About the Maximum Allowable Blood Loss Calculator
The maximum allowable blood loss (MABL) is an estimate of how much blood a patient can lose — while circulating volume is kept up with crystalloid or colloid — before the haematocrit falls to a chosen lower limit. Anaesthetists use it before and during surgery to anticipate when blood loss will become significant and to plan monitoring and blood availability.
Enter the patient’s weight and category to estimate total blood volume (EBV) from conventional mL/kg values, or enter your own mL/kg figure. Then enter the starting haematocrit and the lowest haematocrit you consider acceptable for this patient. The default method is the Gross formula, which uses the average of the starting and final haematocrit to account for progressive dilution; the simple ratio and the exponential (logarithmic) haemodilution model are shown for comparison.
MABL is a planning estimate, not a transfusion trigger. Blood volume per kilogram varies with body habitus, and the acceptable haematocrit depends on the patient’s age, cardiac reserve and the clinical situation. The result is for clinicians and does not recommend whether or when to transfuse.
With the default inputs, the maximum allowable blood loss is 1,750 mL. Change any value above to recalculate instantly.
How to use the maximum allowable blood loss calculator
- 1Enter the patient’s weight in kilograms.
- 2Choose the patient group, or enter a custom blood volume in mL/kg.
- 3Enter the starting haematocrit from the most recent blood count.
- 4Enter the lowest haematocrit you will accept for this patient.
- 5Read the MABL and compare the three formulas.
Formula and method
Estimated blood volume is weight multiplied by an age- and sex-based blood volume per kilogram (about 95 mL/kg in preterm neonates, 85 in term neonates, 80 in infants, 75 in children and adult men and 65 in adult women). These are conventional teaching values; obesity lowers the true mL/kg figure.
Gross (1983) noted that as blood is lost and replaced with fluid, each millilitre lost carries progressively fewer red cells, so he divided the haematocrit drop by the average of the starting and final haematocrit. The simple ratio uses only the starting haematocrit and slightly underestimates the allowable loss, while the logarithmic version models continuous isovolaemic dilution exactly and gives the largest figure. All three assume blood volume is maintained throughout.
- EBV
- Estimated blood volume (mL)
- Hi
- Initial (starting) haematocrit, %
- Hf
- Final (lowest acceptable) haematocrit, %
- MABL
- Maximum allowable blood loss (mL)
Worked examples
70 kg man, Hct 42% to 30%
EBV = 70 × 75 = 5,250 mL. Gross MABL = 5,250 × (42 − 30) ÷ 36 = 1,750 mL. The simple ratio gives 5,250 × 12 ÷ 42 = 1,500 mL and the log formula 5,250 × ln(42/30) ≈ 1,766 mL.
60 kg woman, Hct 38% to 25%
EBV = 60 × 65 = 3,900 mL. The average haematocrit is 31.5%, so MABL = 3,900 × 13 ÷ 31.5 ≈ 1,610 mL, about 41% of her blood volume.
8 kg infant, Hct 33% to 25%
An infant’s blood volume is about 80 mL/kg, so EBV = 640 mL. MABL = 640 × (33 − 25) ÷ 29 ≈ 177 mL — a small absolute volume, which is why careful measurement of losses matters in small children.
Frequently asked questions
What is maximum allowable blood loss?+
MABL is the estimated volume of blood a patient can lose, with circulating volume maintained by fluid replacement, before the haematocrit reaches a chosen lower limit. It helps anaesthetists anticipate when losses become significant.
What blood volume per kilogram should I use?+
Conventional estimates are 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 a lower blood volume per kilogram of total weight.
Which MABL formula is most accurate?+
All three are approximations. The Gross formula, using the average haematocrit, is the most widely taught. The simple ratio is slightly conservative and the logarithmic formula, which models continuous dilution, gives the highest estimate.
Does MABL tell me when to transfuse?+
No. MABL only estimates when a chosen haematocrit will be reached. The decision to transfuse depends on haemodynamic stability, ongoing bleeding, symptoms, comorbidities and local transfusion guidelines.
Why does the starting haematocrit matter so much?+
Patients who start anaemic have less red cell reserve, so a smaller blood loss takes them to the same lower limit. Raising the target haematocrit or starting low both shrink the allowable loss substantially.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.