Convert Centimeters of Water to Atmospheres

Formula

atm = cmH₂O × 0.0009678411053541

or atm = cmH₂O ÷ 1033.2274528

Derivation from SI units:

• 1 cmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.01 m = 98.0665 Pa
• 1 atm = 101 325 Pa (exact, by definition)
• 1 cmH₂O = 98.0665 Pa ÷ 101325 Pa per atm = 0.0009678411053541 atm

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

Worked examples

1. A reading of 6 cmH₂O converts to 0.005807046632 atm.
   6 cmH₂O × 0.0009678411054 = 0.005807046632 atm
2. A value of 30 cmH₂O is 0.02903523316 atm.
   30 cmH₂O × 0.0009678411054 = 0.02903523316 atm

Where it's used

Centimeters of Water are used for mechanical ventilation (PEEP 5–10 cmH₂O, plateau pressure under 30 cmH₂O), while atmospheres appear in chemistry and gas laws (STP and NTP conditions). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Centimeter of Water 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, "centimeter of water, conventional (cmH2O)".
• 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)".

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