What Is Bar? The Bar Unit of Pressure Explained

5 min read•June 1, 2026•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

Bar is a metric unit of pressure defined as exactly 100,000 pascals (100 kPa). It is not an official SI unit but is accepted for use alongside SI and is widely employed in European industry, meteorology, scuba diving, and food processing. One bar is very close to one standard atmosphere (1 atm = 1.01325 bar), making it a convenient round-number reference for pressures near atmospheric.

Bar is commonly seen on pressure gauges in European and international industrial contexts, on scuba tank pressure indicators (which count down from ~200–300 bar), and on tire pressure gauges manufactured outside the US. If you see a pressure spec in bar and need to work in PSI or kPa, the conversion is simple: 1 bar = 14.504 PSI = 100 kPa.

Bar Conversion Formulas

bar → PSI:  PSI = bar × 14.5038
PSI → bar:  bar = PSI ÷ 14.5038
bar → kPa:  kPa = bar × 100  (exact)
kPa → bar:  bar = kPa ÷ 100  (exact)
bar → atm:  atm = bar ÷ 1.01325
atm → bar:  bar = atm × 1.01325

Real-world bar reference:
  1 atm ≈ 1.013 bar
  Espresso pump: 9 bar = 130.5 PSI
  Scuba tank full: 200–300 bar
  Car tire: ~2.2–2.5 bar

Where Bar Is Used

European automotive: tire pressure specifications on European vehicles use bar. A typical passenger car calls for 2.2–2.5 bar (32–36 PSI). Espresso machines: the industry standard for quality espresso extraction is 9 bar (130 PSI) of pump pressure. Scuba diving: tanks are filled to 200 bar (2,900 PSI) or 300 bar (4,350 PSI) depending on the cylinder rating. A diver monitors remaining air in bar.

Industrial applications: pneumatic systems, hydraulic test equipment, compressors, and pressure vessels in Europe and internationally are rated in bar or millibar (mbar). Meteorology: weather stations report barometric pressure in mbar (= hPa); 1013 mbar = 1013 hPa = 1.013 bar = standard atmospheric pressure. Food industry: high-pressure processing (HPP) to preserve food without heat uses pressures up to 6,000 bar.

Bar vs PSI vs kPa

All three measure pressure but are used in different contexts: PSI (US, UK), bar (Europe, industrial, scuba), kPa (strict SI, metric countries, medical). The three form a simple hierarchy: 1 bar = 100 kPa = 14.504 PSI. Standard atmosphere = 1.013 bar = 101.3 kPa = 14.696 PSI. For everyday reference, bar and atm are interchangeable to within 1.3%.

Confusion between these units is most common in automotive contexts: a European vehicle manual specifying '2.3 bar' tire pressure is equivalent to '230 kPa' or '33.4 PSI.' If you have a PSI-only tire gauge, multiply bar by 14.5 to convert. If your gauge reads in kPa, multiply bar by 100.

Frequently Asked Questions

Is bar an SI unit?

No, bar is not an official SI unit. The SI unit of pressure is the pascal (Pa) and its multiples (kPa, MPa). However, bar is recognized by the International Bureau of Weights and Measures (BIPM) as acceptable alongside SI, and it is widely used in industry, meteorology, and diving because its magnitude is convenient — 1 bar ≈ 1 atm.

What is 9 bar in PSI?

9 bar × 14.5038 = 130.5 PSI. This is the standard pressure used in commercial espresso machines for extraction. Home espresso machines typically operate at 9–15 bar.

What is millibar (mbar)?

1 mbar = 0.001 bar = 0.1 kPa = 100 Pa. Millibar is numerically equal to hectopascal (hPa), which weather services use for barometric pressure. Standard atmospheric pressure = 1013.25 mbar = 1013.25 hPa.

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