Inch of mercury (inHg): a historical unit of pressure

Definition and physical basis

By definition an inch of mercury equals the pressure from a 1‑inch (2.54 cm) column of mercury at 0 °C under the standard acceleration of gravity. Because the pressure depends on mercury density and gravity, the value is explicitly tied to the reference temperature and gravity. The symbol inHg is commonly used in technical data and instrument dials.

Conversions and representative values

• 1 inHg ≈ 3386.39 pascals (Pa) — frequently quoted as ≈ 33.8639 hectopascals (hPa) or millibars.
• 1 inHg ≈ 0.4912 pounds per square inch (psi).
• Standard sea‑level atmospheric pressure is about 29.92 inHg (the conventional altimeter setting for ‘‘standard atmosphere’’).

Uses and practical examples

In meteorology, inHg was the traditional unit for barometric pressure in English‑language forecasts and observations; many instruments and historical records use it. In aviation, some national systems (notably the United States) use inHg for altimeter settings and local pressure reports while other jurisdictions use hectopascals. Mechanics and HVAC technicians sometimes report vacuum or gauge pressure in inches of mercury (for example, negative pressures measured in inHg vacuum), and laboratory manometers historically used mercury columns to indicate pressure differences.

History and reasons for decline

Mercury barometers and manometers provided a straightforward physical realization of pressure decades before electrical transducers became common. The widespread availability of mercury instruments made the inch (and millimetre) of mercury convenient. Concerns over mercury toxicity, improved precision of electronic sensors, and international standardization on SI units (the pascal) have driven a decline in routine use of inHg. Many operational practices persist, however, because of legacy instrumentation and training.

Practical considerations and limitations

Because the defined value depends on temperature and gravity, readings taken without correction can differ slightly from the reference value; instrument calibration and temperature compensation are important for precision. Handling mercury instruments also poses environmental and health hazards, which contributed to the unit's replacement in many fields. For general background on pressure units and device types see non‑SI units, meteorology references at weather, and aviation sources at aviation. For SI conversion and definitions consult materials about the pascal and the properties of mercury, the length of an inch, and the role of gravity in pressure measurement.