Pressure Converters — psi, bar, pascal, atmosphere, mmHg
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Pressure conversions span a wider range of units than almost any other physical category because different industries and historical contexts settled on different practical units. The five units in this category cover the major working contexts: pounds per square inch (psi) is the US imperial standard for tire pressure, hydraulic systems, dive equipment, and US industrial gauges; the bar is the metric industrial standard used in European, Japanese, and most international industrial gauges (1 bar ≈ 1 atmosphere ≈ 14.5 psi); the pascal is the formal SI pressure unit (1 Pa = 1 newton per square metre), used in scientific and engineering literature though typical pressures fall in awkward ranges (100,000+ Pa); the atmosphere (atm) is the historical reference pressure equal to standard sea-level air pressure (101,325 Pa = 1.01325 bar = 14.696 psi), still used in chemistry textbooks; and millimetres of mercury (mmHg) is the medical and laboratory unit derived from mercury-column barometers, used universally for blood pressure (120/80 mmHg) and frequently for vacuum-system pressure reporting. Each unit has its own working range, audience, and conversion-precision discipline. Cross-unit conversion runs constantly in international auto repair, dive operations, HVAC service, and chemistry-to-engineering documentation handoffs, with the right unit choice often determined by which gauge or instrument will read the figure downstream rather than by the source of the original measurement.
Units in this category
Pounds per square inch (psi)
One pound per square inch (psi) is the pressure exerted by a force of one pound-force (lbf) acting on an area of one square inch. By substitution from the 1959 International Yard and Pound Agreement values for the pound and the inch, and using standard gravity (9.80665 m/s²) for the conversion of pound-mass to pound-force, one psi equals exactly 6,894.757293168 pascals — typically rounded to 6,894.76 Pa or 6.895 kPa in engineering tables. The conversion to bar is 1 bar = 14.5037738 psi (or, going the other way, 1 psi ≈ 0.0689476 bar); to standard atmospheres 1 atm = 14.6959488 psi; to kilopascals 1 psi = 6.89476 kPa.
Bar (bar)
One bar is defined as exactly 100,000 pascals (100 kPa, or 10⁵ Pa). Equivalently, the bar is one mega-dyne per square centimetre in the older CGS system in which it was originally formulated. The conversion to other commonly-encountered pressure units is: 1 bar = 14.5037738 psi exactly (rounding to five decimal places), 1 bar = 0.986923 standard atmospheres, 1 bar = 750.062 torr (mmHg), and 1 bar = 29.530 inches of mercury.
Pascals (Pa)
One pascal (Pa) is defined as one newton per square metre (1 Pa = 1 N/m²), or equivalently in SI base units one kilogram per metre per second squared (1 Pa = 1 kg·m⁻¹·s⁻²). The pascal is the SI derived unit of pressure and stress, and unlike the bar, the standard atmosphere, the millimetre of mercury and the pound per square inch, it follows directly from the SI base units with no conversion factor. Conversions to other commonly-encountered pressure units are: 1 standard atmosphere = 101,325 Pa exactly (by the 1954 BIPM definition), 1 bar = 100,000 Pa exactly, 1 psi = 6,894.757 Pa, 1 torr (mmHg) = 133.322 Pa, and 1 inch of mercury (inHg) = 3,386.39 Pa.
Standard atmospheres (atm)
One standard atmosphere (atm) is defined as exactly 101,325 pascals (101.325 kPa) by the 10th CGPM resolution of 1954. This value is also equal to exactly 760 millimetres of mercury (mmHg, or torr) at 0 °C under standard gravity — the equivalence is by definition, not by measurement, and was specifically constructed so that the older mercury-column convention and the pascal-based SI convention give the same numerical reference. Conversions to other commonly-encountered pressure units: 1 atm = 1.01325 bar exactly, 1 atm = 14.6959488 psi, 1 atm = 29.9213 inches of mercury, and 1 atm = 1,013.25 millibar / hectopascal.
