What Is Bar?
A bar is a unit that tells us how strong a push or squeeze is on a surface. This push is called pressure. One bar is close to the air pressure at sea level on Earth.
People use bar to talk about things like air pressure in the weather, pressure in car tires, and the pressure of water or gas in pipes and tanks.
Definition
Pressure means how much force pushes on each small area of a surface.
The bar is defined exactly as:
- 1 bar = 100 000 pascals (Pa)
A pascal is the standard SI unit of pressure. So one bar is one hundred thousand pascals. This is a fixed and exact number, not an estimate.
Compared with other common pressure units, 1 bar is about:
- 0.987 atmospheres (atm)
- 14.5 pounds per square inch (psi)
- 750 millimeters of mercury (mmHg)
These values are rounded to make them easy to remember.
History / Origin
The word bar comes from the Greek word baros, which means weight. It was chosen because pressure is like the weight of air or water pressing on a surface.
The bar was created in the early 1900s by meteorologists, the scientists who study weather. They wanted a simple unit that was close to normal air pressure at sea level. This made it easy to talk about and compare weather maps and storms.
At that time, many people used millimeters of mercury from glass tubes filled with liquid. These were hard to read and not very simple for math. The bar and its smaller form, the millibar, gave scientists a clearer and easier way to measure and share pressure values.
Symbol & Abbreviation
The bar is very simple to write.
- Name bar
- Symbol bar
- Plural bar or bars, for example 2 bar or 5 bars
Common related symbols you might see are:
- mbar for millibar, which is one thousandth of a bar
- bar(g) or barg for gauge pressure in bar, pressure above local air pressure
- bar(a) or bara for absolute pressure in bar, total pressure counted from a perfect vacuum
The standard SI unit of pressure is the pascal with symbol Pa, not the bar. However, bar is still widely used together with SI units in many fields.
Current Use Around the World
Even though the official SI unit is the pascal, bar is still popular because it is easy to picture and the numbers are not too big or too small.
Weather and atmosphere
- Meteorologists used millibar for many years to show air pressure on weather maps.
- Today, many weather services use the hectopascal (hPa), which has the same number as the millibar, but some still mention millibar or bar in reports.
Engineering and industry
- Engineers use bar to show pressure in gas systems, pumps, compressors, and hydraulic machines.
- Industrial equipment like pressure gauges and control valves are often marked in bar, sometimes together with psi or kPa.
Diving
- Scuba divers use bar to talk about the pressure of their air tanks and the water pressure at different depths.
- At the surface, pressure is about 1 bar. Underwater it rises by about 1 bar for every 10 meters of depth in seawater.
Everyday life
- Car and bike tire pressures are often written in bar on the tire sidewall or in the car manual.
- Some household appliances like pressure washers and espresso machines show pressure in bar.
Regional habits
- In Europe, bar and kilopascal (kPa) are both very common.
- In the United States, psi is common, but bar is still used in some technical fields and by divers.
- Global scientific papers prefer pascal, but may include bar for clarity.
Example Conversions
Here are some simple conversions to help you understand how big one bar is.
Between bar and pascal
- 1 bar = 100 000 Pa
- 0.5 bar = 50 000 Pa
- 2 bar = 200 000 Pa
Between bar and kilopascal
- 1 bar = 100 kPa
- 2.5 bar = 250 kPa
- 0.8 bar = 80 kPa
Between bar and atmosphere
- 1 bar ≈ 0.987 atm
- 1 atm ≈ 1.013 bar
Between bar and psi
- 1 bar ≈ 14.5 psi
- 2 bar ≈ 29.0 psi
- 3 bar ≈ 43.5 psi
Between bar and millibar
- 1 bar = 1 000 mbar
- 0.995 bar = 995 mbar, this is near average sea level air pressure
Everyday examples
- Typical car tire, about 2 to 2.5 bar
- Mountain bike tire, about 2 to 4 bar
- Road bike tire, often 6 to 8 bar
- Espresso machine pump, around 9 bar at the coffee puck
Related Units
Here are some other units that measure pressure and how they relate to the bar.
- Pascal (Pa) the main SI unit.
- 1 bar = 100 000 Pa
- Kilopascal (kPa) one thousand pascals.
- 1 bar = 100 kPa
- Megapascal (MPa) one million pascals.
- 1 bar = 0.1 MPa
- Atmosphere (atm) based on average air pressure at sea level.
- 1 bar ≈ 0.987 atm
- Pound per square inch (psi) common in the United States.
- 1 bar ≈ 14.5 psi
- Millibar (mbar) one thousandth of a bar, used in weather reports.
- 1 bar = 1 000 mbar
- Hectopascal (hPa) equal in value to a millibar, used in modern meteorology.
- 1 hPa = 1 mbar
- 1 bar = 1 000 hPa
- Millimeter of mercury (mmHg) old unit based on the height of a column of liquid mercury.
- 1 bar ≈ 750 mmHg
FAQs
Is bar an SI unit?
No. Bar is not an SI base unit. The official SI unit for pressure is the pascal (Pa). However, bar is still allowed and widely used together with SI units because it is practical and easy to understand.
Why do many people still use bar instead of pascal?
Pascal is a very small unit. Normal air pressure is about 100 000 Pa, which is a large number. In bar this is just about 1 bar, which is much easier to read and remember. This is why bar and related units like millibar and bar gauge are still popular.
What is normal air pressure in bar?
Average air pressure at sea level on Earth is about 1.013 bar. Many people just say it is around 1 bar. This makes bar a handy unit for comparing different weather conditions and heights.
What is the difference between bar gauge and bar absolute?
Bar gauge (barg) shows how much higher the pressure is than the air around us. At normal air pressure the gauge reads 0 barg, even though the real pressure is about 1 bar.
Bar absolute (bara) measures from a perfect vacuum, which has zero pressure at all. At sea level the absolute pressure is about 1.013 bara.
So, if a pressure gauge shows 2 barg at sea level, the absolute pressure is about 3 bara.
How do I convert bar to psi quickly in my head?
A simple rule is:
- Multiply bar by 15 to get a rough answer in psi.
For example:
- 2 bar × 15 ≈ 30 psi
- 3 bar × 15 ≈ 45 psi
The exact factor is about 14.5, so this shortcut is very close for most everyday uses.
Where will I most likely see bar in daily life?
You will most likely see bar on:
- Car and bike tire pressure labels
- Pressure gauges on air compressors and pumps
- Scuba diving equipment and dive computers
- Espresso machines and some cooking tools like pressure cookers
Is 1 bar exactly equal to 1 atmosphere?
No. They are close but not the same. 1 bar is defined as 100 000 Pa. 1 atmosphere is defined as 101 325 Pa. This means:
- 1 bar ≈ 0.987 atm
- 1 atm ≈ 1.013 bar
For quick estimates they are sometimes treated as almost equal, but for science and engineering, the small difference matters and should be used correctly.
Why did weather reports move from millibar to hectopascal?
Millibar and hectopascal have exactly the same size. 1 mbar equals 1 hPa. Meteorology changed to hectopascal to match the SI system more closely, but the numbers stayed the same, so older data and newer data still match.
How deep underwater is 2 bar of absolute pressure?
At the surface, air pressure is about 1 bar. Underwater, pressure goes up by about 1 bar for every 10 meters of depth in seawater. So 2 bar absolute pressure happens at around 10 meters below the water surface.
This idea helps divers plan safe dives and understand what their equipment readings mean.