Convert Millimeters of Mercury to Atmospheres

The Millimeters of Mercury (symbol: mmHg) is a unit of pressure. A unit of pressure historically defined as the pressure exerted by a column of mercury exactly 1 millimeter tall at the standard acceleration of gravity (9.80665 m/s²) and the standard density of mercury (13,595.1 kg/m³ at 0 °C). The mmHg's contemporary definition rounds to exactly 133.322387415 Pa (NIST SP 811). Despite being non-SI, mmHg remains the international clinical standard for blood pressure measurement ("120 over 80" mmHg) per WHO and ESH guidelines, and is the default unit for intraocular pressure in ophthalmology (normal IOP 10-21 mmHg per Glaucoma Research Foundation) and intracranial pressure monitoring in neurology. Numerically the mmHg is almost identical to the torr (1 mmHg ≈ 1.0000007 torr — the difference is whether one uses the conventional density or the experimentally measured density of mercury). Barometric atmospheric pressure was historically quoted in mmHg before kPa/hPa adoption (760 mmHg = 1 atm = 101.325 kPa).

It belongs to the metric measurement system.

Millimeters of Mercury are commonly used in tire inflation, weather reporting, scuba diving, and engineering.

The Atmosphere (symbol: atm) is a unit of pressure. A non-SI unit of pressure defined as exactly 101,325 pascals, corresponding to the standard sea-level atmospheric pressure as adopted by ISO 2533 and the BIPM SI Brochure. Atmospheres are the default reference pressure in physical chemistry (Standard Temperature and Pressure conventions: 1 atm at 273.15 K under IUPAC's 'STP' definition, though IUPAC has since moved to 1 bar STP), in commercial diving (depths are quoted in atmospheres absolute, with each 10 m of seawater adding ~1 atm), and in pressure-sensitive materials research where vapor pressure and saturation data are tabulated at 1 atm. A common pitfall: the "technical atmosphere" (at, 98,066.5 Pa = 1 kgf/cm²) used in some legacy European and Japanese engineering is NOT the same as the standard atmosphere — older pressure-vessel drawings can read 100 at and mean either depending on context. 1 atm = 1.01325 bar = 14.696 psi = 760 mmHg = 760 torr.

Atmospheres are commonly used in tire inflation, weather reporting, scuba diving, and engineering.