This example problem demonstrates how to convert the pressure units millibar (mbar) 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 and 1 millibar is 1/1000 bar. Combining these factors gives a conversion factor of 1 atm = 1013.25 mbar.

### Key Takeaways: Millibars to Atmospheres Pressure Conversion

- Millibars (mbar) and atmospheres (atm) are two common units of pressure.
- You can use either of two conversion formulas to convert between millibars and atmospheres.
- 1 millibar = 9.869x10
^{-4}atm - 1 atm = 1013.25 mbar
- Remember, the number in mbar will be about a thousand times larger than the equivalent value in atm. Alternatively, converting from mbar to atm will yield a number about a thousand times smaller.
- When performing unit conversions, check your answer to make sure it makes sense, convert it to scientific notation if practical, and use the same number of significant digits as the original value.

### mbar to atm Conversion Problem #1

The air pressure outside a cruising jetliner is approximately 230 mbar. What is this pressure in atmospheres?

**Solution:**

1 atm = 1013.25 mbar

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

pressure in atm = (pressure in mbar) x (1 atm/1013.25 mbar)

pressure in atm = (230/1013.25) atm

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

The air pressure at cruising altitude is 0.227 atm.

### mbar to atm Conversion Problem #2

A gauge reads 4500 mbar. Convert this pressure into atm.

**Solution:**

Again, use the conversion:

1 atm = 1013.25 mbar

Set up the equation to cancel out the mbar units, leaving atm:

pressure in atm = (pressure in mbar) x (1 atm/1013.25 mbar)

pressure in atm = (4500/1013.25) atm

pressure = 4.44 atm

### mbar to atm Conversion Problem #3

Of course, you can use the millibar to atmosphere conversion, too:

1 mbar = 0.000986923267 atm

This may also be written using scientific notation:

1 mbar = 9.869 x 10^{-4} atm

Convert 3.98 x 10^{5} mbar into atm.

**Solution:**

Set up the problem to cancel out the millibar units, leaving the answer in atmospheres:

pressure in atm = pressure in mbar x 9.869 x 10^{-4} atm/mbar

pressure in atm = 3.98 x 10^{5} mbar x 9.869 x 10^{-4} atm/mbar

pressure in atm = 3.9279 x 10^{2} atm

pressure in atm = 39.28 atm

or

pressure in atm = pressure in mbar x 0.000986923267 atm/mbar

pressure in atm = 398000 x 0.000986923267 atm/mbar

pressure in atm = 39.28 atm

Need to work the conversion the other way? Here is how to convert atm to mbar

### About Pressure Conversions

Pressure unit conversions are one of the most common types of conversions because barometers (the instruments used to measure pressure) use any of a number of units, depending on their country of manufacture, the method used to measure pressure, and intended use. Beside mbar and atm, units you may encounter include torr (1/760 atm), millimeters of mercury (mm Hg), centimeters of water (cm H_{2}O), bars, foot sea water (FSW), meter sea water (MSW), Pascal (Pa), newtons per square meter (which is also a Pascal), hectopascal (hPa), ounce-force, pound-force, and pounds per square inch (PSI). A system that is under pressure has the ability to do work, so another way to express pressure is in terms of stored potential energy per unit volume. Thus, there are also units of pressure relating to energy density, such as joules per cubic meter.

The formula for pressure is force per area:

P = F/A

where P is pressure, F is force, and A is area. Pressure is a scalar quantity, meaning it has a magnitude, but not a direction.

### Sources

- Giancoli, Douglas G. (2004).
*Physics: principles with applications*. Upper Saddle River, N.J.: Pearson Education. ISBN 978-0-13-060620-4. - International Bureau of Weights and Measures (2006).
*The International System of Units*(SI), 8th ed. p. 127. ISBN 92-822-2213-6. - Klein, Herbert Arthur. (1988).
*The Science of Measurement: a Historical Survey*. Mineola, NY: Dover Publications 0-4862-5839-4. - McNaught, A. D.; Wilkinson, A.; Nic, M.; Jirat, J.; Kosata, B.; Jenkins, A. (2014).
*IUPAC. Compendium of Chemical Terminology*, 2nd ed. (the "Gold Book"). 2.3.3. Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.P04819 - Resnick, Robert; Halliday, David (1960).
*Physics for Students of Science and Engineering Part 1*. New York: Wiley. p. 364.