Pressure units explained: pascals, bar, psi, atm and mmHg

Pressure is force per unit area. The SI unit is the pascal (Pa), one newton per square metre, which is tiny, so you usually see kilopascals (kPa) or hectopascals (hPa). The other common units relate to it like this: 1 bar = 100 kPa; 1 standard atmosphere = 101.325 kPa = 1.01325 bar; 1 psi = 6.894757 kPa; 1 mmHg ≈ 133.3 Pa. The pressure converter shows a value in every unit at once.

What pressure is

Pressure is how concentrated a force is. Stand on soft snow in boots and you sink; on skis the same weight spreads over a larger area and you float. The air around you presses on every surface with about 101 kPa at sea level, roughly the weight of a 10-tonne mass on each square metre. You do not feel it because it acts equally from all sides, including from inside your body. It falls with altitude, which is why water boils cooler in the mountains, as Why water boils at a lower temperature at altitude explains.

The main units

• Pascal (Pa): 1 N/m², the SI unit. Kilopascals and megapascals are used in engineering.
• Hectopascal (hPa) and millibar (mbar): identical, 100 Pa. Standard sea-level pressure is 1,013.25 hPa.
• Bar: 100,000 Pa, very close to atmospheric pressure but not quite equal to it.
• Standard atmosphere (atm): exactly 101,325 Pa, defined as typical sea-level pressure.
• Pound-force per square inch (psi): 6,894.757 Pa. One atmosphere is about 14.70 psi, and one bar about 14.50 psi.
• Millimetre of mercury (mmHg) and torr: the pressure that supports a 1 mm column of mercury, about 133.32 Pa. A torr is defined as 1/760 of an atmosphere, and the two differ by less than one part in a million.
• Inch of mercury (inHg): about 3,386 Pa. One atmosphere is 29.92 inHg.

Where each unit is used

The choice is mostly by tradition and field. Weather services use hPa or mbar in most of the world, while US weather reports and aviation altimeter settings commonly use inches of mercury. Tyre pressures are given in psi in the US and UK and in bar or kPa across much of Europe and Asia. Blood pressure is still measured in mmHg everywhere, a legacy of mercury sphygmomanometers; a reading of 120 mmHg is about 16 kPa. Diving gives an intuitive feel for the size of an atmosphere: every 10 metres of seawater adds roughly one more atmosphere of pressure, so a diver at 10 m is under about two atmospheres absolute. The blood pressure checker works in mmHg, and What is a normal blood pressure? How to read the numbers explains the categories. Scuba and compressed gas cylinders use bar or psi, and material strengths use MPa.

Gauge versus absolute pressure

Most everyday gauges read gauge pressure, the pressure above the surrounding atmosphere. A tyre at 32 psi on the gauge actually holds about 46.7 psi absolute, because the atmosphere's 14.7 psi is already there before you pump. Absolute pressure is measured from a perfect vacuum, and it is what science, weather and altitude calculations use. In US notation you sometimes see psig and psia to make the difference explicit. A flat tyre reads zero on a gauge, but the air inside is still at about one atmosphere absolute; it simply matches the air outside. The conversion factors are the same either way; the important thing is not to mix the two, for example by adding atmospheric pressure to a value that already includes it.

Worked conversions

• 32 psi = 2.21 bar = 220.6 kPa (a typical car tyre);
• 2.5 bar = 36.3 psi;
• 1 atm = 1,013.25 hPa = 760 mmHg = 29.92 inHg = 14.70 psi;
• 1 bar = 14.50 psi = 750 mmHg.

I built the converter to show every unit at once because people rarely need just one conversion: a tyre spec in kPa, a gauge in psi and a pump in bar is a common combination. The torque units on wheel bolts are a separate story, covered by the torque converter, and for pressure at different heights the boiling point at altitude calculator estimates the air pressure for any elevation.