How To Convert Pressure Units

Pressure is the force applied over a given area. The SI unit is the pascal, defined as one newton per square metre, and it is the base for every other pressure unit.

A kilopascal is a thousand pascals and is common in engineering and meteorology. Standard sea-level atmospheric pressure is about 101.3 kilopascals.

The bar is 100,000 pascals exactly, which is close to one atmosphere and convenient for large pressures. Millibars, a thousandth of a bar, are still used in some weather reporting.

PSI, pounds per square inch, is the customary unit in the United States and equals about 6894.7572932 pascals. It appears on tyre gauges, hydraulic systems and compressed-gas equipment.

The standard atmosphere is defined as exactly 101,325 pascals, the average pressure at sea level. It is the anchor for many other definitions.

The torr is named after Torricelli and is defined as one seven hundred and sixtieth of a standard atmosphere, so 760 torr is exactly one atmosphere, or about 133.3223684 pascals.

The millimetre of mercury is very close to but not identical to the torr. It is the pressure of a one-millimetre column of mercury under standard gravity, about 133.322387415 pascals.

The converter expresses every unit as a size in pascals and converts by multiplying the value by the source factor and dividing by the target factor. That keeps every pair consistent.

The most common engineering conversion is bar to PSI: multiply the bar by 100,000 and divide by 6894.7572932, which gives about 14.5037738 PSI per bar.

Atmospheric conversions use the exact value 101,325 pascals. One atmosphere is 101.325 kilopascals, 1.01325 bar and about 14.6959488 PSI.

Vacuum work uses torr and millimetres of mercury, where small pressures matter. A rough vacuum might reach one torr, a thousandth of an atmosphere.

A frequent error is to treat the torr and the millimetre of mercury as identical. They agree to about five significant figures but diverge beyond that, which matters in precision work.

Tyre pressures are quoted in PSI in the United States and in bar or kilopascals elsewhere. A tyre at 32 PSI is about 2.21 bar, and getting the unit wrong by a factor of fourteen is a safety issue.

Weather reports use hectopascals, which are the same size as millibars. Standard sea-level pressure is about 1013 hectopascals, and a deep low might read 980.

Hydraulic and pneumatic systems are rated in bar, so a workshop manual in PSI needs converting before a pressure gauge is set. One bar is about 14.5 PSI, a factor worth remembering.

Vacuum technology works at the low end of the scale, where torr and millimetres of mercury describe pressures far below atmosphere. A rough vacuum is around one torr, and a good one is far lower.

Absolute and gauge pressures differ by the atmospheric value. A gauge reading of zero means atmospheric pressure, while the absolute pressure is about 14.7 PSI higher.

Diving uses bar and atmospheres interchangeably at depth, where every ten metres of seawater adds about one bar. A tank filled to 200 bar holds about 197 atmospheres of pressure.

Medical equipment quotes blood pressure in millimetres of mercury, where a reading of 120 over 80 is 120 mmHg systolic. Converting that to pascals gives about 16,000, which shows how small the unit is.

Compressed-gas cylinders are stamped with a working pressure in bar and a test pressure higher still, so a conversion error at filling time is a safety concern rather than a rounding detail. Always convert from the cylinder's own marking.

The calculator models the fixed relationships between the units and nothing more. It does not know whether a gauge reads absolute or relative pressure, so treat the output as exact arithmetic on the number you entered.