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Allowable Blood Loss Calculator (Maximum Allowable Blood Loss)
Allowable blood loss (ABL), also called maximum allowable blood loss, is calculated as estimated blood volume × (starting hematocrit − lowest acceptable hematocrit) ÷ starting hematocrit. With an EBV of 4,500 mL, a starting hematocrit of 40 and a lower limit of 30, ABL = 4,500 × 10 ÷ 40 = 1,125 mL. The calculator also shows the average-hematocrit variant.
Prepared by Prospyr · Reviewed October 3, 2026 · Free, no sign-up, runs in your browser
Use the hematocrit your own protocol names as the lower limit. This tool does not suggest one.
Allowable blood loss (starting Hct)
1,200mL
Using EBV 4,800 mL (entered directly). That is 25% of the estimated blood volume.
The other formula gives 1,371 mL.
Decision support only; it does not replace clinical judgment. This is arithmetic on the numbers you enter. It does not set a hematocrit threshold and is not a transfusion recommendation.
Allowable blood loss formula and the mean-hematocrit variant
Two forms are in use. Both start from the same three inputs: EBV, starting hematocrit (Hi) and the final or lowest acceptable hematocrit (Hf).
- Starting-hematocrit form: ABL = EBV × (Hi − Hf) ÷ Hi. This is the form in anesthesia teaching references (as MABL = EBV × (H0 − H1) ÷ H0).
- Mean-hematocrit form: ABL = EBV × (Hi − Hf) ÷ Hmean, where Hmean = (Hi + Hf) ÷ 2. This is the common way of writing the dilution-corrected estimate associated with Gross (1983).
The first is the default. The mean-hematocrit form always returns a larger number. The tool shows the other result under the main one so the difference is visible.
Worked example: maximum allowable blood loss
Patient with an estimated blood volume of 4,500 mL, starting hematocrit 40%, and a lowest acceptable hematocrit of 30% set by the team's protocol.
- Starting form: 4,500 × (40 − 30) ÷ 40 = 4,500 × 0.25 = 1,125 mL.
- Mean-hematocrit form: Hmean = 35. 4,500 × 10 ÷ 35 = 1,286 mL.
Another case: a 10 kg infant with EBV 750 mL (75 mL/kg), Hi 35, Hf 25: 750 × 10 ÷ 35 = 214 mL by the starting form.
Where the EBV comes from
ABL is only as good as the EBV that goes in. Use the estimated blood volume calculator for Nadler and mL/kg values, or choose either method inside this tool. The mL/kg route uses the published ranges and takes the midpoint.
These are all estimates: real blood volume varies with body composition, and the formulas assume bleeding is replaced with fluid so that hematocrit falls steadily.
For aesthetic and plastic surgery practices
Practices that do surgery keep a pre-operative worksheet. This tool produces the ABL figure with the formula and inputs written out, ready to print. The numbers inform a plan the team and anesthesia provider own; the tool offers no thresholds.
Prospyr's plastic surgery practice software keeps these worksheets with the chart, and charting holds the notes.
Frequently asked questions
What is the formula for allowable blood loss?
ABL = EBV × (Hi − Hf) ÷ Hi, where EBV is estimated blood volume, Hi is the starting hematocrit and Hf is the lowest hematocrit your protocol accepts. Pediatric anesthesia references write the same formula as MABL = EBV × (H0 − H1) ÷ H0. This calculator uses it by default.
What is the difference between starting and average hematocrit?
The simple formula divides by the starting hematocrit. The mean-hematocrit variant divides by the average of the starting and final values, ((Hi + Hf) ÷ 2), which gives a larger number because blood is being diluted as it is replaced. With EBV 4,500 mL, Hi 40 and Hf 30, the two are 1,125 mL and 1,286 mL. Say which you used in the record.
How do I find the estimated blood volume to use?
Use your protocol's EBV method. The calculator can take a number you already have, or calculate one with Nadler's equations for adults or a published mL/kg range for neonates, infants, children and adults. See the estimated blood volume calculator for both methods in detail.
What hematocrit should I enter as the lowest acceptable?
That is the threshold your own protocol or the treating provider sets, and this tool does not suggest one. The 30 percent in the default example is only a placeholder so the page shows a result. Replace it with your number.
Is the ABL a transfusion trigger?
This calculator does not make transfusion recommendations. It is arithmetic: how many mL of loss correspond to a drop from one hematocrit to another, given an EBV. Whether and when to transfuse is a clinical decision for the treating provider.
Can I use hemoglobin instead of hematocrit?
The formula works the same with hemoglobin values, as long as the starting and final values use the same unit. This tool is labeled for hematocrit, so convert your inputs first if your protocol states hemoglobin.
Sources and scope
- Pediatric perioperative fluid management. Saudi J Anaesth. 2021 (PMC8477784)
Peer-reviewed. States MABL = EBV × (H0 − H1)/H0 and lists blood volume by age group (mL/kg).
- Children are not little adults: blood transfusion in children with burn injury. Burns Trauma. 2017 (PMC5557478)
Peer-reviewed. Gives the same formula: MABL = [(Hct start − Hct target) ÷ Hct start] × EBV.
- Gross JB. Estimating allowable blood loss: corrected for dilution. Anesthesiology. 1983;58(3):277-280. PMID 6829965
Original paper on the dilution-corrected estimate. The mean-hematocrit form used here is the common way the correction is written; check the paper if you need the exact derivation.
- Nadler SB, Hidalgo JH, Bloch T. Prediction of blood volume in normal human adults. Surgery. 1962;51(2):224-232. PMID 21936146
Source for the Nadler EBV option.
Decision support only; it does not replace clinical judgment. This tool does the arithmetic on numbers you enter. It does not set a hematocrit threshold or recommend transfusion.