# Dead Space Calculator (Bohr–Enghoff)

URL: https://speedycalculator.com/calculators/health/dead-space/
Last updated: 2026-10-30

> The dead space calculator estimates the fraction of each breath that does not take part in gas exchange (VD/VT) from arterial and mixed expired CO₂, with the Bohr–Enghoff equation, and the dead space volume.

## Inputs

- PaCO₂ (mmHg)
- Mixed expired PCO₂ (mmHg)
- Tidal volume (mL, L, fl oz)

## Results

- Dead space fraction (VD/VT)
- Dead space volume
- Alveolar volume per breath

## Formula

```
VD/VT = (PaCO₂ − PECO₂) ÷ PaCO₂
Dead space volume = VD/VT × tidal volume
```

## Explanation

Normally about 30% of each breath stays in the airways. Dead space rises in pulmonary embolism, emphysema, ARDS and with high ventilator pressures; a high dead space fraction in ARDS predicts higher mortality.

### How to use the Dead Space Calculator

- **PaCO₂**
- **Mixed expired PCO₂**
- **Tidal volume**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter arterial PaCO₂, mixed expired PCO₂ (from volumetric capnography or a collected sample) and tidal volume.

### Inputs explained

FieldWhat to enterPaCO₂In mmHgMixed expired PCO₂In mmHgTidal volumeUnits: mL, L, fl oz

### What each result means

- **Dead space fraction (VD/VT)** = ([PaCO₂] - [Mixed expired PCO₂])÷[PaCO₂]×100
- **Dead space volume** = [Dead space fraction (VD÷VT)]÷100×[Tidal volume]
- **Alveolar volume per breath** = [Tidal volume] - [Dead space volume]

### Example values

How the main result changes when **PaCO₂** changes and everything else stays at the example values above:

PaCO₂ (mmHg)Dead space fraction (VD/VT) (%)22.5-2433.7517453856.255067.5599069
*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 — PaCO₂: 45 mmHg; Mixed expired PCO₂: 28 mmHg; Tidal volume: 450 mL.

Results — **Dead space fraction (VD/VT) = 38 %**; Dead space volume = 170 mL; Alveolar volume per breath = 280 mL.

## Key terms

- **VD/VT**: Dead space to tidal volume ratio.

## FAQ

**What is a normal dead space fraction?**
About 0.2–0.35 in healthy adults.

**What raises dead space?**
Pulmonary embolism, emphysema, ARDS, low cardiac output and high PEEP.

**What is the difference between Bohr and Enghoff?**
Bohr uses alveolar PCO₂; Enghoff substitutes arterial PaCO₂.


## Sources

- [Nuckton TJ et al. Pulmonary dead-space fraction as a risk factor for death in ARDS. N Engl J Med, 2002](https://pubmed.ncbi.nlm.nih.gov/11973365/)