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How to Calculate mmHg2011-09-01 13:13:00

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One mmHg is the pressure exerted by a 1 mm vertical column of mercury (Hg) at 0 degree Celsius. One mmHg is virtually equal to 1 torr, which is defined as 1/760 of 1 atmosphere (atm) pressure (i.e., 1 atm = 760 mmHg). The unit of mmHg is considered obsolete, and the SI unit "Pascal" (Pa; 1 atm = 101,325 Pa ) should be used. Nevertheless, mmHg is still widely used in medicine to express blood pressure. The steps below demonstrate several examples of calculating mmHg.Instructions

Things You'll Need

  • Calculator
    • 1

      Calculate the blood pressure of 120 mmHg using the basic definition of mmHg:

      Pressure = Hg density * Standard gravity * Mercury height

      Hg density is 13.5951 g/cm^3 (13595.1 kg/m^3), and standard gravity is 9.80665 m/s^2. Note that 120 mm is 0.12 m.

      Pressure = 13595.1 kg/m^3 * 9.80665 m/s^2 * 0.12 m = 15998.69 Pa

    • 2

      Obtain the relation between 1 Pa and 1 mmHg. Consider that
      1 atm = 101,325 Pa, and 1 atm = 760 mmHg. Hence
      101,325 Pa = 760 mmHg. Multiplying both sides of the equation by 1/760 you would get:

      1 mmHg = 1 Pa * 101,325 / 760

    • 3

      Find the formula to convert pressure in Pa to mmHg using the proportion:

      1 mmHg corresponds to 1 Pa * 101,325/760
      Pressure (mmHg) corresponds to Pressure (Pa)

      The solution of this proportion produces the formula:

      Pressure (mmHg) = Pressure (Pa) * 760 / 101,325 = Pressure (Pa) * 0.0075

    • 4

      Calculate the pressure of 35,000 Pa in mmHg using the formula from Step 3:

      Pressure = 35,000 Pa * 0.0075 = 262.5 mmHg

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