Convert Millimeters of Water to Hectopascals
Convert Millimeters of Water (mmH₂O) to Hectopascals (hPa) instantly and accurately.
Conversion Formula
hPa = mmH₂O × 0.0980665
About Millimeters of Water
One millimetre of water column (mmH₂O) = pressure from 1 mm of fresh water at standard gravity = exactly 9.80665 Pa. Preferred for very low differential pressures in HVAC: ductwork static pressures, fan curves, filter resistance, and building pressurisation are in mm WC. Gas valve operating pressures are typically 0.5-3 mm WC; clean-room cascade differentials 12-25 mm WC. A domestic gas hob manifold operates at ~20 mm WC on natural gas. Also used in spirometer calibration and hospital oxygen delivery systems. 1 mmH₂O = 9.80665 Pa.
About Hectopascals
A hectopascal (hPa) = 100 Pa = 1 mbar - the WMO-official SI-compatible meteorological pressure unit since 1986, replacing millibar with no numerical change. All modern surface observations (METAR, SYNOP), digital barometers, aviation weather, and weather apps report in hPa. QNH altimeter settings outside North America are in hPa. ICAO standard sea-level pressure is 1013.25 hPa. Continental high-pressure record exceeds 1084 hPa; Typhoon Tip (1979) holds the low record at ~870 hPa. 1 hPa = 100 Pa.
Quick Reference Table
| Millimeters of Water (mmH₂O) | Hectopascals (hPa) |
|---|---|
| 1 mmH₂O | 0.0980665 hPa |
| 2 mmH₂O | 0.196133 hPa |
| 5 mmH₂O | 0.490333 hPa |
| 10 mmH₂O | 0.980665 hPa |
| 25 mmH₂O | 2.45166 hPa |
| 50 mmH₂O | 4.90332 hPa |
| 100 mmH₂O | 9.80665 hPa |