Pressure units explained

Pressure is force spread over an area. This category has more competing units than any other on the site, because meteorology, medicine, diving and engineering each settled on a different one and none of them has changed since.

Pascal Pa

Base unit — every factor on this page is measured against it

The SI unit: one newton spread over one square metre. That is a very small amount of pressure, roughly a sheet of paper resting on a table, which is why almost every practical use reaches for the kilopascal or hectopascal instead.

Pa to hPa Pa to kPa Pa to MPa Pa to mbar

Hectopascal hPa

1 hPa = 100 Pa Exact

A hundred pascals, and exactly equal to a millibar. This is the unit on every weather chart. Standard sea-level pressure is 1013.25 hPa, a deep Atlantic low might reach 950, and a strong high sits above 1030.

hPa to Pa hPa to kPa hPa to MPa hPa to mbar

Kilopascal kPa

1 kPa = 1,000 Pa Exact

A thousand pascals. The metric unit for tyre pressures outside Europe and for engineering pressures generally. A car tyre at 32 psi is about 220 kPa; Australia and much of Asia quote tyres this way.

kPa to Pa kPa to hPa kPa to MPa kPa to mbar

Megapascal MPa

1 MPa = 1,000,000 Pa Exact

A million pascals, equal to 10 bar. The scale of hydraulic systems, material strength and concrete testing — concrete is specified as, say, 30 MPa, meaning the pressure it withstands before crushing.

MPa to Pa MPa to hPa MPa to kPa MPa to mbar

Millibar mbar

1 mbar = 100 Pa Exact

Exactly one hectopascal, and the older name for it. Meteorology used millibars for most of the twentieth century, and plenty of barometers, aviation charts and diving computers still say mbar. The change to hPa was a relabelling, not a conversion.

mbar to Pa mbar to hPa mbar to kPa mbar to MPa

Bar bar

1 bar = 100,000 Pa Exact

A hundred kilopascals, chosen to sit close to atmospheric pressure at sea level, which is what makes it convenient. European tyre pressures, espresso machines at 9 bar, and diving cylinders at 200 to 300 bar all use it.

bar to Pa bar to hPa bar to kPa bar to MPa

Standard atmosphere atm

1 atm = 101,325 Pa Exact

The standard atmosphere: exactly 101,325 pascals. A defined reference rather than whatever the air happens to be doing today. It is the baseline for boiling points, chemistry conditions and diving depth, where every 10 metres of seawater adds roughly one.

atm to Pa atm to hPa atm to kPa atm to MPa

Pound per square inch psi

1 psi = 6,894.76 Pa Exact

Pounds of force per square inch, the US and UK tyre and compressor unit. Exact, because the pound-force and the square inch are both exactly defined. Atmospheric pressure is about 14.7 psi, which is why a tyre gauge reading 32 actually holds nearer 47 in absolute terms.

psi to Pa psi to hPa psi to kPa psi to MPa

Torr Torr

1 Torr = 133.322368 Pa Exact

Exactly one seven-hundred-and-sixtieth of a standard atmosphere, named for Torricelli, who built the first barometer. It is the unit of vacuum work: a rough vacuum is around 1 torr, a high vacuum below a millionth of one.

Torr to Pa Torr to hPa Torr to kPa Torr to MPa

Millimetre of mercury mmHg

1 mmHg = 133.322387 Pa Exact

The height of a mercury column that the pressure would support. Blood pressure is quoted in it and shows no sign of changing — 120 over 80 means systolic and diastolic pressure in mmHg. Defined as exactly 133.322387415 Pa.

mmHg to Pa mmHg to hPa mmHg to kPa mmHg to MPa

Inch of mercury inHg

1 inHg = 3,386.39 Pa Exact

Inches of mercury, the same idea in imperial. Used by US weather reports, where standard sea-level pressure is 29.92 inHg, and by aviation wherever altimeters are set in inches rather than hectopascals.

inHg to Pa inHg to hPa inHg to kPa inHg to MPa

Convert pressure

bar to psi psi to bar kPa to psi atm to bar psi to kPa