Millimeter Of Water (mmH₂O)

ScientificCurrent unit

Exactly 9.80665 pascals by convention. Used for very low pressures.

1 millimeter of water = 9.80665 pascals (exactly).

About the millimeter of water

A column of water one millimetre tall presses down with 9.80665 Pa under the usual convention, which assumes a density of 1,000 kg/m3 for water at its densest, near 4 degrees Celsius, and standard gravity. NIST marks this conventional millimeter of water as an exact factor. Using the true density of water at 4 degrees Celsius, 999.972 kg/m3, gives about 9.80638 Pa instead, so the two differ in the fifth digit.

Because the unit is so small it suits very low pressures, such as the draft in a duct or a chimney, filter drops and sensitive gauges. It also gives waterproof ratings for fabric, where a tent or jacket rated to some number of millimetres can resist that height of water before leaking. Pressure in this form is often written mmH2O, mmWG or mmWC. Ten millimetres of water is about 98 Pa, or roughly 0.1 percent of one standard atmosphere.

Sources: Wikipedia: Millimetre of water; NIST SP 811, Appendix B.8

Millimeter Of Water conversion chart

1 millimeter of water expressed in every other pressure unit:

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