Pediatric Blood Volume Calculator (mL/kg by age and allowable blood loss)

Understanding the pediatric blood volume calculator — concept and principle

This calculator estimates a child's circulating blood volume from weight and age group, and the maximum allowable blood loss (MABL) before the haematocrit falls to a chosen lower limit. Anaesthetists and surgical teams use these figures to plan fluid and transfusion during operations.

Weight × mL per kg: with the default inputs the result is 1,305 mL (MABL 399 mL).

How it is calculated

Estimated blood volume (EBV) = weight (kg) × mL/kg for the age group (middle of the range)

MABL = EBV × (current haematocrit − lowest accepted haematocrit) ÷ current haematocrit

mL/kg ranges: preterm newborn 90–100, full-term newborn 80–90, infant 70–80, child 70–75, adolescent or adult 65–70.

The principle

Blood makes up a larger share of body weight in newborns and falls with age. Multiplying weight by a typical value per kilogram gives a working estimate. The MABL formula is a simple proportion: if blood is lost and replaced with fluid that has no red cells, the haematocrit falls in proportion to the share of blood lost. The formula estimates how much loss brings the haematocrit down to the lowest value the team accepts.

Worked example

Default inputs: child 1 year or older, 18 kg, haematocrit 36%, lowest accepted 25%.

mL/kg used = (70 + 75) ÷ 2 = 72.5.
EBV = 18 × 72.5 = 1,305 mL (range 18 × 70 = 1,260 to 18 × 75 = 1,350).
MABL = 1,305 × (36 − 25) ÷ 36 = 398.75 → 399 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)	Estimated blood volume (mL)
4.5	326
9	653
13.5	979
18	1,305
22.5	1,631
27	1,958
36	2,610

How to read the result

The MABL is a planning guide, not a strict limit. During surgery the team watches vital signs, measured haemoglobin and ongoing loss. Actual blood volume varies between children; in children with obesity it is lower per kilogram of total weight, so using total weight overestimates it.

Common mistakes

Using adult mL/kg values for infants.
Forgetting that the simple MABL formula assumes losses are replaced with fluids without red cells.
Treating MABL as a transfusion trigger on its own.

Keep in mind: This is a planning estimate for trained clinicians, not a substitute for clinical monitoring and judgement.

Frequently asked questions

How much blood does a newborn have?

About 80–90 mL per kg at term, so roughly 270–315 mL for a 3.5 kg baby.

Who sets the lowest accepted haematocrit?

The treating team, based on the child's age, condition and the operation.

Is there a more accurate formula?

Some use a logarithmic version that accounts for continuous dilution, but the simple formula is widely used.

Sources: Roseff SD, Luban NL, Manno CS, "Guidelines for assessing appropriateness of pediatric transfusion", Transfusion 2002;42(11):1398–1413 · University of Iowa Carver College of Medicine, Iowa Head and Neck Protocols, "Maximum Allowable Blood Loss" (2017)

Checked 9 Nov 2026. Iowa Head and Neck Protocols, University of Iowa Carver College of Medicine, “Maximum Allowable Blood Loss” (2017): EBV table from Roseff SD, Luban NL, Manno CS, Transfusion 2002;42(11):1398 and Strauss RG, Transfusion 2008;48(2):209, and the MABL formula.

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