# Allowable Blood Loss Calculator

URL: https://speedycalculator.com/calculators/health/blood-loss/
Last updated: 2026-10-30

> The allowable blood loss calculator estimates how much blood can be lost during surgery before a transfusion is needed, from estimated blood volume and the starting and lowest acceptable haematocrit.

## Inputs

- Patient group (mL/kg)
- Weight (kg, lb, st, g)
- Starting haematocrit (%)
- Lowest acceptable haematocrit (%)

## Results

- Estimated blood volume
- Allowable blood loss
- As a share of blood volume

## Formula

```
EBV = weight × mL/kg (75 men, 65 women, 80 infants, 70 children)
Allowable blood loss = EBV × (Hct initial − Hct target) ÷ average Hct
```

## Explanation

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

FieldWhat to enterPatient group (mL/kg)Choose one: Adult man (75), Adult woman (65), Child (70), Infant (80) …WeightUnits: kg, lb, st, gStarting haematocritIn %Lowest acceptable haematocritIn %

### 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)351,33252.51,998702,66487.53,3301053,9961405,328
*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.*

## 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.

## FAQ

**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.


## Sources

- [Gross JB. Estimating allowable blood loss: corrected for dilution. Anesthesiology, 1983](https://pubmed.ncbi.nlm.nih.gov/6614542/)