Enter the patient’s weight, initial hematocrit, and final hematocrit into the calculator to determine the allowable blood loss (ABL) in milliliters. This calculator helps in estimating the maximum volume of blood that can be lost without requiring a transfusion.

Allowable Blood Loss Formula

The following formula is used to calculate the allowable blood loss (ABL):

ABL = (Patient Weight * (Initial Hct - Final Hct) * 70) / Initial Hct


  • ABL is the allowable blood loss (mL)
  • Patient Weight is the weight of the patient (kg)
  • Initial Hct is the initial hematocrit (%)
  • Final Hct is the final hematocrit (%) after which a transfusion is considered

To calculate the allowable blood loss, multiply the patient’s weight by the difference between the initial and final hematocrit levels, then multiply by 70 and divide by the initial hematocrit level.

What is Allowable Blood Loss?

Allowable blood loss is the volume of blood that a patient can safely lose during surgery or a medical procedure without the need for a blood transfusion. It is based on the patient’s weight and hematocrit levels. Hematocrit is the percentage of red blood cells in the blood, and it is a key factor in determining the oxygen-carrying capacity of the blood. The ABL is an important consideration in preoperative planning and patient safety.

How to Calculate Allowable Blood Loss?

The following steps outline how to calculate the Allowable Blood Loss.

  1. First, determine the patient’s weight in kilograms.
  2. Next, determine the initial hematocrit level (Initial Hct) in percentage.
  3. Then, determine the final hematocrit level (Final Hct) in percentage.
  4. Use the formula to calculate the Allowable Blood Loss (ABL) in milliliters.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Patient Weight = 70 kg

Initial Hematocrit (Initial Hct) = 45%

Final Hematocrit (Final Hct) = 30%