Pressure Unit Converter for Lab and Clinical Use
Pressure Unit Converter for Lab and Clinical Use
Convert mmHg, kPa, cmH2O, atmospheres, bar and psi through the pascal, with the exact definitions and the blood gas unit split that makes the number meaningless on its own.
Pressure converter
Any unit → mmHg100 kPa, converted to mmHg through the pascal
Formula and the exact definitions
1 mmHg = 133.322387415 Pa exactly · 1 cmH₂O = 98.0665 Pa exactly
1 atm = 101,325 Pa exactly = 760 mmHg exactly · 1 bar = 100,000 Pa
1 psi = 6,894.757293 Pa · 1 kPa = 7.50062 mmHg
- Pa
- the pascal, the SI unit of pressure: one newton per square metre. Every other unit here is defined as a fixed number of pascals
- mmHg
- defined by convention rather than by an actual mercury column, at 133.322387415 Pa exactly, so that it no longer depends on local gravity or on the temperature of the mercury
- cmH₂O
- 98.0665 Pa exactly, fixed by the same convention using water of density 1 g/cm³ and standard gravity of 9.80665 m/s². The convention for airway and CSF pressures
- torr
- one 760th of a standard atmosphere. It differs from the mmHg by about one part in seven million and the two are used interchangeably
Worked example
100 kPa, converted to mmHg through the pascal
100 × 1,000 = 100,000 Pa, since a kilopascal is 1,000 Pa exactly
100,000 ÷ 133.322387415 = 750.06 mmHg
Equivalently, 1 kPa = 7.50062 mmHg, so 100 × 7.50062 = 750.06 mmHg
100 kPa is exactly 1 bar, and just under one standard atmosphere of 101.325 kPa or 760 mmHg
The same factor applied to a blood gas: a PaO₂ of 10 kPa is 75.0 mmHg, and a PaCO₂ of 5.3 kPa is 39.8 mmHg
One of each unit, expressed in the others
| 1 unit of | Pa | mmHg | kPa | cmH₂O |
|---|---|---|---|---|
| mmHg (torr) | 133.322 | 1 | 0.13332 | 1.3595 |
| kPa | 1,000 | 7.5006 | 1 | 10.197 |
| cmH₂O | 98.0665 | 0.73556 | 0.098067 | 1 |
| atm | 101,325 | 760 | 101.325 | 1,033.2 |
| bar | 100,000 | 750.06 | 100 | 1,019.7 |
| psi | 6,894.76 | 51.715 | 6.8948 | 70.307 |
Where each unit is actually used
| Measurement | Conventional unit | The same value in the other unit |
|---|---|---|
| Arterial oxygen tension, normal on room air | 10.6–13.3 kPa or 80–100 mmHg | The two are the same range in different units |
| Arterial carbon dioxide tension, normal | 4.7–6.0 kPa or 35–45 mmHg | A PaCO₂ of 5 means nothing without its unit |
| Systolic blood pressure 120 | mmHg, everywhere | 16.0 kPa — kPa is essentially never used for blood pressure |
| CSF opening pressure 10–25 | cmH₂O | 7.4–18.4 mmHg, or 0.98–2.45 kPa |
| Positive end-expiratory pressure 5 | cmH₂O | 3.7 mmHg, or 0.49 kPa |
| One standard atmosphere | 101.325 kPa | 760 mmHg, 1,033 cmH₂O, 14.70 psi |
Why the unit matters more than the number
Blood gas results are reported in kilopascals across most of Europe and in millimetres of mercury in the United States and much of Asia, and the two conventions produce numbers that overlap dangerously. The factor between them is 7.50062, small enough that the two ranges nearly touch: an arterial oxygen tension of 10 is unremarkable in kPa, where it corresponds to 75 mmHg, and incompatible with life in mmHg, where it is 1.3 kPa. The same is true at the other end, where a carbon dioxide tension of 5 is normal in kPa and profoundly low in mmHg. Nothing in the figure itself signals which convention produced it, so a blood gas value transcribed without its unit is not a result but a hazard.
The definitions underneath are exact, which is a comfort when checking arithmetic. The pascal is the SI unit, one newton per square metre, and every other unit here is fixed as a defined number of pascals rather than measured against a physical column. A millimetre of mercury is 133.322387415 Pa by convention, so it no longer depends on local gravity or on the temperature of the mercury. A standard atmosphere is 101,325 Pa exactly, which makes it 760 mmHg exactly. Converting through the pascal, as this calculator does, keeps those exact relationships intact rather than chaining approximate factors together.
Centimetres of water survive because the pressures they describe are small and the unit gives them convenient magnitudes. Airway pressures, ventilator settings, positive end-expiratory pressure and central venous pressure are conventionally in cmH₂O, as is cerebrospinal fluid opening pressure, where the normal adult range of roughly 10 to 25 cmH₂O becomes an awkward 7 to 18 mmHg. The unit is fixed at 98.0665 Pa using water of density 1 g/cm³ and standard gravity, so it too is exact by definition rather than dependent on the temperature of the water in a real manometer.
Torr and millimetres of mercury can be treated as the same unit. The torr is defined as one 760th of a standard atmosphere and the mmHg by its pascal value, and the two differ by about one part in seven million — far below the resolution of any instrument in a clinical laboratory. Bar and pounds per square inch belong to the engineering side of the laboratory, on gas cylinders, regulators and autoclaves, and there the question to ask is not the conversion factor but whether the reading is absolute or gauge pressure, since a gauge reads zero at one atmosphere.
Frequently asked questions
How do I convert kPa to mmHg?
Multiply by 7.50062, since 1 kPa is 1,000 Pa and 1 mmHg is 133.322387415 Pa. A PaO2 of 13.3 kPa is 100 mmHg, and a PaCO2 of 5.3 kPa is 40 mmHg.
Is a PaO2 of 10 normal?
In kilopascals, yes — 10 kPa is about 75 mmHg. In millimetres of mercury it would be 1.3 kPa, which is incompatible with life. The unit carries more information than the number, so blood gas values should never be transcribed without it.
Why are airway and CSF pressures in cmH2O?
Because the pressures involved are small and centimetres of water give them convenient whole numbers. A CSF opening pressure of 10 to 25 cmH2O is only 7 to 18 mmHg, and ventilator pressures such as a PEEP of 5 cmH2O would be under 4 mmHg.
Are torr and mmHg the same thing?
For every practical purpose, yes. The torr is one 760th of a standard atmosphere and the mmHg is defined at 133.322387415 Pa; they differ by roughly one part in seven million, which no clinical instrument can resolve.
Why convert through pascals rather than directly?
Because the pascal is the SI unit and every other pressure unit is defined as an exact number of pascals. Going through it uses one exact definition in each direction instead of chaining rounded conversion factors together.
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References
- Bureau International des Poids et Mesures. The International System of Units (SI Brochure). 9th ed. — the pascal, and non-SI units accepted for use with the SI.
- Thompson A, Taylor BN. Guide for the Use of the International System of Units (SI). NIST Special Publication 811 — exact conversion factors for mmHg, atmosphere, bar and psi.
- Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements. CLSI guideline C46.