# Centimetre of water

A centimetre of water (US spelling centimeter of water, abbreviated cmH
2
O
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 × 1 cm = 98.063754138 Pa98.0638 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.

 1 cmH2O (conventional) = 98.0665 pascals = 0.01 metre water (mH2O), metre water column (m wc) or metre water gauge (m wg) = 10 mm wg = 0.980665 mbar or hPa ≈ 0.393700787401575 inH2O ≈ 0.000967838 atm ≈ 0.735559240069085 torr ≈ 0.73555913527668 mm Hg ≈ 0.0289590210738851 inHg ≈ 0.0142233433071196 psi