Millimeters Of Water to Pascals (mmH₂O to Pa)

1 millimeter of water equals 9.80665 pascals exactly.

How to convert millimeters of water to pascals

To convert millimeters of water to pascals, multiply the number of millimeters of water by 9.80665. This factor is exact. To go the other way, multiply pascals by 0.1019716213 (rounded).

pascals = millimeters of water × 9.80665

Example: 10 millimeters of water = 98.0665 pascals.

About these units

Millimeter Of Water (mmH₂O)

ScientificCurrent unit

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

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.

1 millimeter of water = 9.80665 pascals (exactly).

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

Pascal (Pa)

Metric (SI)Current unit

The SI unit of pressure: one newton per square meter. A very small unit; atmospheric pressure is about 101,325 Pa.

Blaise Pascal never saw a pascal, because the name was adopted by the 14th CGPM in 1971 for the newton per square meter. He did, however, earn the honor. In 1647 he published experiments on the vacuum, and on 19 September 1648 his brother-in-law Florin Périer carried a mercury barometer up the Puy de Dôme near Clermont, where the column stood at about 23 inches against about 26 at the base. Pascal had reasoned that air pressure should fall with altitude, and the result confirmed it.

One pascal is a tiny pressure, so the unit is mostly met with prefixes. Standard atmospheric pressure is 101,325 Pa, a typical sea-level reading is about 1,013 hPa, and the threshold of hearing in air is taken as a reference pressure of 20 micropascals. Building airtightness tests, in contrast, are run at 50 Pa.

Pascal is also remembered for hydrostatics, where he showed that pressure depends on height difference rather than the weight of the fluid, and for the hydraulic press.

The pascal is the reference unit for pressure on this site.

Sources: Wikipedia: Pascal (unit); Wikipedia: Blaise Pascal

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