Blood Loss Calculator — Estimated Haemorrhage & ATLS Class
Estimate how much blood has been lost based on the drop in haemoglobin concentration. Enter patient sex, weight, and the pre- and post-event haemoglobin values to get the estimated blood loss in mL, percentage of blood volume, and ATLS haemorrhage classification.
Sex
kg
g/dL
g/dL
Based on haemoglobin drop and estimated blood volume (Gross formula)
- 1
EBV factor for sex
75 mL per kg body weight - 2
Estimated blood volume (EBV)
70 × 75 = 5,250 - 3
Haemoglobin drop
14 − 10 = 4 - 4
Estimated blood loss
5,250 × 4 ÷ 14 = 1,500
How does this calculator work?
Estimated blood loss = EBV × (Hb_initial − Hb_current) / Hb_initial, where EBV = weight × 75 mL/kg (male) or 65 mL/kg (female). The result is classified into ATLS Classes I–IV by percentage of blood volume lost (Class I ≤15%, Class IV >40%). Best used before significant IV fluid resuscitation.
Formula
How this is calculated
The Gross formula estimates blood loss from the fall in haemoglobin concentration. First, the estimated blood volume (EBV) is calculated from body weight using accepted population averages: approximately 75 mL/kg for adult males, 65 mL/kg for adult females, and 80 mL/kg for children. These figures reflect average red-cell mass per kilogram and were established in the anaesthesia literature.
The blood loss in mL is then EBV × (Hb_initial − Hb_current) / Hb_initial. This assumes haemoglobin concentration changes proportionally to red-cell mass lost — a reasonable approximation when fluid resuscitation is minimal, but haemodynamic resuscitation dilutes haemoglobin and leads to underestimation of true blood loss.
The result is classified using the Advanced Trauma Life Support (ATLS) haemorrhage schema: Class I (up to 15% blood volume lost, compensated), Class II (15–30%, tachycardia likely), Class III (30–40%, hypotension, confusion), and Class IV (>40%, immediately life-threatening). These thresholds guide clinical response; however, ATLS 10th edition has moved toward individual patient assessment over rigid class boundaries. This calculator is an educational estimate — clinical decisions must incorporate vital signs, coagulation status, and clinical context.
Frequently asked questions
Men typically have a higher proportion of lean body mass and circulating red-cell volume per kilogram than women, giving an average EBV of ~75 mL/kg versus ~65 mL/kg. Children have a higher metabolic rate and a relatively larger circulating volume, hence ~80 mL/kg.
Yes. IV fluids dilute the remaining blood, lowering haemoglobin more than the blood loss alone would — causing the formula to overestimate actual red-cell loss. The Gross formula is most accurate before significant resuscitation begins. For fluid-resuscitated patients, cumulative surgical suction volumes, swab weights, and drain output are used alongside haemoglobin to estimate true loss.
Advanced Trauma Life Support (ATLS, American College of Surgeons) classifies haemorrhagic shock into four classes based on volume of blood lost and clinical signs. Class I (≤15%) is usually compensated with no treatment; Class IV (>40%) requires immediate blood product transfusion and likely surgical intervention.
TG we-Calculate Editorial Team. (2026). Blood Loss Calculator — Estimated Haemorrhage & ATLS Class [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/blood-loss-calculator
TG we-Calculate Editorial Team. "Blood Loss Calculator — Estimated Haemorrhage & ATLS Class." TG we-Calculate. 2026. https://we-calculate.com/calculator/blood-loss-calculator.
TG we-Calculate Editorial Team, "Blood Loss Calculator — Estimated Haemorrhage & ATLS Class," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/blood-loss-calculator
@misc{wecalculate_blood_loss_calculator, title = {Blood Loss Calculator — Estimated Haemorrhage & ATLS Class}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/blood-loss-calculator}}, year = {2026}, note = {TG we-Calculate} }
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