# Millimeters, water gauge

Millimeters, water gauge, also known as a millimetre of water (US spelling millimeter of water) or millimetres water column and abbreviated to mmwg, mmH2O or mmwc, respectively, is a less commonly used 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 Pa9.80638 Pa, but conventionally a nominal maximum water density of 1000 kg/m³ is used, giving 9.80665 Pa.

 1 mmH2O (conventional) = 9.80665 pascals[1] = 0.001 metre water (mH2O), metre water column (m.wc) or metre water gauge (m wg) = 0.1 cm wg = 0.0980665 mbar or hPa ≈ 0.0393700787401575 inH2O ≈ 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/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 rainwear or other outerwear can take or how much water a tent can resist without leaking.