What this calculator does
Physiologic dead space (Bohr equation) works out share of each breath that does not take part in gas exchange. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaVD/VT = (PaCO2 − PECO2) / PaCO2 (Enghoff modification, using arterial CO2); VD = (VD/VT) × VT
The inputs explained
| Field | What to enter |
|---|---|
| Arterial PaCO₂ (mmHg) | A number, measured in mmHg. Starts at 40. |
| Mixed expired CO₂ (PECO2) (mmHg) | A number, measured in mmHg. Starts at 28. |
| Tidal volume (mL) | A number, measured in mL. Starts at 500. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with arterial paco₂
Every other input is held at the calculator’s starting values while arterial paco₂ varies. Select any row to load that scenario into the calculator.
How the answer changes with arterial paco₂
| Arterial PaCO₂ (mmHg) | Dead space fraction (VD/VT) | Physiologic dead space volume | Typical range, spontaneously breathing adult |
|---|---|---|---|
| 20 | -40.0% | -200 mL | 0.2 - 0.4 (20% - 40%) |
| 30 | 6.67% | 33 mL | 0.2 - 0.4 (20% - 40%) |
| 40 | 30.0% | 150 mL | 0.2 - 0.4 (20% - 40%) |
| 60 | 53.3% | 267 mL | 0.2 - 0.4 (20% - 40%) |
| 80 | 65.0% | 325 mL | 0.2 - 0.4 (20% - 40%) |
| 120 | 76.7% | 383 mL | 0.2 - 0.4 (20% - 40%) |