcher rule; neonates 85–90 mL/kg.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.