Convert Millimeters of Water to Inches of Water
Convert Millimeters of Water (mmH₂O) to Inches of Water (inH₂O) instantly and accurately.
Conversion Formula
inH₂O = mmH₂O × 0.03937007874
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 Inches of Water
One inch of water column (inH₂O or in WC) = pressure from 1 inch of fresh water at standard gravity ≈ 249.09 Pa. The standard unit for low-pressure measurement in US HVAC and plumbing: ductwork static pressure, fan performance, filter ratings, and gas appliance manifold pressures are all specified in inches WC. Household natural gas supply pressure is typically 7 in WC (0.25 psi). Blower door tests for building air-tightness use 50 Pa ≈ 0.2 in WC test pressure. 1 inH₂O = 249.089 Pa.
Quick Reference Table
| Millimeters of Water (mmH₂O) | Inches of Water (inH₂O) |
|---|---|
| 1 mmH₂O | 0.0393701 inH₂O |
| 2 mmH₂O | 0.0787402 inH₂O |
| 5 mmH₂O | 0.19685 inH₂O |
| 10 mmH₂O | 0.393701 inH₂O |
| 25 mmH₂O | 0.984252 inH₂O |
| 50 mmH₂O | 1.9685 inH₂O |
| 100 mmH₂O | 3.93701 inH₂O |