Calculate maximum allowable blood loss (MABL) from patient weight, an estimated blood volume factor and the initial and target hematocrit or hemoglobin.

Typical blood volume per kg by patient group. Values vary by source; use your protocol’s factor if it differs.

The lowest acceptable hematocrit set by your protocol. This calculator does not recommend one.

Optional settings

Allowable Blood Loss Formula

This calculator uses a simple initial-denominator ratio formula for an educational blood-loss estimate:

EBV = Weight (kg) × Selected Factor (mL / kg) ABL = EBV × (Initial Hct - Comparison Hct) / Initial Hct

Variables:

  • ABL is the blood-loss estimate from this formula (mL), not an established safe-loss limit
  • Patient Weight is the weight of the patient (kg)
  • Initial Hct is the initial hematocrit (%)
  • Comparison Hct is a supplied comparison value (%); this calculator does not set a transfusion threshold

First estimate blood volume from weight and the selected mL/kg factor. Multiply EBV by (initial minus comparison hematocrit) divided by initial hematocrit. The former factor of 70 mL/kg is one possible assumption, not a universal patient value. Hemoglobin mode uses the same fractional-decrease arithmetic with matching hemoglobin units. Reverse mode solves comparison Hct = initial Hct × (1 − loss/EBV).

What is Allowable Blood Loss?

The term allowable blood loss is used for preoperative formula estimates based on an assumed blood volume and comparison hematocrit or hemoglobin. The result does not establish how much blood a patient can safely lose or whether a transfusion is required. Clinical assessment includes active bleeding, circulation, oxygen delivery, comorbidity and response to treatment. Fluid replacement and redistribution are not modeled here. Mean-hematocrit and logarithmic dilution formulas are different models and can give different estimates.

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) as a percentage.
  3. Then, supply the comparison hematocrit selected for the relevant clinical context; this calculator does not recommend one.
  4. Select or enter the blood-volume factor and apply the formula to calculate the estimate 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; select custom EBV factor = 70 mL/kg, giving EBV = 4,900 mL.

Initial Hematocrit (Initial Hct) = 45%

Comparison Hematocrit = 30%. The estimate is 4,900 × (45 − 30) ÷ 45 = 1,633.33 mL. This is an arithmetic example, not a safe-loss or transfusion recommendation.