Millimeter Water (4 °C) to Inch Mercury
mmH2O
inHg
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 mmH2O (Millimeter Water (4 °C)) → 0.002895821 inHg (Inch Mercury) Just now · Rounding: Up to 8 significant figures | Recalculate this conversion using your current rounding settings. |
Quick Reference Table (Millimeter Water (4 °C) to Inch Mercury)
| Millimeter Water (4 °C) (mmH2O) | Inch Mercury (inHg) |
|---|---|
| 1 | 0.002895821 |
| 10 | 0.02895821 |
| 25 | 0.072395526 |
| 100 | 0.2895821 |
| 250 | 0.72395526 |
| 1,000 | 2.895821 |
| 10,332 | 29.919623 |
Results rounded to a maximum of 8 significant figures
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.
About Inch Mercury (inHg)
The inch of mercury (inHg) is a conventional pressure unit based on a 1-inch column of mercury at 32 °F (0 °C) under standard gravity, using the same adopted mercury density as mmHg. This converter preserves the full conventional factor: 133.322387415 Pa/mmHg × 25.4 mm/in = 3,386.388640341 Pa/inHg. Thus 1 inHg equals exactly 25.4 mmHg under the shared convention. Neither factor is rounded to a conversion-table approximation; the exactness describes the adopted reference, not the physical accuracy of a mercury-column measurement. It is the standard unit for atmospheric pressure and altimeter settings in US aviation and meteorology. Weather forecasts in the US report barometric pressure in inHg; aircraft altimeters in the US are set to inHg, with standard sea-level pressure at approximately 29.921 inHg. HVAC refrigeration technicians also use inHg for vacuum measurements below atmospheric pressure.
Standard sea-level atmospheric pressure is approximately 29.921 inHg. A major hurricane may lower barometric pressure below 27 inHg.
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.
Inch Mercury – Frequently Asked Questions
Why do US weather reports give barometric pressure in inches of mercury?
The US National Weather Service inherited the convention from early American meteorology, which used mercury barometers calibrated in inches. A typical sea-level reading of 29.92 inHg is easy to remember and fits weather maps without decimal clutter. Most other countries switched to millibars or hectopascals, but the US stuck with inHg for the same reason it kept Fahrenheit — familiarity and institutional inertia.
What is the altimeter setting that pilots hear in US aviation?
US air traffic controllers broadcast the local barometric pressure in inches of mercury — for example, "altimeter two niner niner two" means 29.92 inHg. Pilots dial this into their altimeter so the instrument reads correct altitude above sea level. If the setting is wrong by just 0.1 inHg, the altimeter reads roughly 100 feet off — enough to matter during instrument approaches in fog.
What inHg reading counts as "low pressure" versus "high pressure"?
At sea level, 29.92 inHg is standard. Readings above 30.20 inHg are high-pressure (clear skies, calm winds). Below 29.50 inHg is considered low pressure and often signals approaching storms. The lowest sea-level pressure ever recorded was Typhoon Tip in 1979 at 25.69 inHg (870 mbar). A household barometer swinging from 30.50 down to 29.30 is a reliable sign that weather is deteriorating.
How do HVAC technicians use inches of mercury for vacuum readings?
Refrigeration techs evacuate AC system lines to remove moisture before charging with refrigerant. They measure the vacuum in inHg below atmospheric pressure — a reading of 29 inHg (out of 29.92 max) means near-total vacuum. Industry best practice requires pulling to at least 29.92 inHg (or equivalently, below 500 microns on a micron gauge) to ensure all moisture has boiled off at room temperature.
How do you convert inches of mercury to millibars or psi?
1 inHg ≈ 33.86 mbar ≈ 0.491 psi. So standard atmosphere (29.92 inHg) is about 1013 mbar or 14.7 psi. For quick mental math: multiply inHg by 34 to get millibars, or divide by 2 to get a rough psi estimate. These conversions come up constantly when comparing US weather data with international sources or converting aviation altimeter settings for foreign aircraft.