Convert Inches of Water to Standard Atmospheres
Convert Inches of Water (inH₂O) to Standard Atmospheres (atm) instantly and accurately.
Conversion Formula
atm = inH₂O × 0.002458316408
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 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
| Inches of Water (inH₂O) | Standard Atmospheres (atm) |
|---|---|
| 1 inH₂O | 0.00245832 atm |
| 2 inH₂O | 0.00491663 atm |
| 5 inH₂O | 0.0122916 atm |
| 10 inH₂O | 0.0245832 atm |
| 25 inH₂O | 0.0614579 atm |
| 50 inH₂O | 0.122916 atm |
| 100 inH₂O | 0.245832 atm |