This allowable blood loss (ABL) calculator estimates the quantity of blood that can be lost during surgery based on patient weight, ABV and hematocrit. There is in depth information on the parameters and the formulas used below the form.

Initial hematocrit:
Final hematocrit:
Average blood volume (ABV) in mL/kg:

How does this allowable blood loss (ABL) calculator work?

This health tool determines the allowable blood loss based on patient data and helps medical specialists make provisions for blood loss during surgery and other procedures.

The parameters taken into account are the following:

■ Type of patient – there is a selection available from premature neonate to adult man and woman. This selection helps introduce the recommended variable for Average blood volume as explained below.

■ Weight – patient weight in either kg or lbs. The default selection is on kilograms because this is the measurement unit directly used in the ABL formula.

■ Initial hematocrit – the percentage blood composition of patient before intervention.

■ Final hematocrit – the percentage blood composition of patient expected after intervention.

■ Average blood volume – quantity of blood in mL per kg according to guidelines, for different types of patients.

■ ABVs table:

Patient Blood volume in mL/kg
Premature neonate 95
Full term neonate 85
Infant 80
Adult woman 65
Adult man 75

The first computation in the allowable blood loss (ABL) calculator is to retrieve estimated blood volume:

EBV in mL = Weight in kg x Average blood volume in mL/kg

EBV is then introduced with hematocrit values, to determine the allowable blood loss:

ABL = (EBV x (Hi-Hf))/Hi


■ Hi – initial hematocrit;

■ Hf – final hematocrit.

The equations above have been developed after the Gross formula to guide hemorrhage management, in both internal and external blood leakage. ABL is also referred to as maximum allowable blood loss as it marks the amount after which hemorrhage complications start occurring.

Blood loss and hematocrit changes

It is important to develop methods for the accurate recording of blood loss in order to control and initiate replacement therapy, often with crystalloid or colloid solutions in order to reobtain normovolemia (normal intravascular volumes).

The threshold from which IV solutions are replaced with blood transfusions is set in some guidelines (see Morgan and Mikhail 1996) at a hemoglobin between 7 and 10 mg/dL, corresponding to hematocrit between 21 and 30%.

In many clinical settings blood loss is underestimated, for instance in obstetrics, postpartum blood loss is underestimated by 30-50%.

These are the main stages and their corresponding changes in blood pressure:

■ Compensation BL: 500 – 1,000 mL with no change in BP;

■ Mild BL: 1,000 – 1,5000 mL with slight fall in BP (to 80 – 100 mmHg);

■ Moderate BL: 1,500 – 2,000 mL with marked fall in BP (to 70 – 80 mmHg);

■ Severe BL: over 2,000 mL with profound fall in BP (to 50 – 70 mmHg).

Over 2,000 mL blood loss can lead to hypotension, tachycardia and installation of shock.

In acute hemorrhage, hematocrit changes may not reflect the real extent of blood loss, for example, after 1000 mL blood loss, Ht only drops 3% in first hour.

Complete compensation of hematocrit after blood loss and subsequent transfusion occurs in 48h, while significant changes can be observed after 4 hours.

Example of a calculation

Taking the case of a female weighing 60 kg with an average blood volume of 65 mL/kg (refer to the above average blood volumes table), initial hematocrit 41% and final hematocrit 35%. The calculations are:

EBV = 60 kg x 65 mL/kg = 3,900 mL

ABL = (3,900 mL x (41-35))/41=570.7 mL (rounded to 571 mL allowable blood loss).


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3) Hahn RG. (1989) Estimating allowable blood loss with correction for variations in blood volume. Acta Anaesthesiologica Scandinavica; 33(6), 508–512.

4) Lorentz A, Gasteiger P, Osswald PM. (1987) Calculation of the allowable blood loss before transfusion with a programmable pocket calculator. Anaesthesist; 36(6):306-12.

5) Parsons SJ, Helm AT, Maclaughlin E, Bale RS (2003) The Use Of A Maximum Allowable Blood Loss Protocol To Reduce Blood Transfusion Requirements In Elective Primary Lower Limb Arthroplasty. Bone&Joint Orthopaedic Proceedings; 85-B(SUPP II), 171.

09 Jul, 2016