Millimetres of mercury (mmHg)
One millimetre of mercury (mmHg) is defined as the pressure exerted at the base of a column of mercury exactly one millimetre high at 0 °C under standard gravity (9.80665 m/s²), with the density of mercury at that temperature taken as 13,595.1 kg/m³. By substitution, 1 mmHg = 133.322387415 Pa. The standard atmosphere was historically defined to be exactly 760 mmHg, and the modern numerical value of 1 atm = 101,325 Pa was chosen to preserve that 760-mm equivalence to within the resolution of practical mercury barometers.
Kilopascals (kPa)
The kilopascal (kPa) is exactly 1000 pascals, where one pascal equals one newton of force distributed over one square metre of area (1 Pa = 1 N/m²). The relationship is fixed by SI prefix and SI-derived-unit definition. Standard atmospheric pressure at sea level is exactly 101,325 Pa or 101.325 kPa.
Megapascals (MPa)
The megapascal (MPa) is exactly 1,000,000 pascals or exactly 1000 kilopascals, where one pascal equals one newton of force distributed over one square metre of area. The relationship is fixed by SI prefix and SI-derived-unit definition, with the mega- prefix denoting exactly 10⁶. Standard atmospheric pressure at sea level (101,325 Pa) equals approximately 0.101 MPa — too small a figure for atmospheric reporting, where kPa or hPa is the natural unit.
Torr (Torr)
One Torr is defined as exactly 1/760 of a standard atmosphere, which equals exactly 101,325/760 = 133.322368... pascals. The relationship to the millimetre of mercury (mmHg) is very close but technically distinct: 1 mmHg = 133.322387415 Pa per the BIPM definition based on standard gravity acting on a column of mercury, while 1 Torr = 133.322368...
History of pressure measurement
Pressure measurement began with Evangelista Torricelli's 1643 mercury barometer, which established that atmospheric pressure could support a column of mercury 760 mm tall — directly seeding the mmHg unit and the concept of standard atmospheric pressure. Blaise Pascal's 1648 Puy-de-Dôme experiment confirmed that air pressure decreases with altitude, and his name was later attached to the SI pressure unit. The atmosphere was standardised at exactly 101,325 Pa by the 9th General Conference on Weights and Measures in 1948, fixing what had previously been a measurement of average sea-level air pressure to a defined reference value. The bar was introduced in 1909 by Norwegian meteorologist Vilhelm Bjerknes, defined as exactly 100,000 Pa, to give meteorology a clean SI-aligned pressure unit. Pounds per square inch derives directly from the imperial pound and inch and has been used in US engineering since the early industrial era. Modern pressure metrology uses ITS-90 calibration alongside primary-standard piston gauges and fixed-point cells.
Where pressure conversions matter
Pressure conversions appear constantly in cross-system industrial work. Automotive tire-pressure ratings list psi on US-market vehicles and bar on European and Japanese vehicles, with international auto repair shops handling US imports converting at every routine service. HVAC and refrigeration handles psi-vs-bar in the manifold-gauge sets of cross-border service teams, particularly around US-built Carrier and Trane chillers installed in metric-using countries. Medical practice keeps mmHg universally for blood pressure (120/80 mmHg is the reference healthy reading worldwide regardless of host-country measurement system), with pulmonary and intracranial pressures also in mmHg or cmH₂O. Diving equipment runs in psi or bar depending on tank origin (US 3000 psi tanks vs European 232 bar tanks), with international dive operators converting at every fill. Aviation reports altimeter setting in inches of mercury (inHg, US convention) or hectopascals (hPa, international), with cockpit altimeters offering both. Chemistry laboratory work uses atm or bar for gas-law calculations and Torr (equivalent to mmHg) for vacuum-system reporting. Industrial hydraulics ratings range from low bar values for HVAC fluid systems up to thousands of bar (or tens of thousands of psi) for high-pressure proof testing. Homebrewing and craft-beer service uses bar for European-imported CO2 regulators against psi-marked US gauges, and weather services worldwide aggregate pressure data in millibar or hectopascal regardless of the local consumer-facing convention.
