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 pound per square inch?
Pound per square inch (psi) is an imperial pressure unit widely used in the United States for tire inflation, hydraulic system pressures, gas cylinder ratings, and plumbing specifications.
Real-world uses
PSI is the standard pressure unit in the United States for tyre inflation (typically 30–35 psi for cars), compressed air tools, hydraulic systems, and water pressure. HVAC systems, fire extinguisher ratings, and pressure cooker specifications use psi in North America.
History
Pounds per square inch emerged naturally from the Imperial/US customary system, combining the pound-force with the square inch. It became the dominant pressure unit in American industry and remains deeply embedded in US engineering standards and building codes.
Common mistakes
Not distinguishing between psi (absolute, psia) and psig (gauge, relative to atmosphere). A tyre reading of 32 psig actually means about 46.7 psia. Also, confusing psi with kPa when using equipment from metric countries.
When is this conversion used?
Tyre pressure gauges, HVAC systems, and industrial equipment may use different pressure units depending on the manufacturer and region. Converting between psi, bar, and kPa is common in automotive and engineering work.
Worked examples
1 inch of mercury = 0.4911542 pound per square inch
1 pound per square inch = 6,894.75729317 pascal
How to convert inch of mercury to pound per square inch
To convert inch of mercury to pound per square inch, multiply the value by 0.4911542.
To convert pound per square inch back to inch of mercury, multiply by 2.03602046.
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 pound per square inch
| inch of mercury | pound per square inch |
|---|---|
| 0.1 | 0.04911542 |
| 0.5 | 0.2455771 |
| 1 | 0.4911542 |
| 2 | 0.9823084 |
| 5 | 2.455771 |
| 10 | 4.911542 |
| 25 | 12.27885499 |
| 50 | 24.55770998 |
| 100 | 49.11541996 |
| 250 | 122.78854991 |
| 500 | 245.57709982 |
| 1,000 | 491.15419963 |
Common values
| inch of mercury | pound per square inch | |
|---|---|---|
| Car tyre | 64.96595636 inch of mercury | 31.9083023 pound per square inch |
| Standard atmosphere | 29.9212524 inch of mercury | 14.69594878 pound per square inch |
| Blood pressure (systolic) | 4.72479683 inch of mercury | 2.3206038 pound per square inch |
| Deep-sea submersible | 32,482.97818118 inch of mercury | 15,954.15115032 pound per square inch |
| Bicycle tyre | 183.08587702 inch of mercury | 89.92339739 pound per square inch |
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