mmHg – Millimetres of Mercury at 0 deg C Pressure Unit

Millimeters of Mercury at a temperature of zero degrees Celsius is a small Metric pressure unit which is derived from the hydrostatic pressure generated by a 1 mm tall column of mercury liquid. 1 Millimeter of Mercury equals 133.322… Pascals.

The mmHg pressure unit is mostly used in medical industry particularly for measuring blood pressure where tradionally a liquid manometer was used as part of blood pressure testing equipment. Electronic means of measuring blood pressure are now more commonplace but the units continues to be used by the medical profession.

The pressure that is generated by a fixed height of mercury which will vary depending on the ambient temperature and local gravity, therefore it is not an absolute pressure conversion and the preferred reference values of temperature and density can vary from country to country. The conversion factors listed below are derived from mercury with a density of 13,595.1 kg/m³ and a local gravity of 9.80665 m/s².

Please use the table below to find conversion factors in other units for 1 Millimetre of Mercury.

To convert into mmHg from other pressure units please follow the relevant link below.

Multiple calculations can be conducted easily using our online pressure conversion tool.

See how mmHg is calculated from SI units or check the various forms of writing mmHg.

mmHg Pressure Conversion Factors

Please note that the conversion factors above are accurate to 6 significant figures.

mmHg Derivation

The calculation below shows how the pressure unit Millimetres of Mercury (mmHg) is derived from SI Units.


  • Pressure = Force / Area
  • Force = Mass x Acceleration
  • Mass = Density x Volume
  • Volume = Area x Height
  • Acceleration = Distance / (Second x Second)

SI Units

  • Mass: Kilogram (kg)
  • Distance: Metre (m)
  • Time: Second (s)
  • Force: Newton (N)
  • Pressure: Pascal (Pa)

Input Values

  • Density = Mercury Density at 0degC = 13595.1 kg/m³
  • Area = 1 m²
  • Height = 1 mm = 0.001 m
  • Acceleration = Standard Gravity = 9.80665 m/s²


  • 1 mmHg Mass = 13595.1 kg/m³ x 1 m² x 0.001 m = 13.5951 kg
  • 1 mmHg Force = 13.5951 kg x 9.80665 m/s² = 133.3223874 N
  • 1 mmHg Pressure = 133.3223874 N / 1 m² = 133.3223874 Pa

mmHg Alternate Descriptions

These are the different versions used for identifying mmHg that you may find elsewhere.

  • Millimetres of Mercury
  • Millimeters of Mercury
  • Millimetres of Mercury Column
  • Millimeters of Mercury Column
  • mmHg
  • mm Hg

mmHg Help

Do you have a question about mmHg?

870 mmHg to atm

What is the conversion of 870 mmHg into atm units?

870 mmHg = 1.14474 atm.

0-1740mmHg to atm Conversion Scale

Does Millimetres of Mercury (mmHg) = Torr

Are mmHg units really equal to Torr? I have the following information that appears to contradict this:

1 mmHg = 1.00000015001 Torr
1 Torr = 0.9999998499900226 mmHg

1 hPa = 0.750061561303 mmHg
1 hPa = 0.750061673821 Torr

mmHg to Torr conversion

There is also this:

1 Pa = 0.0075006168271 Torr
1 Pa = 0.0075006157585 mmHg

Torr discussion on wiki

Though I think most just use these for the sake of easy math:

1 hPa = .75 mmHg
1 hPa = .02953 inHg

The value of 1 Torr is an unambiguous calculation with no variable environmental factors and is clearly defined as the 1 atm/760 or 101325Pa/760 which equals 133.322368421052631578947 Pascals (calculate this on Wolfram Alpha).

So in theory every conversion should be the same albeit some variation in precision.  For example our pressure converter uses a 9 digit precision and calculates 133.322387 Pascals as the answer.

Millimetres of Mercury (mmHg) is a little more tricky since different values are used for the density of mercury.  For our conversions we use a widely accepted figure of 13,595.1 kg/m³ which is the density of mercury at 0 degrees Celsius.  If you then use standard local gravity of 9.80665 m/s², the conversion from liquid column to pressure is  133.322387415 Pascals.

So there is a small difference between 1 Torr and 1 mmHg, but it only shows if you are comparing conversion values of a precision of 8 significant figures or more.  This level of precision is much greater than what most are looking for, so many resources show the two values as the same e.g. 133.322 Pascals.

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