Allowable Blood Loss Calculator — Maximum ABL Before Transfusion
Calculate the maximum allowable blood loss (ABL) before transfusion from weight, patient type (estimated blood volume), starting and lowest acceptable hematocrit — Gross formula, simple and logarithmic methods.
How it works
EBV = weight (kg) × blood volume factor (mL/kg): adult man 75, adult woman 65, child 75, infant 80, full-term neonate 85, preterm neonate 95. Gross: ABL = EBV × (Hi − Hf) / H_avg, H_avg = (Hi + Hf)/2. Simple: ABL = EBV × (Hi − Hf) / Hi. Logarithmic (isovolaemic haemodilution): ABL = EBV × ln(Hi / Hf).
The allowable blood loss estimates how much blood can be lost — with volume replaced by crystalloid or colloid — before the hematocrit falls to a chosen transfusion threshold. Gross’s formula (Anesthesiology 1983) uses the average of the starting and final hematocrit; the logarithmic form models continuous dilution and gives a similar result, while the simple ratio underestimates it. Blood volume factors are textbook averages (Morgan & Mikhail), so the result is a planning guide, not a transfusion trigger.
Worked example
An adult man weighing 70 kg has an estimated blood volume of 70 × 75 = 5,250 mL. Going from a hematocrit of 40% to 30%: ABL = 5,250 × (40 − 30) / 35 = 1,500 mL (Gross). The simple method gives 1,313 mL and the logarithmic method 1,510 mL.
Sources: Calculation methods for intraoperative blood loss: a literature review (BMC Surgery 2024) · StatPearls — Physiology, Blood Volume
Frequently asked questions
What blood volume per kg should I use?
Common anaesthesia references use about 95 mL/kg for preterm neonates, 85 for term neonates, 80 for infants, 75 for children and adult men and 65 for adult women. Obese patients have less blood volume per kg of total weight.
What is the lowest acceptable hematocrit?
It depends on the patient. Restrictive transfusion strategies typically use a haemoglobin of about 7 g/dL (hematocrit around 21%) in stable adults, and 8 g/dL in cardiac surgery or with cardiovascular disease.
Can I use haemoglobin instead of hematocrit?
Yes. The formulas use a ratio, so any consistent measure works — for example a haemoglobin of 13 g/dL falling to 9 g/dL.
Does ABL mean I should transfuse at that point?
No. It is an estimate for planning. Transfusion decisions also depend on ongoing bleeding, haemodynamics, oxygen delivery and repeated laboratory measurements.
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For trained clinicians only. Always confirm doses and measurements against the product label, local protocol and the full clinical picture — this calculator does not replace professional judgement.