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 millibar?
A millibar is a pressure unit equal to one thousandth of a bar. It is the standard unit for atmospheric pressure in meteorology and weather forecasting, particularly in aviation and marine contexts.
Real-world uses
Millibars are the traditional unit in meteorology for reporting atmospheric pressure, particularly in weather maps and hurricane tracking. Standard atmospheric pressure is 1013.25 mbar, and tropical cyclones can have central pressures below 900 mbar.
History
The millibar was introduced by Sir Napier Shaw in 1909 for meteorological use. It dominated weather reporting throughout the 20th century. Since 1982, the WMO has officially recommended hectopascals instead, though both represent exactly 100 Pa.
Common mistakes
Not realizing that 1 millibar = 1 hectopascal exactly. Meteorological services have largely switched to hPa, but the numerical values are identical. Also, assuming lower millibar values mean calmer weather—lower pressure often indicates storms.
When is this conversion used?
Converting between pound per square inch and millibar is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region. 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 = 68.94757293 millibar
1 millibar = 100 pascal
How to convert pound per square inch to millibar
To convert pound per square inch to millibar, multiply the value by 68.94757293.
To convert millibar back to pound per square inch, multiply by 0.01450377.
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 millibar
| pound per square inch | millibar |
|---|---|
| 0.1 | 6.89475729 |
| 0.5 | 34.47378647 |
| 1 | 68.94757293 |
| 2 | 137.89514586 |
| 5 | 344.73786466 |
| 10 | 689.47572932 |
| 25 | 1,723.68932329 |
| 50 | 3,447.37864658 |
| 100 | 6,894.75729317 |
| 250 | 17,236.89323292 |
| 500 | 34,473.78646584 |
| 1,000 | 68,947.57293168 |
Common values
| pound per square inch | millibar | |
|---|---|---|
| Car tyre | 31.9083023 pound per square inch | 2,200 millibar |
| Standard atmosphere | 14.69594878 pound per square inch | 1,013.25 millibar |
| Blood pressure (systolic) | 2.3206038 pound per square inch | 160 millibar |
| Deep-sea submersible | 15,954.15115032 pound per square inch | 1,100,000 millibar |
| Bicycle tyre | 89.92339739 pound per square inch | 6,200 millibar |
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