These example problems demonstrate how to convert the pressure unit bar (bar) to atmospheres (atm). Atmosphere originally was a unit related to the air pressure at sea level. It was later defined as 1.01325 x 10^{5} pascals. A bar is a pressure unit defined as 100 kilopascals. This makes one atmosphere nearly equal to one bar, specifically: 1 atm = 1.01325 bar. But, sometimes you need to find the exact conversion.

## Helpful Tip Convert bar to atm

When converting bar to atm, the answer in atmospheres should be slightly lower than the original value in bars.

## Bar to Atm Pressure Conversion Problem #1

The air pressure outside a cruising jetliner is approximately 0.23 bar. What is this pressure in atmospheres?

**Solution:**

1 atm = 1.01325 bar

Set up the conversion to the desired unit will be canceled out. In this case, we want atm to be the remaining unit.

pressure in atm = (pressure in bar) x (1 atm/1.01325 bar)

pressure in atm = (0.23/1.01325) atm

pressure in atm = 0.227 atm**Answer:**

The air pressure at cruising altitude is 0.227 atm.

Check your answer. The answer in atmospheres should be slightly less than the answer in bars.

bar > atm

0.23 bar > 0.227 atm

## Bar to Atm Pressure Conversion Problem #2

Convert 55.6 bars into the atmosphere.

**Use the conversion factor:**

1 atm = 1.01325 bar

Again, set up the problem so the bar units cancel out, leaving atm:

pressure in atm = (pressure in bar) x (1 atm/1.01325 bar)

pressure in atm = (55.6/1.01325) atm

pressure in atm =54.87 atm

bar > atm (numerically)

55.6 bar > 54.87 atm

## Bar to Atm Pressure Conversion Problem #3

You can also use the bar to atm conversion factor:

1 bar = 0.986923267 atm

Convert 3.77 bar into atmospheres.

pressure in atm = (pressure in bar) x (0.9869 atm/bar)

pressure in atm = 3.77 bar x 0.9869 atm/bar

pressure in atm = 3.72 atm

## Notes About Units

The atmosphere is considered to be an established constant. This does not mean that the actual pressure at any point at sea level actually will be identical to 1 atm. Similarly, STP or Standard Temperature and Pressure is a standard or defined value, not necessarily equal to actual values. STP is 1 atm at 273 K.

When looking at pressure units and their abbreviations, be careful not to confuse bar with barye. Barye is the centimeter-gram-second of CGS unit of pressure, equal to 0.1 Pa or 1x10^{-6} bar. The abbreviation for the barye unit is Ba.

Another potentially confusing unit is Bar(g) or barg. This is a unit of gauge pressure or pressure in bars above atmospheric pressure.

The units bar and millibar were introduced in 1909 by the British meteorologist William Napier Shaw. Although the bar is still an accepted unit by some European Union countries, it has largely been deprecated in favor of other pressure units. Engineers largely use a bar as a unit when recording data in pascals would produce large numbers. The boost of turbo-powered engines is often expressed in bars. Oceanographers may measure the pressure of seawater in decibars because the pressure in the ocean increases roughly 1 dbar per meter.