Convert Centimeters of Water to Inches of Water

Formula

inH₂O = cmH₂O × 0.3937007874016

or inH₂O = cmH₂O ÷ 2.54

Derivation from SI units:

• 1 cmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.01 m = 98.0665 Pa
• 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
• 1 cmH₂O = 98.0665 Pa ÷ 249.08891 Pa per inH₂O = 0.3937007874016 inH₂O

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

Worked examples

1. A reading of 6 cmH₂O converts to 2.362204724 inH₂O.
   6 cmH₂O × 0.3937007874 = 2.362204724 inH₂O
2. A value of 30 cmH₂O is 11.81102362 inH₂O.
   30 cmH₂O × 0.3937007874 = 11.81102362 inH₂O

Where it's used

Centimeters of Water are used for mechanical ventilation (PEEP 5–10 cmH₂O, plateau pressure under 30 cmH₂O), 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.

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, "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.