The atmospheric pressure is often given in different units such as millimeters of mercury (mmHg), atmospheres (atm), and Pascals (Pa), with 1 atm being equal to 760 mmHg and 101,325 Pa. To convert atmospheric pressure to mm H₂O, one would multiply the mmHg value by approximately 13.6, reflecting the difference in density between mercury and water.
Atmospheric or barometric pressure is commonly reported in various units, such as millimeters of mercury (mmHg), atmospheres (atm), or Pascals (Pa). To convert from one pressure unit to another, we use established equivalencies: 1 atm is exactly equal to 760 mmHg, which is also equivalent to 101,325 Pa or 101.3 kPa.
A common unit to find atmospheric pressure reported in is millibars (mbar), which are directly related to Pascals. One millibar is equal to 100 Pascals, and since average atmospheric pressure at sea level is approximately 1.013 x 105 Pa, this is also equal to 1013 mbar or 1 atm. When we need to convert these units to mmHg or mm H2O (millimeters of water), we rely on these relationships. For example, to convert millibars to mmHg, you would first convert millibars to atm then multiply by 760 to find the equivalent in mmHg.
To convert atmospheric pressure to mm H2O, a different conversion is used because water is less dense than mercury. A commonly used conversion factor is that 1 mmHg is equivalent to approximately 13.6 mm H2O, since mercury is approximately 13.6 times denser than water. Therefore, multiplying the value in mmHg by 13.6 gives the approximate value in mm H2O.
1 atm = 760 mmHg = 1013 mbar, and when converting to mm H2O, we multiply the mmHg value by 13.6 to account for the density difference between mercury and water.