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The bar is a metric unit of pressure.
It is defined as exactly 100 kilopascals (kPa), or 100,000 Pascals (Pa).
Although it is not an official part of the International System of Units (SI), it is widely accepted for use with the SI. The bar is a popular unit for measuring pressure because it is very close to the average atmospheric pressure on Earth at sea level.
The term "bar" comes from the Greek word βάρος (baros), which means "weight."
The unit was introduced by British meteorologist William Napier Shaw in 1909. It is still widely used in meteorology, oceanography, and engineering.
A common point of confusion is the difference between a bar and a standard atmosphere (atm).
While they are very close in value, they are not the same:
This means 1 bar is approximately equal to 0.987 atm. Because it's so close to atmospheric pressure and is a round number (100 kPa), the bar is a very convenient unit for many applications.
The bar is a versatile unit used to measure pressure in many industrial and everyday contexts.
Common examples include: