A centimetre of water (US spelling centimeter of water, abbreviated cm H2O or cm H2O) is a less commonly used unit of pressure derived from pressure head calculations using metrology. It may be defined as the pressure exerted by a column of water of 1 cm in height at 4 °C (temperature of maximum density) at the standard acceleration of gravity, so that 1 cmH2O (4°C) = 999.9720 kg/m3 × 9.80665 m/s2 / 100 = 98.0638 Pa, but conventionally a nominal maximum water density of 1000 kg/m³ is used, giving 98.0665 Pa.
It is frequently used to measure the central venous pressure, the intracranial pressure while sampling cerebrospinal fluid, as well as determining pressures during mechanical ventilation or in water supply networks (then usually in metres water column). It is also a common unit of pressure in the speech sciences. This unit is commonly used to specify the pressure to which a CPAP machine is set after a polysomnogram.