Allowable Blood Loss Calculator

Created by Małgorzata Koperska, MD and Dominika Śmiałek, MD, PhD candidate
Reviewed by Bogna Szyk and Steven Wooding
Based on research by
Gross J. B. Estimating allowable blood loss: corrected for dilution Anesthesiology (March 1983)
Last updated: Sep 07, 2023

The allowable blood loss calculation is an essential part of the pre-surgery routine. It estimates how much blood a patient can lose before reaching a dangerously low level of hemoglobin. In this article, you can find information on what is the maximum allowable blood loss and what formula we use.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a health care provider.

How to use the allowable blood loss calculator?

  1. Choose the sex/age category of the patient – the calculator uses several different coordinates, whether they're a newborn baby or an adolescent male.

  2. Enter their weight in the preferred unit – our calculator converts it to kilograms by itself.

  3. Enter their hemoglobin (Hgb) level in g/dL.

  4. Enter the lowest allowable hemoglobin level – depending on the patient, it's usually between 7 and 10 g/dL.

  5. The allowable blood loss is calculated in mL, but you can switch to other units just by clicking on the unit.

What is the allowable blood loss formula?

Here, the blood volume is already included in the equation:

ABL = weight (kg) × age_sex_factor × [initial_hemoglobin (g/dL) - final_hemoglobin (g/dL)] / initial_hemoglobin (g/dL)

The age_sex_factor is the amount of blood per kilogram of body weight for the chosen category. It is estimated approximately as:

  • 100 mL/kg for premature infants;
  • 85 mL/kg for babies younger than 3 months;
  • 75 mL/kg for children over the age of 3 months;
  • 70 mL/kg for male adolescents;
  • 65 mL/kg for female adolescents and adults; and
  • 75 mL/kg for male adults.

What is the maximum allowable blood loss volume?

The allowable blood loss depends on the total blood volume. You can estimate it using our blood volume calculator for adults and the pediatric blood volume calculator for kids.

For example, for a relatively healthy adult male, with a hemoglobin level of 15 g/dL and 80 kg of weight, the allowable blood loss would be around 2,400 ml, whereas for a teenager with a hemoglobin level of 12 g/dL and 60 kilograms of weight, it would only be 1,050 ml.

According to Morgan and Mikhail's Clinical Anesthesiology:

“Ideally, blood loss should be replaced with crystalloid or colloid solutions to maintain intravascular volume (normovolemia) until the danger of anemia outweighs the risks of transfusion. At that point, further blood loss is replaced with transfusions of red blood cells to maintain hemoglobin concentration (or hematocrit) at that level. For most patients, that point corresponds to a hemoglobin between 7 and 10 g/dL or a hematocrit of 21-30%.”

For information about blood transfer, check out our blood donor calculator. And to discover the basics of the use of blood in forensic science, head to our angle of impact calculator.

Małgorzata Koperska, MD and Dominika Śmiałek, MD, PhD candidate
Age/sex category
Adult female
Initial hemoglobin of the patient
Lowest acceptable hemoglobin
Allowable blood loss
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