Common Pressure Units: PSI, Bar, kPa, atm, and mmHg Explained

Pressure measurement carries more competing units than almost any other everyday physical quantity. Pounds per square inch (psi) dominates US tire-pressure gauges, hydraulic systems, and dive equipment. The bar dominates European and Asian industrial gauges. The pascal is the formal SI unit but appears in everyday work usually as kilopascals (kPa) or megapascals (MPa). The atmosphere (atm) appears in chemistry textbooks and laboratory work. Millimetres of mercury (mmHg) is the universal medical unit for blood pressure and a common laboratory unit for vacuum systems. Each unit has its own working range, audience, and conversion-precision discipline, and cross-unit work is constant in international engineering, medical practice, and consumer-product retail.

Pressure is force per unit area, with the force being the perpendicular component of any push on a surface. The SI definition is straightforward: one pascal equals one newton of force distributed over one square metre, or 1 Pa = 1 N/m². Typical practical pressures fall in awkward pascal ranges — atmospheric pressure is 101,325 Pa, tire pressure is 200,000 to 300,000 Pa, hydraulic pressure is millions of Pa — so the bar (defined as exactly 100,000 Pa) and its multiples (kPa, MPa) are usually preferred for human-readable industrial reporting. The bar was introduced in 1909 specifically to give meteorology and industrial work a clean SI-aligned pressure unit at the right magnitude.

Pounds per square inch derives directly from the imperial pound and inch and has been the standard US industrial-pressure unit since the early industrial era. A passenger-car tire-placard pressure typically falls in the 30–35 psi range; a residential water-supply main runs at 50–80 psi; an industrial hydraulic system can run at 2000–5000 psi for proof-test pressures. US-built scuba tanks rate to 3000 psi service pressure; US fire-hose hydrant pressures run at 50–100 psi at the outlet. The unit is universal in US automotive, hydraulic, and pneumatic specifications, and US-trained technicians read psi gauges by inspection without conversion.

The bar dominates European and Asian industrial gauges because it lands at convenient magnitudes for most everyday pressure work. One bar approximately equals one atmosphere, so atmospheric pressure reads as roughly 1 bar absolute on metric gauges. European tire-placard pressures typically run in 2.0–2.5 bar (the metric equivalent of 30–35 psi), European steel scuba tanks rate to 232 bar, and European hydraulic systems run at 100–700 bar working pressures. The conversion between bar and psi is approximately 1 bar = 14.5 psi, and the "14.5 psi per bar" mental shortcut is the most useful single pressure-unit conversion to memorise for cross-system technical work.

The atmosphere (atm) was defined as the average sea-level air pressure and was standardised at exactly 101,325 Pa by the 9th General Conference on Weights and Measures in 1948. Its primary modern use is in chemistry textbook problems where the ideal gas law (PV = nRT) is written with R = 0.0821 L·atm/mol·K — the form of the gas constant that gives clean arithmetic when pressure is in atmospheres. Modern chemistry has largely shifted to bar-format equilibrium constants (with R = 0.08314 L·bar/mol·K), but the atm tradition persists in older texts and in some specialty fields like high-pressure physics where pressures are quoted in thousands of atmospheres.

Blood-pressure measurement uses millimetres of mercury (mmHg) universally, regardless of whether the host country otherwise uses metric or imperial. The reference healthy reading of 120/80 mmHg has been calibrated against decades of population health data, and the threshold values for hypertension diagnosis are defined in mmHg. The unit derives from Torricelli's 1643 mercury barometer, which established that atmospheric pressure could support a column of mercury 760 mm tall at sea level. Modern aneroid and digital sphygmomanometers no longer contain mercury but still display in mmHg to keep clinical-decision rules consistent across instrument generations. Pulmonary pressures, intraocular pressures, and intracranial pressures are also measured in mmHg in most modern medical practice.

The standard reference relationship is 1 atmosphere = 1.01325 bar = 14.696 psi = 760 mmHg = 101,325 Pa. Each conversion factor is exact by international agreement, and the relationships chain together cleanly. To convert psi to bar, divide by 14.5038 (the precise factor). To convert bar to kPa, multiply by 100. To convert atm to bar, multiply by 1.01325 — the small 1.3% gap is what distinguishes the two units and matters in chemistry-precision work. To convert mmHg to kPa, multiply by 0.13332.

A common pressure-measurement subtlety is the distinction between gauge and absolute pressure. Gauge pressure measures pressure above the local atmospheric pressure (so 0 gauge = 1 atmosphere absolute). Absolute pressure measures pressure above absolute vacuum. A car tire reading "32 psi" on a gauge means 32 psi above atmospheric, equivalent to about 46.7 psi absolute at sea level. The distinction matters in chemistry, vacuum systems, and high-pressure engineering because gas-law equations require absolute pressure; everyday tire-pressure and HVAC work uses gauge pressure throughout. European service documents distinguish "bar g" (gauge) from "bar a" (absolute) explicitly, with a 1.013 bar offset between them.

Aviation uses inches of mercury (inHg) for altimeter setting in US convention and hectopascals (hPa) in international convention; the two are interchangeable through 1 inHg = 33.86 hPa. Vacuum technology uses Torr (essentially identical to mmHg, with 1 Torr = 1/760 atm by definition) for low-pressure work, with ranges spanning ultra-high vacuum at 10⁻¹² Torr through rough vacuum at 1 Torr. Weather services worldwide aggregate pressure in millibar or hectopascal regardless of local consumer convention; standard sea-level pressure is 1013.25 mb or 1013.25 hPa, both numerically identical figures.

Use psi when working with US-built equipment, US-spec tire pressures, US hydraulic systems, or US-customary engineering documentation. Use bar when working with European or Japanese industrial equipment and in any international industrial context. Use kPa or MPa when SI-format scientific or regulatory reporting is required. Use atm in chemistry-textbook problems and high-pressure-physics work. Use mmHg for blood pressure universally and for laboratory vacuum-system reporting in many contexts. Knowing which unit the audience expects is half the battle in pressure-unit work; the conversion arithmetic is the easy part.