A centimetre or millimetre of water (US spelling centimeter or millimeter of water) are less commonly used measures of pressure based on the pressure head of water.
A centimetre of water[1] is a unit of pressure. 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 Pa ≈ 98.0638 Pa, but conventionally a nominal maximum water density of 1000 kg/m3 is used, giving 98.0665 Pa.
The centimetre of water unit 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.
Millimetre of water (US spelling millimeter of water)[3] is a unit of pressure. It may be defined as the pressure exerted by a column of water of 1 mm in height at 4 °C (temperature of maximum density) at the standard acceleration of gravity, so that 1 mmH2O (4 °C) = 999.9720 kg/m3 × 9.80665 m/s2 × 1 mm = 9.8063754138 Pa ≈ 9.80638 Pa, but conventionally a nominal maximum water density of 1000 kg/m3 is used, giving 9.80665 Pa.
In limited and largely historic contexts it may vary with temperature, using the equation:
The unit is often used to describe how much water rainwear or other outerwear can take or how much water a tent can resist without leaking.