Convert Millimeters of Water to Standard Atmospheres
Convert Millimeters of Water (mmH₂O) to Standard Atmospheres (atm) instantly and accurately.
Conversion Formula
atm = mmH₂O × 9.678411054e-5
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 Standard Atmospheres
The standard atmosphere (atm) = exactly 101,325 Pa - defined by the 10th CGPM (1954) as the reference air pressure at sea level. Used to define STP (0°C and 1 atm) for gas law and thermochemistry. Chemical equilibrium constants (Kp), Henry's law solubility, and thermodynamic tables are tabulated at 1 atm. ISA aviation standard: sea-level 1 atm / 15°C, pressure halving every ~5.5 km. Diving physics: +1 atm per 10 m depth (fresh water) so 30 m = 4 atm absolute. Diamond formation in Earth's mantle: ~50,000 atm. 1 atm = 101,325 Pa.
Quick Reference Table
| Millimeters of Water (mmH₂O) | Standard Atmospheres (atm) |
|---|---|
| 1 mmH₂O | 9.678 × 10-5 atm |
| 2 mmH₂O | 0.000193568 atm |
| 5 mmH₂O | 0.000483921 atm |
| 10 mmH₂O | 0.000967841 atm |
| 25 mmH₂O | 0.0024196 atm |
| 50 mmH₂O | 0.00483921 atm |
| 100 mmH₂O | 0.00967841 atm |