What Is a Bar?
The complete guide to the bar as a unit of pressure, including conversions and practical applications.
Last updated: 2026-03-15
Definition
A bar is a metric unit of pressure defined as exactly 100,000 pascals (100 kPa or 0.1 MPa). It is not an SI unit but is accepted for use with the SI system.
1 bar = 100,000 Pa = 100 kPa = 0.986923 atm ≈ 14.5038 PSI
The name comes from the Greek word "baros" meaning weight or pressure.
Bar Conversions
| Unit | 1 Bar Equals |
|---|---|
| Pascals (Pa) | 100,000 |
| Kilopascals (kPa) | 100 |
| Atmospheres (atm) | 0.9869 |
| PSI | 14.5038 |
| mmHg (Torr) | 750.06 |
| Millibar (mbar) | 1,000 |
Where the Bar Is Used
Tire Pressure
European vehicles display recommended tire pressure in bar (typically 2.0-3.0 bar for passenger cars). US gauges show PSI. A tire at 2.5 bar is at about 36 PSI.
Weather Forecasting
Meteorologists use millibars (mbar) or hectopascals (hPa) for atmospheric pressure. Standard sea-level pressure is 1,013.25 mbar. Low pressure systems (storms) might drop to 980-990 mbar.
Scuba Diving
Dive tanks are rated in bar (typically 200-300 bar). Divers monitor tank pressure throughout a dive. A full aluminum tank holds about 200 bar.
Coffee Machines
Espresso machines operate at 9-15 bar of pressure. This high pressure forces water through finely ground coffee to produce espresso. The "9 bar" standard is considered optimal for extraction.
History
The bar was introduced by Norwegian meteorologist Vilhelm Bjerknes in 1909. Though the SI system uses pascals as the official pressure unit, the bar remains popular because 1 bar is conveniently close to 1 atmosphere, making it intuitive for everyday pressure readings.
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