Pressure Conversion Guide

Übersetzung in Arbeit.

Diese Anleitung ist noch nicht in DE verfügbar. Die englische Fassung steht unten.

Pressure converters are where physics and engineering shorthand collide. A car tyre at 32 psi is at about 2.2 bar; a blood-pressure cuff at 120 mmHg is at about 16 kilopascals; a weather barometer at 1,013 millibar is at 1 standard atmosphere. The numerical gaps are small enough that people skip the conversion and copy the wrong unit, then get the wrong answer by a factor of seven (mmHg vs kPa) or by 1% (bar vs atm). This guide covers every pressure unit we support, the modern SI base, and the absolute-vs-gauge convention that traps roughly half of all engineers exactly once.

What is pressure?

Pressure is force divided by area — newtons per square metre, in SI units. One newton per square metre is one pascal (Pa), the SI unit. Pressure scales the way you would expect: doubling the force on a fixed area doubles the pressure, and halving the area while keeping the force constant doubles the pressure. That second relationship is why a knife edge cuts and a spoon edge does not: same hand force, much smaller contact area.

Everyday pressures span many orders of magnitude. Atmospheric pressure at sea level is about 101,325 Pa. Car tyres run at roughly 200,000 Pa above atmospheric. Hydraulic systems run at 20–70 megapascals. Industrial steam pipes can reach 10 megapascals. Each domain settled on a unit that put the numbers in a comfortable range, which is why bar, psi, atmospheres, and mmHg all coexist alongside the SI pascal.

History of pressure measurement

Pressure measurement starts with Torricelli's barometer in 1644. A column of mercury 760 mm tall, sealed at the top, balanced the weight of the atmosphere — and that height became a unit. The torr and the millimetre of mercury are the same value to within 7 parts per million: 1 Torr is defined as 1/760 of a standard atmosphere exactly, and 1 mmHg is defined as the pressure exerted by a 1 mm column of mercury at standard gravity, which works out to 133.322 Pa. The two units are numerically interchangeable for any practical purpose; we route them through the same factor in our converter.

The bar arrived in 1909, defined as exactly 100,000 Pa — a round metric number close to one atmosphere of pressure. Meteorologists adopted the millibar (100 Pa, also called the hectopascal) because typical sea-level pressure variation falls in the 950–1,050 mbar band, a comfortable three-digit range on a forecast chart. The pascal itself was named for Blaise Pascal in 1971; before that, SI pressure was quoted as N/m² without a special name.

Imperial pressure uses the pound per square inch (psi). Defined by the pound-force (1 lbf = 4.4482216152605 N exactly) divided by the square inch, 1 psi works out to 6,894.757 Pa. US automotive, hydraulic, and pneumatic industries are built around psi; the rest of the world is built around bar or kilopascals.

Units we cover

Our pressure category supports ten units. SI: pascal (Pa), kilopascal (kPa), megapascal (MPa). Metric weather and industrial: bar, millibar (also written hPa, hectopascal). Imperial: PSI. Historical and medical: atmosphere (atm, defined as exactly 101,325 Pa), millimetre of mercury (mmHg), inch of mercury (inHg, the unit on US barometers), torr. The mmHg and torr are numerically identical to seven decimal places, so we route them through the same conversion factor.

We do not include 'absolute' versus 'gauge' as separate units — that distinction is a convention applied on top of any pressure unit, not a different scale. See the gotchas section below.

How to convert

Every pressure conversion routes through the pascal, the SI base. Each unit declares how many pascals it represents: 1 bar = 100,000 Pa, 1 psi = 6,894.757 Pa, 1 atm = 101,325 Pa, 1 mmHg = 133.322 Pa. The methodology page lists every factor and cites NIST SP 811. The standard atmosphere value of 101,325 Pa was fixed by the CIPM in 1954 (CGPM Resolution 10) and is the same value used by NIST.

Useful approximate factors: 1 bar ≈ 14.504 psi. 1 atm ≈ 14.696 psi ≈ 1.01325 bar. 1 mmHg ≈ 0.01934 psi. 1 kPa ≈ 0.145 psi. For sanity-check work: tyre pressures in the 30–35 psi range are roughly 2.0–2.4 bar; healthy systolic blood pressure of 120 mmHg is roughly 16 kPa or 0.16 bar; standard sea-level atmospheric pressure of 1,013 mbar is 1 atm.

Hydraulic and pneumatic systems live higher up the scale. A typical hydraulic press operates at 20 MPa (about 200 bar, or 2,900 psi). Industrial compressed-air systems run at 600–1,000 kPa (6–10 bar). Vacuum systems run below atmospheric: a rough vacuum sits around 1 mbar, a high vacuum around 10⁻⁶ mbar, and an ultra-high vacuum around 10⁻⁹ mbar. The full pressure span in everyday use stretches across about thirteen orders of magnitude, which is why the SI prefix ladder runs from pascals all the way to megapascals on the high side.

Common conversions you'll need

Automotive and engineering dominate pressure searches. PSI to bar and bar to PSI handle tyre pressure between US and European cars — a German car with a 2.2 bar tyre spec needs about 32 psi in the US. Pascals to kilopascals and atmospheres to pascals handle scientific work. For meteorologists and aviation, inches of mercury and millibars convert through pascals: a US 'altimeter setting' of 29.92 inHg is the same as the European 1,013.25 mbar reference, both of which are standard atmospheric pressure at sea level.

The full pressure comparison table lists every supported unit with its pascal factor and a worked sample.

Pitfalls and gotchas

Gauge vs absolute pressure: gauge pressure is measured relative to atmospheric pressure; absolute pressure is measured relative to a perfect vacuum. A tyre at '32 psi' is at 32 psi gauge (psig), which is 32 + 14.696 = 46.7 psi absolute (psia). Industrial steam and refrigeration specs usually quote absolute pressure; tyres, hydraulics, and consumer-product specs usually quote gauge pressure. The two differ by one standard atmosphere — a non-trivial gap, especially below atmospheric where the labels matter most. Our converter assumes whatever unit you input is in the same gauge or absolute frame as the unit you select for output.

Bar is not equal to atm: 1 bar = 100,000 Pa exactly, but 1 atm = 101,325 Pa exactly. They differ by 1.325% — close enough that meteorologists historically used both, but the gap matters for engineering specs. If your input source is a weather report, treat the units carefully; some use 'mbar' to mean exactly 100 Pa, and some use 'atm' as a colloquial stand-in for sea-level pressure rounded to 1.000.

Torr and mmHg: officially distinct, in practice identical. The torr is defined as exactly 1/760 atm = 133.32236842105 Pa; the mmHg in modern use is defined as 133.322387415 Pa. The two differ by about 7 ppm, which is below the precision of any practical pressure gauge. We treat them as a single conversion factor.

PSI versus PSIA versus PSIG: 'PSI' alone is ambiguous; 'PSIA' is absolute, 'PSIG' is gauge. If a spec sheet just says 'PSI', you usually have to read context — automotive specs are almost always gauge, scientific specs are almost always absolute. Calling the manufacturer is faster than guessing.

Blood pressure stays in mmHg: clinical blood-pressure measurement is one of the few domains that did not transition to SI. A reading of 120/80 mmHg is the global standard, and 'kPa' versions appear only in research papers. Do not assume that a hospital is using SI just because the rest of the country is.