Convert Inches of Water to Millimeters of Water
Convert Inches of Water (inH₂O) to Millimeters of Water (mmH₂O) instantly and accurately.
Conversion Formula
mmH₂O = inH₂O × 25.4
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.
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 Water (inH₂O) | Millimeters of Water (mmH₂O) |
|---|---|
| 1 inH₂O | 25.4 mmH₂O |
| 2 inH₂O | 50.8 mmH₂O |
| 5 inH₂O | 127 mmH₂O |
| 10 inH₂O | 254 mmH₂O |
| 25 inH₂O | 635 mmH₂O |
| 50 inH₂O | 1270 mmH₂O |
| 100 inH₂O | 2540 mmH₂O |