Convert Inches of Mercury to Millimeters of Water
Convert Inches of Mercury (inHg) to Millimeters of Water (mmH₂O) instantly and accurately.
Conversion Formula
mmH₂O = inHg × 345.31554
About Inches of Mercury
One inch of mercury (inHg) ≈ 3,386 Pa - the primary barometric pressure unit in US aviation and meteorology. ASOS weather stations, METARs, and altimeter settings in the US report in inHg. Standard sea-level pressure is 29.92 inHg. Altimeters are set to local QNH in inHg to maintain altitude reference. US barometers for home weather stations display inHg. Typical pressure range: 28.5-31.0 inHg in continental weather systems. Record low: ~25.70 inHg (Typhoon Tip, 1979). 1 inHg = 3,386.39 Pa.
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.
Quick Reference Table
| Inches of Mercury (inHg) | Millimeters of Water (mmH₂O) |
|---|---|
| 1 inHg | 345.316 mmH₂O |
| 2 inHg | 690.631 mmH₂O |
| 5 inHg | 1726.58 mmH₂O |
| 10 inHg | 3453.16 mmH₂O |
| 25 inHg | 8632.89 mmH₂O |
| 50 inHg | 17265.8 mmH₂O |
| 100 inHg | 34531.6 mmH₂O |