pCO₂ Unit Converter (mmHg to kPa)
pCO₂ Unit Converter (mmHg to kPa)
Convert arterial pCO₂ from mmHg to kPa, using the same factor as pO₂, with the type 2 respiratory failure threshold.
pCO₂ Unit Converter (mmHg to kPa)
mmHg → kPapCO₂ 40 mmHg
Formula
- 0.133322
- the same mmHg-to-kPa factor used for every partial pressure — it depends only on the definition of the atmosphere, not the gas
- reverse
- kPa × 7.50062 = mmHg
Worked example
pCO₂ 40 mmHg
40 × 0.133322 = 5.33 kPa
pCO₂ reference points
| mmHg | kPa | Meaning |
|---|---|---|
| 35 | 4.7 | Lower limit of normal |
| 40 | 5.3 | Typical healthy adult |
| 45 | 6.0 | Upper limit of normal |
| 45+ | > 6.0 | Type 2 respiratory failure threshold, with hypoxaemia |
The same factor, a different gas
The mmHg-to-kPa conversion factor is identical for every partial pressure — 0.133322 — because it comes from the definition of the atmosphere, not from any property of the gas itself. The same arithmetic that converts pO₂ converts pCO₂, and the same national divide applies: the United States reports mmHg, most of Europe reports kPa.
Normal arterial pCO₂ is 35–45 mmHg, or 4.7–6.0 kPa, and unlike pO₂ it does not drift with age — a raised or lowered pCO₂ at any age reflects alveolar ventilation, not a normal physiological trend. A pCO₂ below the range signals hyperventilation, which may be primary (anxiety, pain, sepsis, pulmonary embolism) or compensatory, as the respiratory system blows off CO₂ to correct a metabolic acidosis. A pCO₂ above the range signals hypoventilation — from sedation, neuromuscular weakness, severe airway obstruction, or exhaustion in a patient who has been compensating and can no longer sustain the extra work of breathing.
Type 2 respiratory failure is defined by a pCO₂ above 6.0 kPa (45 mmHg) occurring together with hypoxaemia — a pO₂ below 8 kPa. The pCO₂ alone does not diagnose it; the two values are always read together, alongside pH and bicarbonate to establish whether the picture is acute, chronic, or acute-on-chronic.
Frequently asked questions
How do I convert pCO₂ from mmHg to kPa?
Multiply by 0.133322 — the same factor used for pO₂. A pCO₂ of 40 mmHg is 5.33 kPa.
What is a normal pCO₂?
35–45 mmHg, or 4.7–6.0 kPa, in arterial blood. Unlike pO₂ this range does not shift with age.
What pCO₂ defines type 2 respiratory failure?
A pCO₂ above 6.0 kPa (45 mmHg) together with a pO₂ below 8 kPa (60 mmHg). The pCO₂ elevation alone, without hypoxaemia, does not meet the definition.
Why is my pCO₂ low?
Hyperventilation — either a primary drive from anxiety, pain, sepsis or pulmonary embolism, or a compensatory response as the lungs blow off CO₂ to offset a metabolic acidosis. Interpreting it needs the pH and bicarbonate alongside it.
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References
- Sasse SA, Jaffe MB, Chen PA. Arterial blood gas interpretation. In: StatPearls. StatPearls Publishing; 2024.
- BIPM. The International System of Units (SI), 9th edition — definition of the kilopascal and its relation to mmHg.