Convert Standard Atmospheres to Inches of Water
Convert Standard Atmospheres (atm) to Inches of Water (inH₂O) instantly and accurately.
Conversion Formula
inH₂O = atm × 406.7824617
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.
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
| Standard Atmospheres (atm) | Inches of Water (inH₂O) |
|---|---|
| 1 atm | 406.782 inH₂O |
| 2 atm | 813.565 inH₂O |
| 5 atm | 2033.91 inH₂O |
| 10 atm | 4067.82 inH₂O |
| 25 atm | 10169.6 inH₂O |
| 50 atm | 20339.1 inH₂O |
| 100 atm | 40678.2 inH₂O |