Anaesthetists calculate allowable blood loss before major surgery to plan when to transfuse. The lowest acceptable haematocrit depends on age, heart disease and the operation.
How to use the Allowable Blood Loss Calculator
- Patient group (mL/kg)
- Weight
- Starting haematocrit
- Lowest acceptable haematocrit
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter patient group, weight and the starting and lowest acceptable haematocrit.
Inputs explained
| Field | What to enter |
|---|---|
| Patient group (mL/kg) | Choose one: Adult man (75), Adult woman (65), Child (70), Infant (80) … |
| Weight | Units: kg, lb, st, g |
| Starting haematocrit | In % |
| Lowest acceptable haematocrit | In % |
What each result means
- Estimated blood volume = [Patient group (mL÷kg)]×[Weight]
- Allowable blood loss = [Estimated blood volume]×([Starting haematocrit] – [Lowest acceptable haematocrit])÷(([Starting haematocrit] + [Lowest acceptable haematocrit])÷2)
- As a share of blood volume = [Allowable blood loss]÷[Estimated blood volume]×100
Example values
How the main result changes when Weight changes and everything else stays at the example values above:
| Weight (kg) | Allowable blood loss (mL) |
|---|---|
| 35 | 1,332 |
| 52.5 | 1,998 |
| 70 | 2,664 |
| 87.5 | 3,330 |
| 105 | 3,996 |
| 140 | 5,328 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
EBV = weight × mL/kg (75 men, 65 women, 80 infants, 70 children)
Allowable blood loss = EBV × (Hct initial − Hct target) ÷ average Hct
Worked example
Inputs — Patient group (mL/kg): Adult man (75); Weight: 70 kg; Starting haematocrit: 42 %; Lowest acceptable haematocrit: 25 %.
Results — Estimated blood volume = 5,250 mL; Allowable blood loss = 2,664 mL; As a share of blood volume = 51 %.
Key terms
- ABL
- Allowable blood loss.
- EBV
- Estimated blood volume.
Frequently asked questions
What is a safe transfusion threshold?
Many patients tolerate haemoglobin down to 7 g/dL (8 with heart disease), roughly haematocrit 21–24%.
Why use average haematocrit?
Blood lost later contains fewer red cells as haematocrit falls.
Does this include fluid replacement?
It assumes blood volume is maintained with fluids.