Inch Water (4 °C) to Millimeter Mercury
inH2O
mmHg
Conversion History
| Conversion | Reuse | Delete |
|---|---|---|
1 inH2O (Inch Water (4 °C)) → 1.8682679 mmHg (Millimeter Mercury) Just now · Rounding: Up to 8 significant figures | Recalculate this conversion using your current rounding settings. |
Quick Reference Table (Inch Water (4 °C) to Millimeter Mercury)
| Inch Water (4 °C) (inH2O) | Millimeter Mercury (mmHg) |
|---|---|
| 0.1 | 0.18682679 |
| 0.5 | 0.93413395 |
| 1 | 1.8682679 |
| 2 | 3.7365358 |
| 4 | 7.4730716 |
| 10 | 18.682679 |
| 407 | 760.38503 |
Results rounded to a maximum of 8 significant figures
About Inch Water (4 °C) (inH2O)
The inch of water at 4 °C (inH₂O) equals 249.08193551052 pascals under the fixed-density reference and standard gravity. This converter derives the factor from 9.8063754138 Pa/mm × exactly 25.4 mm/in. It is the standard low-pressure unit in US HVAC engineering, duct design, and building mechanical systems. Static pressure in supply and return ducts, air filter resistance, and fan performance curves are specified in inches of water column (often written "in. w.c." or "in. w.g."). US medical ventilators and flow bench testing also use inH₂O.
A residential furnace filter creates a pressure drop of 0.1–0.5 inH₂O. Commercial HVAC systems typically operate at 1–4 inH₂O of static pressure.
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.
Inch Water (4 °C) – Frequently Asked Questions
Why does the US HVAC industry measure duct pressure in inches of water?
American HVAC systems inherited the inch-pound measurement system, and duct static pressures fall neatly in the 0.1–4 inH₂O range — tidy numbers that are easy to read on a manometer or Magnehelic gauge. Converting to pascals (25–1,000 Pa) gives larger, less memorable values. Since the entire US supply chain — ductwork charts, fan curves, filter specs — is calibrated in inH₂O, switching would mean rewriting decades of engineering tables.
What is a normal static pressure reading for a residential HVAC system?
Total external static pressure should generally stay below 0.5 inH₂O for most residential furnaces. Supply-side static pressure is usually 0.2–0.3 inH₂O and return-side 0.1–0.2 inH₂O. Readings above 0.7 inH₂O indicate a problem — dirty filters, undersized ducts, or too many sharp bends. High static pressure forces the blower motor to work harder, raising energy bills and shortening equipment life.
How do you convert inches of water to pascals or psi?
1 inH₂O ≈ 249 Pa ≈ 0.0361 psi. The pascal conversion is handy for international specs: a 2 inH₂O reading is about 498 Pa. The psi conversion shows how small HVAC pressures are — 4 inH₂O is only 0.14 psi, which is why psi gauges are useless for duct work (the needle would barely leave zero). Inches of water occupy the Goldilocks zone for air-handling pressures.
What does "in. w.g." mean on a furnace spec sheet?
It stands for "inches water gauge" — the same as inH₂O. "Gauge" means the reading is relative to atmospheric pressure (not absolute). You may also see "in. w.c." (inches water column). All three abbreviations — inH₂O, in. w.g., in. w.c. — refer to exactly the same unit. European equivalents would be listed in pascals or mmH₂O.
Can a homeowner measure inH₂O without professional tools?
Yes, with a cheap U-tube manometer (under $20) or a digital differential pressure gauge. Drill a small test port in the supply and return plenums, connect the manometer with vinyl tubing, and read the water level difference. Many energy auditors and HVAC DIY forums recommend this as a first diagnostic step — high static pressure is the single most common cause of poor airflow and uneven room temperatures.
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.