What is inch of mercury?
Inch of mercury (inHg) is a pressure unit representing the pressure exerted by a one-inch column of mercury. It is used in aviation altimetry, barometric pressure readings, and vacuum measurements in the United States.
Real-world uses
Inches of mercury are used in US aviation for altimeter settings (standard is 29.92 inHg), weather reporting on American TV broadcasts, and HVAC system vacuum measurements. American barometers are traditionally scaled in inHg.
History
Inches of mercury originate from the mercury barometer invented by Torricelli in 1644. The use of inches (rather than millimetres) reflects the Imperial measurement tradition. US aviation adopted inHg for altimeter settings, while most other countries use hPa.
Common mistakes
Confusing inches of mercury with millimetres of mercury—29.92 inHg = 760 mmHg = 1 atm. Also, assuming the conversion from inHg to psi is simple; 1 inHg ≈ 0.491 psi, not a round number.
What is torr?
A torr is a pressure unit equal to approximately 1/760 of an atmosphere. It is used in vacuum technology, low-pressure experimental physics, and blood pressure measurement in older clinical literature.
Real-world uses
Torr is used in vacuum technology, semiconductor manufacturing, and blood pressure measurement (mmHg ≈ torr). Vacuum chambers for electron beam welding operate at about 10⁻⁵ torr, and freeze-drying processes typically operate at 0.1–1 torr.
History
Named after Evangelista Torricelli, who invented the mercury barometer in 1644. The torr was defined to approximate the pressure exerted by 1 mm of mercury. It was formally standardized as 1/760 of a standard atmosphere.
Common mistakes
Assuming torr and mmHg are exactly the same—they are nearly identical but differ by about 0.000015%. For most purposes they are interchangeable, but ultra-precise vacuum work may require distinction.
When is this conversion used?
Converting inch of mercury to torr is useful in the pressure domain when comparing values across different measurement standards or applying formulas that require a specific unit.
Worked examples
1 inch of mercury = 25.40000632 torr
1 torr = 133.32236842 pascal
How to convert inch of mercury to torr
To convert inch of mercury to torr, multiply the value by 25.40000632.
To convert torr back to inch of mercury, multiply by 0.03937007.
Measurement standards
The pascal (Pa) is the SI derived unit of pressure, defined as one newton per square metre. The World Meteorological Organization mandates the hectopascal (hPa) for atmospheric pressure reporting in aviation and weather services.
Did you know?
The pressure at the bottom of the Mariana Trench, nearly 11 km deep, exceeds 1,086 bar — over a thousand times atmospheric pressure. At this depth, water is compressed by about 5%, making it measurably denser than at the surface.
Quick reference: inch of mercury to torr
| inch of mercury | torr |
|---|---|
| 0.1 | 2.54000063 |
| 0.5 | 12.70000316 |
| 1 | 25.40000632 |
| 2 | 50.80001263 |
| 5 | 127.00003158 |
| 10 | 254.00006316 |
| 25 | 635.00015791 |
| 50 | 1,270.00031581 |
| 100 | 2,540.00063163 |
| 250 | 6,350.00157907 |
| 500 | 12,700.00315815 |
| 1,000 | 25,400.0063163 |
Common values
| inch of mercury | torr | |
|---|---|---|
| Car tyre | 64.96595636 inch of mercury | 1,650.13570195 torr |
| Standard atmosphere | 29.9212524 inch of mercury | 760 torr |
| Blood pressure (systolic) | 4.72479683 inch of mercury | 120.00986923 torr |
| Deep-sea submersible | 32,482.97818118 inch of mercury | 825,067.85097429 torr |
| Bicycle tyre | 183.08587702 inch of mercury | 4,650.38243276 torr |
Available Pressure units
More inch of mercury conversions
- Convert inch of mercury to pascal
- Convert inch of mercury to kilopascal
- Convert inch of mercury to bar
- Convert inch of mercury to pound per square inch
- Convert inch of mercury to atmosphere
- Convert inch of mercury to millibar
- Convert inch of mercury to hectopascal
- Convert inch of mercury to torr
- Convert inch of mercury to megapascal