Pressure Converter – Bar, PSI, Pascal and Atmospheres

Pressure units at a glance

Pressure is force per area. The SI unit is the pascal (Pa), one newton per square metre – a very small amount, so kilopascals (kPa) and megapascals (MPa) are common. The converter uses these definitions:

• 1 bar = 100,000 Pa = 100 kPa
• 1 millibar (mbar) = 100 Pa, the same as a hectopascal (hPa)
• 1 standard atmosphere (atm) = 101,325 Pa
• 1 psi (pound-force per square inch) = 6,894.757 Pa
• 1 torr = 1/760 atm ≈ 133.32 Pa, practically equal to 1 mmHg

Tyre pressure: bar, psi and kPa

Tyre pressures in Europe are usually given in bar, in the US in psi and in Canada and Australia in kPa. One bar is 14.5 psi, so 2.2 bar is 32 psi and 2.5 bar is 36 psi. Tyre gauges show gauge pressure – the pressure above the surrounding air – so a reading of 0 means the tyre is at atmospheric pressure, not empty.

Weather and air pressure

Weather maps show air pressure in hectopascals or millibars, which are the same. Average sea-level pressure is 1,013.25 hPa, or one atmosphere. Low-pressure systems below roughly 1,000 hPa tend to bring wind and rain; high pressure above 1,020 hPa usually means settled weather. Aviation in the United States uses inches of mercury instead.

Blood pressure and vacuum

Blood pressure is measured in millimetres of mercury, which is practically the same as torr: a reading of 120 mmHg is about 16 kPa. Vacuum systems also use torr or millibar.

High pressures

Hydraulic systems, gas cylinders and pressure washers work at hundreds of bar. A scuba tank filled to 200 bar holds about 2,900 psi, and concrete strength is given in megapascals (MPa). One MPa equals 10 bar or about 145 psi.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.