Inches of water column (inWC) and millimeters of water column (mmWC) are non-SI units of pressure that express the pressure exerted at the base of a column of water. One inch of water column is equal to the pressure at the base of a water column exactly one inch high at 4°C, while one millimeter of water column corresponds to a water column exactly one millimeter high at the same temperature. One inch of water column is approximately equal to 249.089 Pascals. These units have been used to measure pressure since the early days of fluid mechanics and are particularly useful in applications where low-pressure differentials are involved.