Millimeter Mercury to Millimeter Water (4 °C)
mmHg
mmH2O
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 mmHg (Millimeter Mercury) → 13.595481 mmH2O (Millimeter Water (4 °C)) Just now · Rounding: Up to 8 significant figures | Recalculate this conversion using your current rounding settings. |
Quick Reference Table (Millimeter Mercury to Millimeter Water (4 °C))
| Millimeter Mercury (mmHg) | Millimeter Water (4 °C) (mmH2O) |
|---|---|
| 1 | 13.595481 |
| 10 | 135.95481 |
| 80 | 1,087.6385 |
| 120 | 1,631.4577 |
| 760 | 10,332.565 |
| 1,000 | 13,595.481 |
| 3,000 | 40,786.442 |
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 Millimeter Water (4 °C) (mmH2O)
The millimeter of water at 4 °C (mmH₂O) is the pressure exerted by a 1 mm column of pure water at 4 °C under standard gravity. Using the fixed reference density of 999.972 kg/m³, it equals 9.8063754138 pascals. It is used for very low pressure measurements where even pascals give large numbers: HVAC duct static pressures, spirometry and respiratory mechanics, building ventilation system balancing, and manometer readings in laboratory work. The 4 °C reference specifies a temperature close to water's maximum density for reproducible reference conversions.
HVAC supply duct static pressures typically range from 25 to 250 mmH₂O. A forced exhalation against resistance generates roughly 10–50 mmH₂O.
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.
Millimeter Water (4 °C) – Frequently Asked Questions
Why is HVAC duct pressure measured in millimeters of water instead of pascals?
HVAC technicians originally measured duct pressure with a simple U-tube manometer filled with water — you literally read the height difference in millimeters. One mmH₂O ≈ 9.81 Pa, so a typical 25–250 mmH₂O duct pressure range corresponds to 245–2,450 Pa. The water column scale is still used because the instruments are cheap, intuitive, and field-rugged, even though digital gauges now display the same numbers electronically.
What does the "at 4 °C" part of the unit mean?
Water is densest near 4 °C, but its density is not exactly 1 gram per cubic centimeter. Column pressure follows p = ρgh, so both density and gravity matter. This converter combines a fixed reference density of 999.972 kg/m³ with standard gravity (9.80665 m/s²), giving 98.063754138 Pa per centimeter without rounding the reference calculation. NIST publishes the shorter approximate value 98.0638 Pa/cm. The result also differs from the conventional 98.0665 Pa/cm factor that assumes a density of 1,000 kg/m³. Real manometer readings may require temperature and local-gravity corrections.
How do you read a water manometer in mmH₂O?
Connect one side of a U-tube to the duct and leave the other open to atmosphere. The water level drops on the pressurized side and rises on the open side. The total height difference in millimeters is the gauge pressure in mmH₂O. Inclined (slant) manometers amplify small readings by tilting the tube — a 10:1 slope makes each millimeter of travel represent 0.1 mmH₂O, improving resolution for filter pressure-drop testing.
What mmH₂O range indicates a clogged HVAC filter?
A clean residential furnace filter creates 12–50 mmH₂O of pressure drop. When the drop exceeds 125–250 mmH₂O (varies by manufacturer), the filter is restricting airflow enough to hurt efficiency and strain the blower motor. Commercial systems set alarms at specific mmH₂O thresholds — when the differential pressure sensor hits the limit, a "replace filter" indicator lights up on the building management system.
How does mmH₂O relate to inches of water column (inH₂O)?
For water-column units with the same temperature and gravity reference, 1 inch of water = 25.4 mmH₂O, since 1 inch = 25.4 mm. Thus 0.5 inches of water at 4 °C equals 12.7 mmH₂O at 4 °C. US HVAC specs often use inches of water gauge ("in. w.g."); European and Asian specs use mmH₂O. Check the stated reference before comparing factors: a water-column unit at 4 °C is slightly different from one at another temperature or the conventional density-based unit.