# Centimetre 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 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.

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[1] = 0.01 metre water (mH2O), metre water column (m.wc) or metre water gauge (m wg) = 10 mm wg ≈ 0.980665 mbar or hPa ≈ 0.39370 inH2O ≈ 0.000967838 atm ≈ 0.73556 torr ≈ 0.735559 mm Hg ≈ 0.0289590 inHg ≈ 0.0142233 psi