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.
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.
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 pound per square inch = 2.03602046 inch of mercury
1 inch of mercury = 3,386.389 pascal
How to convert pound per square inch to inch of mercury
To convert pound per square inch to inch of mercury, multiply the value by 2.03602046.
To convert inch of mercury back to pound per square inch, multiply by 0.4911542.
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: pound per square inch to inch of mercury
| pound per square inch | inch of mercury |
|---|---|
| 0.1 | 0.20360205 |
| 0.5 | 1.01801023 |
| 1 | 2.03602046 |
| 2 | 4.07204092 |
| 5 | 10.18010231 |
| 10 | 20.36020461 |
| 25 | 50.90051153 |
| 50 | 101.80102305 |
| 100 | 203.60204611 |
| 250 | 509.00511527 |
| 500 | 1,018.01023054 |
| 1,000 | 2,036.02046108 |
Common values
| pound per square inch | inch of mercury | |
|---|---|---|
| Car tyre | 31.9083023 pound per square inch | 64.96595636 inch of mercury |
| Standard atmosphere | 14.69594878 pound per square inch | 29.9212524 inch of mercury |
| Blood pressure (systolic) | 2.3206038 pound per square inch | 4.72479683 inch of mercury |
| Deep-sea submersible | 15,954.15115032 pound per square inch | 32,482.97818118 inch of mercury |
| Bicycle tyre | 89.92339739 pound per square inch | 183.08587702 inch of mercury |
Available Pressure units
More pound per square inch conversions
- Convert pound per square inch to pascal
- Convert pound per square inch to kilopascal
- Convert pound per square inch to bar
- Convert pound per square inch to atmosphere
- Convert pound per square inch to millibar
- Convert pound per square inch to hectopascal
- Convert pound per square inch to torr
- Convert pound per square inch to inch of mercury
- Convert pound per square inch to megapascal