Convert Atmospheres to Inches of Water

Formula

inH₂O = atm × 406.7824617322

Derivation from SI units:

• 1 atm = 101 325 Pa (exact, by definition)
• 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
• 1 atm = 101325 Pa ÷ 249.08891 Pa per inH₂O = 406.7824617322 inH₂O

Both units are defined exactly, so the factor is exact. Atmosphere definition: atmosphere; Inch of Water definition: inch of water.

Worked examples

1. A reading of 1.5 atm converts to 610.1736926 inH₂O.
   1.5 atm × 406.7824617 = 610.1736926 inH₂O
2. A value of 10 atm is 4067.824617 inH₂O.
   10 atm × 406.7824617 = 4067.824617 inH₂O

Where it's used

Atmospheres are used for chemistry and gas laws (STP and NTP conditions), while inches of water appear in HVAC total external static pressure (residential ≈ 0.5 inH₂O). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Atmosphere to other pressure units

Frequently asked questions

Sources

• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "atmosphere, standard (atm)".
• Guide for the Use of the International System of Units (SI), NIST Special Publication 811, 2008 Edition, National Institute of Standards and Technology , Appendix B.8, "inch of water, conventional (inH2O)".

Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.