Millimeter Mercury to Centimeter Water (4 °C)
mmHg
cmH2O
Conversion History
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Quick Reference Table (Millimeter Mercury to Centimeter Water (4 °C))
| Millimeter Mercury (mmHg) | Centimeter Water (4 °C) (cmH2O) |
|---|---|
| 1 | 1.3595481 |
| 10 | 13.595481 |
| 80 | 108.76385 |
| 120 | 163.14577 |
| 760 | 1,033.2565 |
| 1,000 | 1,359.5481 |
| 3,000 | 4,078.6442 |
Results rounded to a maximum of 8 significant figures
About Millimeter Mercury (mmHg)
The millimeter of mercury (mmHg) is a conventional pressure unit based on a 1 mm column of mercury at an adopted density of 13,595.1 kg/m³, representing mercury at 0 °C, under standard gravity of 9.80665 m/s². This converter preserves the full product: 1 mmHg = 133.322387415 pascals exactly under that convention. The adopted density is a reference value, not a claim that real mercury always has precisely that density. It is the universal unit for clinical blood pressure measurement and intraocular pressure in ophthalmology. Normal blood pressure is approximately 120/80 mmHg (systolic/diastolic). The unit is also used in vacuum technology, barometry, and respiratory physiology for reporting partial pressures of oxygen and carbon dioxide in blood. It remains entrenched in clinical medicine globally despite SI adoption.
Normal human blood pressure is about 120/80 mmHg. Standard atmospheric pressure is approximately 760 mmHg.
Etymology: Derives from Evangelista Torricelli's 1643 mercury barometer experiment, in which he first measured atmospheric pressure as the height of mercury column it could support — approximately 760 mm. The unit is named after the instrument's working fluid rather than its inventor.
About Centimeter Water (4 °C) (cmH2O)
The centimeter of water at 4 °C (cmH₂O) equals 98.063754138 pascals under the fixed-density reference and standard gravity — ten times the mmH₂O unit with the same reference conditions. It is the standard pressure unit in respiratory medicine and intensive care: positive end-expiratory pressure (PEEP), peak inspiratory pressure, and continuous positive airway pressure (CPAP) for sleep apnoea are all specified in cmH₂O. Cerebrospinal fluid pressure measurements also use cmH₂O. The unit spans a clinically convenient range, covering both physiological pressures and therapeutic ventilator settings.
CPAP therapy for sleep apnoea is prescribed at 4–20 cmH₂O. Normal cerebrospinal fluid pressure is about 10–18 cmH₂O in the lying position.
Millimeter Mercury – Frequently Asked Questions
Why is blood pressure still measured in mmHg instead of kilopascals?
Clinical medicine is deeply conservative about units because misreadings kill people. Doctors, nurses, and patients worldwide have memorized "120/80 is normal" in mmHg. Converting to kPa (16.0/10.7) would require retraining millions of clinicians and rewriting every guideline. The WHO considered the switch and decided the risk of transcription errors during transition outweighed the elegance of SI compliance. So mmHg stays — likely for decades more.
What do the two numbers in a blood pressure reading actually mean?
The top number (systolic) is the peak pressure when the heart contracts and pushes blood into the arteries — typically 90–120 mmHg. The bottom number (diastolic) is the lowest pressure between beats when the heart relaxes — typically 60–80 mmHg. A reading of 140/90 mmHg or above is classified as hypertension. The gap between the two (pulse pressure) also matters: a wide gap above 60 mmHg may signal stiff arteries.
How did Torricelli's mercury barometer lead to the mmHg unit?
In 1643, Evangelista Torricelli filled a glass tube with mercury, inverted it into a dish of mercury, and watched the column drop to about 760 mm. The empty space above was the first laboratory vacuum. The height of the mercury column became the measurement of atmospheric pressure — approximately 760 mmHg at sea level. Nearly 400 years later, we still use his column height as a pressure unit in medicine and vacuum science.
What is the difference between mmHg and torr?
They closely agree, but their definitions are different. One torr is exactly 1/760 of a standard atmosphere: 101,325/760 Pa, or approximately 133.322368421 Pa. The conventional mmHg instead adopts a mercury density of 13,595.1 kg/m³ and standard gravity of 9.80665 m/s², giving exactly 133.322387415 Pa for a 1 mm column. This converter preserves that full conventional factor, so 1 mmHg converts to approximately 1.000000142466 torr. Values such as 133.3224 Pa/mmHg in conversion tables are rounded approximations, not a different reference frame. The exactness belongs to the adopted convention, not to the accuracy of a real mercury barometer. Medicine commonly uses mmHg and vacuum science uses torr; precise conversions keep the units distinct.
What mmHg values are important in medicine beyond blood pressure?
Intraocular pressure (glaucoma screening): normal is 10–21 mmHg, above 21 is suspicious. Partial pressure of oxygen in arterial blood (PaO₂): normal is 80–100 mmHg. Central venous pressure: 2–6 mmHg. Intracranial pressure: normal below 15 mmHg, dangerous above 20 mmHg. Carbon dioxide in blood (PaCO₂): 35–45 mmHg. The unit pervades clinical monitoring far beyond the blood pressure cuff.
Centimeter Water (4 °C) – Frequently Asked Questions
Why is CPAP pressure prescribed in cmH₂O and not in a more standard unit?
Respiratory medicine adopted cmH₂O because the original ventilators literally used water columns to regulate pressure — a jar of water with a submerged tube set the pressure at whatever depth the tube was immersed. A CPAP setting of 10 cmH₂O meant the air bubbled out at 10 cm depth. The unit stuck even after electronics replaced water seals, because clinicians, patients, and device manuals all speak the same scale.
What CPAP pressure in cmH₂O do most sleep apnoea patients use?
Most adults are prescribed between 6 and 14 cmH₂O, with 10 cmH₂O being a common starting point. Severe obstructive sleep apnoea may require 15–20 cmH₂O. Auto-titrating (APAP) machines vary pressure within a set range — typically 4–20 cmH₂O — adjusting breath by breath. Higher pressures are more effective at splinting the airway open but can cause discomfort and air swallowing.
How does cmH₂O relate to the ventilator pressures used in intensive care?
ICU ventilators also use cmH₂O. Positive end-expiratory pressure (PEEP) is usually set at 5–15 cmH₂O to keep alveoli open. Peak inspiratory pressure above 30–35 cmH₂O raises the risk of lung injury. Plateau pressures are monitored to stay below 30 cmH₂O. The entire field of mechanical ventilation runs on this single unit because it directly corresponds to the pressures inside the lung.
What is a normal cerebrospinal fluid pressure in cmH₂O?
Measured via lumbar puncture with the patient lying on their side, normal CSF pressure is 10–18 cmH₂O in adults. Above 25 cmH₂O suggests raised intracranial pressure — potentially from a tumor, meningitis, or hydrocephalus. Below 6 cmH₂O indicates low pressure, often from a CSF leak. Neurologists use cmH₂O rather than mmHg because spinal fluid is essentially water, making the unit a direct physical analogue.
How do you convert cmH₂O to millibars or kilopascals?
1 cmH₂O ≈ 0.981 mbar ≈ 0.0981 kPa. For bedside estimates, 1 cmH₂O ≈ 1 mbar is close enough (error under 2%). A CPAP setting of 12 cmH₂O is about 11.8 mbar or 1.18 kPa. Since respiratory equipment universally reads cmH₂O, conversion is mainly needed when interfacing with industrial instruments or when charting pressures alongside blood gas data reported in mmHg.