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*)^{[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 cmH_{2}O (4°C) = 999.9720 kg/m^{3} × 9.80665 m/s^{2} × 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.

1 cmH _{2}O (conventional)= 98.0665 pascals ^{[2]}= 0.01 metre water (mH _{2}O), metre water column (m wc) or metre water gauge (m wg)= 10 mm wg = 0.980665 mbar or hPa ≈ 0.393700787401575 inH _{2}O≈ 0.000967838 atm ≈ 0.735559240069085 torr ≈ 0.73555913527668 mm Hg ≈ 0.0289590210738851 inHg ≈ 0.0142233433071196 psi

## Millimetre of water

*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 mmH_{2}O (4°C) = 999.9720 kg/m^{3} × 9.80665 m/s^{2} × 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.

1 mmH _{2}O (conventional)= 9.80665 pascals ^{[2]}= 0.001 metre water (mH _{2}O), metre water column (m.wc) or metre water gauge (m wg)= 0.1 cm wg = 0.0980665 mbar or hPa ≈ 0.0393700787401575 inH _{2}O≈ 9.67841105354059E-05 atm ≈ 0.0735559240069085 torr ≈ 0.073555913527668 mm Hg ≈ 0.00289590210738851 inHg ≈ 0.00142233433071196 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/m
^{3}at 4°C) - g: acceleration due to gravity (conventionally 9.80665 m/s
^{2}but sometimes locally determined) - h: water height in millimetres

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.

## See also

## References

## External links

- Pressure conversion calculator at Cornell University website