### Centimetres of water

A centimetre of water (US spelling centimeter of water, abbreviated cmH2O or cm H2O) is a less commonly used unit of pressure derived from pressure head calculations using metrology. 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.

It is 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. That is 999.9720 [kg/m3] @ 4 °C × 9.80665 [m/s2] / 100 = 1 cmH2O (4°C) = 98.0638 Pa. Using the approximation 1000 [kg/m³] instead results in 98.0665 Pa.

This unit is commonly used to specify the pressure to which a CPAP machine is set after a polysomnogram.

 1 cmH2O (conventional) = 98.0665 pascals = 0.01 metres water column or meters, water gauge (m.Wg) = 10 mm.Wg ≈ 0.980665 mbar or hectopascals ≈ 0.39370 inH2O ≈ 0.000967838 atm ≈ 0.73556 torr ≈ 0.735559 mmHg ≈ 0.0289590 inHg ≈ 0.0142233 psi