Convert Atmosphere to Millimeter of Mercury (atm → mmHg)

Converting atmospheres to millimeters of mercury is straightforward: multiply by the conversion factor. Follow these three steps to do it by hand or in your head.

1. 1
   Take your value in atmospheres

   Start with the number of atmospheres (atm) you want to convert.
2. 2
   Multiply by 759.99989

   The conversion factor from atm to mmHg is 759.99989. Multiply your value by this number.
3. 3
   Read the result in millimeters of mercury

   The result is your value in millimeters of mercury (mmHg).

Multiply the value in atmospheres by 759.99989. For the reverse direction, multiply by 0.001316.

• ✓1 atm = 101,325 Pa = 1.01325 bar = 14.696 psi.
• ✓Note: atm ≠ bar. Close but not identical.
• ✓Modern SI prefers pascals; atm is a legacy reference.

• ✗Using 1 atm = 1 bar exactly — off by 1.3%.
• ✗Confusing standard atm with technical atmosphere (98,066.5 Pa) or atmosphere-absolute in diving.
• ✗Applying 1 atm outside sea level without correction.

What is the Atmosphere?

The atmosphere equals exactly 101,325 pascals (the average atmospheric pressure at sea level, latitude 45°) and is a reference unit in chemistry, diving, and engineering. Defined for scientific convenience to represent 'standard atmospheric pressure,' it is widely used in chemistry (gas laws, reaction conditions), aviation (cabin pressure relative to ambient), and scuba diving (depth pressure: every 10 m of seawater adds about 1 atm). Standard reference conditions in chemistry often specify 1 atm and 25°C. The atmosphere relates to the pascal (101,325 Pa = 1 atm), the kilopascal (101.325 kPa = 1 atm), the bar (1.01325 bar = 1 atm), the psi (14.696 psi = 1 atm), the torr (760 torr = 1 atm), and the meter of seawater (10.33 mH₂O = 1 atm). The 'technical atmosphere' (1 at = 98.066 kPa = 1 kgf/cm²) is a slightly different historical unit no longer in use.

• Chemistry standard conditions
• Aviation cockpit pressurisation references
• Pressure tank and vessel ratings

What is the Millimeter of Mercury?

The millimeter of mercury equals approximately 133.322 pascals and is the universal medical unit for blood pressure measurements worldwide. Normal blood pressure is around 120/80 mmHg (systolic/diastolic). The unit's origin is the manometer: a column of mercury rises 1 mm for every 133 Pa of pressure difference. Mercury barometers historically measured atmospheric pressure (760 mmHg = 1 atm at sea level). Beyond medicine, mmHg appears in vacuum-system specifications, certain laboratory contexts, and altimeter settings (in inHg in the US, in mmHg or hPa elsewhere). The mmHg relates to the torr (1 mmHg ≈ 1.0000003 torr — essentially identical), the pascal (1 mmHg ≈ 133.3 Pa), the atmosphere (760 mmHg = 1 atm), and inches of mercury (1 inHg = 25.4 mmHg). The unit's persistence in medicine reflects historical inertia and the precision of mercury manometers.

• Blood pressure measurement globally
• Vacuum-system specifications
• Historical scientific texts