Convert Millimeter of Water to Inch of mercury (mmH2O to inHg)

Pressure is the force applied perpendicular to a surface per unit area: P = F / A. The SI unit is the pascal (Pa), equal to one newton per square metre (1 Pa = 1 N/m²). Practical applications use larger units: the bar (1 bar = 100,000 Pa, close to standard atmospheric pressure), atmosphere (atm) (1 atm = 101,325 Pa), and psi (pounds per square inch, used in the US for tire and gas pressures). Blood pressure is measured in mmHg (millimetres of mercury), also called torr.

The millimeter of water (symbol: mmH₂O or mmH2O) is a non-SI unit of pressure defined as the pressure exerted by a one-millimeter column of water under standard gravity. It is commonly used in ventilation systems, clean rooms, HVAC engineering, and laboratory instruments to measure small pressure differences. Although the pascal is the SI unit of pressure, the millimeter of water remains a practical unit for low-pressure applications. It is based on the concept of a water column.

One millimeter of water is equal to 9.80665 pascals, 0.1 centimeter of water, 0.0000980665 bar, 0.00142233 pounds per square inch, and 0.00980665 kilopascal. The millimeter of water is widely used for measuring very low gas and liquid pressures.

The inch of mercury (symbol: inHg) is a non-SI unit of pressure based on the height of a mercury column one inch high under standard gravity. It is widely used in aviation, meteorology, and vacuum measurement, particularly in the United States and Canada. Although the pascal is the SI unit of pressure, the inch of mercury remains an important unit in many industries. More information is available in the inch of mercury reference.

One inch of mercury is equal to 3,386.39 pascals, 3.38639 kilopascals, 0.0338639 bar, 0.491154 psi, and 25.4 millimeters of mercury. The inch of mercury is commonly used for barometric pressure and vacuum measurements.