Allowable Blood Loss Calculator (MABL from blood volume and hematocrit)

Understanding the allowable blood loss calculator — concept and principle

This calculator estimates the maximum allowable blood loss (MABL): how much blood can be lost before the haematocrit falls to a chosen lower limit. It uses body weight, a blood volume per kilogram for the patient group, and the starting and lowest acceptable haematocrit. Anaesthesia trainees, nurses and students use it for surgical planning exercises.

Blood volume × haematocrit drop — with the default inputs: 1,500 mL.

How it is calculated

Estimated blood volume EBV = weight (kg) × blood volume per kg
Volume per kg used: adult man 75, adult woman 65, ages 12–18 70, children 1–12 72.5, infants 77.5, term newborns 85, premature babies 95 mL/kg (midpoints of published ranges)
MABL = EBV × (Hct initial − Hct final) ÷ Hct initial
Share of blood volume = (Hct initial − Hct final) ÷ Hct initial × 100

The principle

If lost blood is replaced with fluid containing no red cells, blood volume stays roughly constant while red cells are diluted. The haematocrit therefore falls in proportion to the fraction of red cells lost. The simple formula assumes this linear dilution, which slightly underestimates the true allowable loss when replacement happens continuously, but it gives a practical planning figure. Blood volume per kilogram is higher in babies than adults and lower in women than men.

Worked example

Defaults: adult man (75 mL/kg), 70 kg, starting haematocrit 42%, lowest acceptable 30%.

EBV = 70 × 75 = 5,250 mL; factor used 75 mL/kg.
Fraction = (42 − 30) ÷ 42 = 28.6% of blood volume.
MABL = 5,250 × 12 ÷ 42 = 1,500 mL.

Each point is the calculator re-run with only “Body weight (kg)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Body weight (kg)	Maximum allowable blood loss (mL)
17.5	375
35	750
52.5	1,125
70	1,500
87.5	1,875
105	2,250
140	3,000

How to read the result

The figure helps the team anticipate when transfusion might be considered, but it is an estimate. Transfusion decisions also depend on how fast bleeding happens, heart function, symptoms and lab results. In obesity, blood volume per kilogram of total weight is lower, so EBV based on total weight overestimates.

Common mistakes

Using total body weight in obesity without adjustment.
Applying adult volumes to children.
Treating the final haematocrit as a fixed rule rather than a team decision.
Ignoring ongoing losses that are not measured, such as into tissues.

Keep in mind: Not medical advice. This is a planning estimate for trained clinicians; real decisions use monitoring and clinical judgement.

Frequently asked questions

What is a normal blood volume?

Roughly 75 mL/kg in adult men and 65 mL/kg in adult women, according to common anaesthesia tables.

Why does the formula use haematocrit?

It tracks how much red cell concentration falls as blood is lost and replaced with fluid.

Is the MABL a transfusion trigger?

No. It is one planning number; triggers depend on the patient and guidelines.

Sources: OpenAnesthesia (International Anesthesia Research Society), "Maximum allowable blood loss"

Checked 9 Nov 2026. OpenAnesthesia (International Anesthesia Research Society), “Maximum allowable blood loss” — MABL = EBV × (HCTi − HCTf)/HCTi and estimated blood volume table (mL/kg by age group).

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