How to convert pressure units
The foundational pressure conversions tie back to standard atmospheric pressure: 1 atm = 1.01325 bar = 14.696 psi = 760 mmHg = 101,325 Pa. Each pair conversion is a fixed multiplicative factor with no offset (unlike temperature). Within metric, the conversions are decimal shifts: 1 bar = 100,000 Pa = 100 kPa = 0.1 MPa. The most useful single mental anchor is "1 bar is about 14.5 psi" — the actual factor is 14.5038, accurate to within 0.05% across typical industrial pressures. The 1 atm vs 1 bar distinction matters in chemistry and precision-engineering contexts because the 1.3% gap accumulates in equilibrium-constant and gas-law calculations; the casual "1 atm ≈ 1 bar" approximation works for tire-pressure intuition but fails in precision work. Gauge pressure (above atmospheric) and absolute pressure (above vacuum) are distinct quantities, and European service documents distinguish "bar g" from "bar a" with a 1.013 bar offset between them — this is the pressure-conversion equivalent of the temperature-scale offset issue.
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Frequently asked questions
Why do US tire gauges use psi but European gauges use bar?
US-built automotive components, including tire-pressure gauges and TPMS sensors, were standardised on psi during the early-to-mid 20th-century formation of the US auto industry, and the 1975 Metric Conversion Act left consumer tooling voluntary. European auto manufacturing standardised on bar in the post-war metrication era. The two industries have parallel infrastructure — psi-marked gauges, hoses, fittings, and TPMS calibration in the US; bar-marked equivalents in Europe — and cross-system service work requires the conversion at every interface.
What is the difference between gauge pressure and absolute pressure?
Gauge pressure measures pressure above the local atmospheric pressure (so 0 gauge = 1 atmosphere absolute = 1.013 bar absolute), while 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. The distinction matters in chemistry, vacuum systems, and high-pressure engineering because gas-law and thermodynamic equations require absolute pressure; everyday tire-pressure and HVAC work uses gauge pressure throughout.
Why is blood pressure measured in mmHg?
Blood pressure measurement using mercury sphygmomanometers became the medical standard in the early 20th century, and the resulting unit (millimetres of mercury column height) was preserved when mercury manometers were replaced by aneroid and digital instruments. The reference healthy reading of 120/80 mmHg has been calibrated against decades of population health data, and changing the unit would require recalibrating clinical decision rules. Modern instruments display mmHg even though they no longer contain mercury.
Is 1 atmosphere exactly 1 bar?
No — 1 standard atmosphere equals exactly 1.01325 bar, about 1.3% larger than 1 bar. The two units originate independently: the atmosphere was defined as average sea-level air pressure, and the bar was defined as exactly 100,000 Pa to give a clean SI-aligned pressure unit. The 1.3% gap is small enough to ignore in tire-pressure and casual industrial work but matters in chemistry, vacuum technology, and precision-engineering calculations.
What is the pascal and why is it the SI unit?
One pascal equals one newton of force distributed over one square metre of area (1 Pa = 1 N/m²), making it the formal SI derived unit for pressure. The pascal is named after Blaise Pascal in recognition of his foundational atmospheric-pressure experiments. Typical practical pressures fall in awkward Pa ranges — atmospheric pressure is 101,325 Pa, tire pressure is 200,000+ Pa, hydraulic pressure is millions of Pa — so the bar (100,000 Pa) and kPa or MPa multiples are usually preferred for human-readable reporting.
How many psi is normal car tire pressure?
US passenger-car tire-placard pressures typically fall in the 30–35 psi range (approximately 2.1–2.4 bar), set by the vehicle manufacturer based on tire size, vehicle weight, and load conditions. The placard inside the driver's door jamb specifies the manufacturer-recommended figure, which differs from the maximum pressure printed on the tire sidewall. European factory specifications cluster in the same physical range but are written in bar (2.0–2.5 bar typical), and the conversion runs at every cross-border service.
What is a Torr and how does it relate to mmHg?
One Torr is defined as exactly 1/760 of a standard atmosphere, which equals 0.99999 mmHg — so for practical purposes Torr and mmHg are interchangeable, with the tiny discrepancy mattering only in high-precision metrology. Vacuum technology uses Torr extensively for low-pressure work below atmospheric (1 Torr = 133.322 Pa), with ranges spanning ultra-high vacuum at 10⁻¹² Torr through rough vacuum at 1 Torr. The unit was named after Torricelli, who demonstrated atmospheric pressure with the first mercury barometer.