mmH₂O
millimeter of water column
Pressureexact by definition
The millimetre of water column is the pressure at the base of a 1 mm column of water, taken conventionally as density 1000 kg/m³ under standard gravity, giving exactly 9.80665 pascals. It is the unit of low-pressure gas work, draft measurement and manometry, where a pascal is too small to read off a tube and a kilopascal is far too large.
Watch out: Real water is not exactly 1000 kg/m³, so an actual manometer reads slightly differently from the conventional value depending on temperature. The conventional figure is the one used for unit conversion and is what this site applies to every water column unit, which keeps mm, cm, m, inches and feet of water mutually consistent.
| 1 mmH₂O | 0.00980665 kPa |
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