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

The pascal (Pa) is the SI base unit of pressure, defined as one newton per square meter (N/m²). One pascal is very small — normal atmospheric pressure is about 101,325 Pa — so the kilopascal (kPa = 1,000 Pa) is the practical unit for most metric applications. Tire pressure in kPa typically runs 200–250 kPa for passenger cars. Medical blood pressure cuffs sometimes report in kPa alongside mmHg. Meteorologists report atmospheric pressure in hectopascals (hPa), which are numerically identical to millibars (1 hPa = 1 mbar).

Because kPa is an SI unit, it is the standard in scientific and engineering calculations worldwide. European and Australian vehicle manuals list tire pressure in kPa. Compressed gas regulators in metric countries display kPa or MPa (megapascal = 1,000 kPa). The advantage of kPa is that it scales cleanly: atmospheric pressure ≈ 101.3 kPa, and the math is straightforward.

PSI (pounds per square inch) is the dominant pressure unit in the United States for consumer and industrial applications. Tire pressure gauges, air compressors, hydraulic systems, water pressure regulators, and natural gas lines are all commonly rated in PSI in North America. One PSI equals approximately 6.895 kPa, so converting between systems is a multiplication by that factor or its reciprocal.

Typical reference values: car tire pressure 30–35 PSI; bicycle tire 80–130 PSI (road) or 25–35 PSI (mountain); residential water pressure 40–80 PSI; industrial hydraulics 1,000–5,000 PSI; deep-sea diving pressures can reach tens of thousands of PSI at extreme depths. Despite global adoption of SI units, PSI remains deeply embedded in US infrastructure and consumer products.

Bar is a metric unit equal to exactly 100,000 Pa (100 kPa), which makes it close to — but not exactly — one standard atmosphere (1 atm = 101.325 kPa ≈ 1.01325 bar). The bar is widely used in European industry, HVAC, hydraulics, and weather reports. Tire pressure stickers on European vehicles commonly list bar alongside PSI. Scuba regulators and compressed gas cylinders in Europe are rated in bar.

The millibar (mbar = 0.001 bar = 100 Pa = 1 hPa) is the standard unit for atmospheric pressure in weather forecasting worldwide, including in the United States. Sea-level atmospheric pressure is approximately 1013.25 mbar. One bar converts to 14.5038 PSI. The relationship 1 bar ≈ 14.5 PSI is a useful mental anchor.

One standard atmosphere (1 atm) is defined as exactly 101,325 Pa (101.325 kPa or 1.01325 bar). It was historically defined as the mean atmospheric pressure at sea level, and it remains the reference point for many scientific tables: gas laws, solubility tables, diving decompression tables, and altitude calculations all reference standard conditions at 1 atm.

In practice, actual atmospheric pressure varies by altitude and weather. At sea level, normal pressure ranges from about 0.98 to 1.03 atm depending on weather systems. At the summit of Everest (8,849 m), atmospheric pressure is roughly 0.33 atm. Pressure altitude (used in aviation) is referenced to the standard atmosphere. The conversion: 1 atm = 14.696 PSI = 101.325 kPa = 1.01325 bar = 760 mmHg.

Millimeters of mercury (mmHg) measures pressure as the height of a mercury column it can support. It originates from Torricelli's 1643 barometer experiment. The torr (symbol Torr) is nearly identical: 1 Torr = 1/760 atm ≈ 1.000014 mmHg — close enough that they are interchangeable in most practical contexts.

mmHg dominates in medicine. Blood pressure is universally reported in mmHg: a normal reading of 120/80 means 120 mmHg systolic, 80 mmHg diastolic. Intraocular pressure (glaucoma monitoring) is measured in mmHg. Gas partial pressures in physiology and diving medicine use mmHg or Torr. In chemistry labs, vacuum pressures and vapor pressures are often listed in Torr. One mmHg = 133.322 Pa = 0.133 kPa ≈ 0.0193 PSI.