How this calculator works
Gross described an allowable-loss approximation corrected for dilution that divides the hemoglobin drop by the mean of the initial and final concentrations. The blood-volume estimate is weight times a coefficient. Where reference coefficients are ranges, this page returns a range rather than choosing an unsupported midpoint. Age groups follow the cited anaesthesia table; the newborn period and infancy have different estimates. The final hemoglobin is a clinician-selected input, not a universal transfusion threshold. Fluid replacement, ongoing haemorrhage and individual physiology can invalidate the estimate. The result is an estimate for information only and is not medical advice.
Formula
Gross approximation: allowable loss = estimated blood volume x (initial Hb - final Hb) / [(initial Hb + final Hb)/2]. Estimated blood volume = weight x mL/kg coefficient. The formula assumes maintained circulating volume during dilution.
Worked example
An adult female at 70 kg has estimated blood volume 4550 mL. A fall from Hb 12 to 9 g/dL gives 4550 x 3 / 10.5 = 1300 mL. An 80 kg adult male falling from 15 to 10 gives 6000 x 5 / 12.5 = 2400 mL.
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Frequently asked questions
Is allowable loss a safe amount to lose?
No. It is a planning estimate under assumptions. Bleeding rate, circulation, illness and clinical observations determine care.
Why might another calculator give a smaller result?
Some use initial hemoglobin as the denominator. This page explicitly uses the Gross mean-concentration approximation.
Why are infant results ranges?
Published blood-volume estimates vary by age and maturity. A range preserves that uncertainty; a clinician can enter a specific coefficient if justified.