Millibar (mbar)

What Is Millibar (mbar)?

A millibar, written as mbar, is a unit used to measure pressure. Pressure tells us how strongly air or a fluid pushes on a surface. Weather reports and pilots often use millibars to show how heavy the air is at a place and time.

When you hear that air pressure is 1013 millibars, it means the air is pressing on every part of the ground with a certain known strength that scientists can compare anywhere in the world.

Definition

A millibar is a metric unit of pressure. It is a very small part of a bar, which is a larger pressure unit.

• 1 bar equals 1000 millibars.
• 1 millibar equals 0.001 bar.
• 1 millibar equals 100 pascals, the main SI pressure unit.
• 1 millibar equals 1 hectopascal, so their numbers are the same.

Standard air pressure at sea level is about 1013.25 millibars. Many weather maps round this to 1013 millibars.

History / Origin

The word bar comes from the Greek word baros, which means weight. Scientists chose this word because pressure can be thought of as the weight of air or fluid pressing on a surface.

The bar and millibar were introduced in the early 1900s for weather science. Meteorologists needed a clear and handy way to show air pressure on maps, and the millibar gave numbers that were easy to read and write, usually around a few hundred or a little over one thousand.

Later, the International System of Units, called SI, chose the pascal as the main pressure unit. This made the bar and millibar non SI units. Even so, the millibar stayed very common in weather work. To match SI better, many weather centers now use the hectopascal. Since 1 millibar equals 1 hectopascal, the numbers on maps did not need to change.

Symbol & Abbreviation

The standard way to write millibar is:

• Name: millibar
• Symbol: mbar

Examples:

• 1013 mbar
• 950 mbar
• 500 mbar

Always write the number first, then a space, then mbar. Do not add extra letters or punctuation after the symbol.

Current Use Around the World

Millibar is not an official SI unit, but it is still widely used in practice, especially in meteorology and aviation.

Weather and climate

• Weather maps often show air pressure in millibars or in hectopascals, with the same numeric values.
• High pressure areas on maps might be around 1020 to 1030 mbar.
• Low pressure storms may drop below 1000 mbar, and very strong storms can go below 950 mbar.
• Upper air charts use levels like 500 mbar or 850 mbar to describe pressure at different heights above the ground.

Aviation

• Pilots use pressure settings for their altimeters, which show height.
• In many countries, these settings match sea level pressure, expressed in millibars or hectopascals.
• Standard pressure for flight levels is 1013.25 mbar, used worldwide for high altitude routes.

Science and education

• Some school and college books still mention millibar along with pascal.
• Climate records and old research papers often list air pressure in millibars, so students and scientists must understand the unit.

In many modern technical standards, the pascal is preferred. Still, in everyday weather information for the public, millibar or hectopascal values remain very common.

Example Conversions

To use millibar well, it helps to know how to convert it to other pressure units. Here are some simple rules and examples.

Basic relationships

• 1 mbar equals 100 Pa.
• 1 mbar equals 1 hPa.
• 1 bar equals 1000 mbar.
• 1 atm is about 1013.25 mbar.
• 1 mbar is about 0.0145 psi.

Millibar to pascal

Multiply by 100.

• 10 mbar equals 10 × 100 Pa equals 1000 Pa.
• 500 mbar equals 500 × 100 Pa equals 50000 Pa.
• 1013 mbar equals 1013 × 100 Pa equals 101300 Pa.

Millibar to hectopascal

The number stays the same because 1 mbar equals 1 hPa.

• 950 mbar equals 950 hPa.
• 1000 mbar equals 1000 hPa.
• 1013 mbar equals 1013 hPa.

Millibar to bar

Divide by 1000.

• 1 mbar equals 0.001 bar.
• 500 mbar equals 0.5 bar.
• 1013 mbar is about 1.013 bar.

Millibar to atmosphere

Divide by 1013.25.

• 1013.25 mbar equals 1 atm.
• 1013 mbar is about 1013 ÷ 1013.25 atm, which is very close to 1 atm.
• 900 mbar is about 900 ÷ 1013.25 atm, which is about 0.89 atm.

Millibar to psi

Multiply by about 0.0145 to get pounds per square inch, psi.

• 1 mbar is about 0.0145 psi.
• 1000 mbar is about 14.5 psi.
• 1013 mbar is about 14.7 psi, which matches normal air pressure at sea level.

Related Units

Millibar is part of a group of pressure units. These are closely related and often appear together.

• Pascal, Pa The main SI unit for pressure. 1 mbar equals 100 Pa.
• Hectopascal, hPa 1 hPa equals 100 Pa. Same size as 1 mbar. Weather reports often write hPa instead of mbar.
• Bar, bar A larger pressure unit. 1 bar equals 1000 mbar and 100000 Pa. Close to normal air pressure at sea level.
• Standard atmosphere, atm Based on average sea level pressure. 1 atm is defined as 101325 Pa, which is 1013.25 mbar.
• Pound per square inch, psi A pressure unit used mainly in the United States and in engineering. 1 mbar is about 0.0145 psi.
• Millimeter of mercury, mmHg or torr Older units used in medicine and physics. 1 atm equals 760 mmHg, which also equals 1013.25 mbar.

FAQs

What is a millibar in simple words

A millibar is a unit used to tell how strong the air or a gas is pressing on something. It is a way to measure air pressure. Weather people use it every day.

Is millibar the same as hectopascal

Yes, they are the same size. 1 millibar equals 1 hectopascal. Only the names and symbols are different. If you see 1000 mbar or 1000 hPa, the pressure is the same.

Is millibar an SI unit

No, the millibar is not an official SI unit. The correct SI unit for pressure is the pascal, Pa. Still, millibar is allowed for use with SI and is very common in meteorology.

What is normal air pressure in millibars

Normal air pressure at sea level is about 1013 millibars. It is a little more exactly 1013.25 millibars, but people usually round it to 1013.

Why do weather forecasts still use millibars

Weather professionals have used millibars for many years. The numbers are easy to work with, usually around 900 to 1050. Because 1 millibar equals 1 hectopascal, many services switched the name to hectopascal while keeping the same values.

How can I convert millibars to pascals quickly

To change millibars to pascals, just multiply by 100. For example, 980 mbar equals 980 × 100 Pa equals 98000 Pa. To go back, divide pascals by 100.

Where do I see millibars in real life

You might see millibars on weather apps, in TV weather reports, on printed weather maps, in pilot training books, and in climate data used at school or in science projects.

Can pressure be below 0 millibar

No, real physical pressure cannot go below zero. Zero pressure would mean no particles at all, a perfect vacuum. In practice, air pressure on Earth is always above zero millibar.