Converting Millibars to Atmospheres

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 10⁵ pascals. The 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.

Problem:

The air pressure outside a cruising jet liner 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 cancelled 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.