Convert Pressure from bar to millibar (bar to mbar)

What is a bar (bar)?

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.

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Origin of the Bar

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.

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Bar vs. Standard Atmosphere (atm)

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:

• 1 bar = 100,000 Pascals (Pa)
• 1 atmosphere (atm) = 101,325 Pascals (Pa)

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.

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Common Applications of the Bar

The bar is a versatile unit used to measure pressure in many industrial and everyday contexts.

Common examples include:

• Automotive: Measuring tire pressure (often listed on a car's door jamb alongside PSI).
• Scuba Diving: Expressing the amount of air pressure inside diving tanks.
• Weather: Used by meteorologists for weather forecasting and reading atmospheric pressure.
• Plumbing & Pumps: Measuring water pressure in pipes and pump systems.
• Industrial: Measuring air pressure in pneumatic equipment and gas pressure in cylinders for welding.

What is a millibar (mbar)?

A millibar (mbar) is a metric unit used to measure atmospheric pressure.

It's most commonly used in weather forecasting and reporting.

(Technically, one millibar is equal to 0.1 kilopascal, or kPa).

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The Role of Millibars in Weather Forecasting

Meteorologists use millibars as the primary unit to measure and report atmospheric pressure.

On weather maps, you'll see lines called isobars, which connect areas of equal pressure, drawn in millibar intervals.

Standard atmospheric pressure at sea level is about 1013 mbar.

This number is a key benchmark for weather conditions:

• High-Pressure System (Anticyclone): Generally above 1013 mbar, associated with clear skies and calm weather.
• Low-Pressure System (Cyclone): Generally below 1013 mbar, often bringing clouds, wind, and storms.

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Are Millibars and Hectopascals (hPa) the Same?

Yes, for all practical purposes, a millibar (mbar) and a hectopascal (hPa) are the same.

The two units are used interchangeably.

The conversion is simple: 1 mbar = 1 hPa.

While you'll still see millibars in public weather reports (especially in the U.S.), the hectopascal is the preferred SI (International System of Units) unit for global scientific consistency.

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How to Read Barometric Pressure in Millibars

A barometer measures barometric pressure—the weight of the air above you—using millibars.

Watching the change in this reading is key to predicting the weather:

• A falling barometer (decreasing mbar value) suggests a low-pressure system is approaching, bringing a higher chance of rain or storms.
• A rising barometer (increasing mbar value) typically signals a high-pressure system is moving in, meaning improving or fair weather.