A centimetre or millimetre of water (US spelling centimeter or millimeter of water) are less commonly used measures of pressure derived from pressure head. Centimetre of water A centimetre of water (US spelling centimeter of water) 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 cm = ≈ , but conventionally a nominal maximum water density of is used, giving . 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. {| |- | || = 98.0665 pascals |- |rowspan=9| |= 0.01 metre water (mH2O), metre water column (m wc) or metre water gauge (m wg) |- |= 10 mm wg |- |= mbar or hPa |- |≈ inH2O |- |≈ atm |- |≈ torr |- |≈ mm Hg |- |≈ inHg |- |≈ psi |} Millimetre of water Millimetre of water (US spelling millimeter of water) 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 mm = ≈ , but conventionally a nominal maximum water density of is used, giving . {| |- | || = 9.80665 pascals |- |rowspan=9| |= 0.001 metre water (mH2O), metre water column (m.wc) or metre water gauge (m wg) |- |= 0.1 cm wg |- |= mbar or hPa |- |≈ inH2O |- |≈ |- |≈ torr |- |≈ mmHg |- |≈ inHg |- |≈ psi |} In limited and largely historic contexts it may vary with temperature, using the equation: P = ρ·g·h/1000, where P: pressure in Pa ρ: density of water (conventionally 1000 kg/m3 at 4 °C) g: acceleration due to gravity (conventionally 9.80665 m/s2 but sometimes locally determined) h: water height in millimetres. The unit is often used to describe how much water