Convert Centimeters of Water to Inches of Mercury

Formula

inHg = cmH₂O × 0.02895902107389

or inHg = cmH₂O ÷ 34.531554

Derivation from SI units:

• 1 cmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.01 m = 98.0665 Pa
• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 cmH₂O = 98.0665 Pa ÷ 3386.388640341 Pa per inHg = 0.02895902107389 inHg

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

Worked examples

1. A reading of 6 cmH₂O converts to 0.1737541264 inHg.
   6 cmH₂O × 0.02895902107 = 0.1737541264 inHg
2. A value of 30 cmH₂O is 0.8687706322 inHg.
   30 cmH₂O × 0.02895902107 = 0.8687706322 inHg

Where it's used

Centimeters of Water are used for mechanical ventilation (PEEP 5–10 cmH₂O, plateau pressure under 30 cmH₂O), while inches of mercury appear in aircraft altimeter settings (standard 29.92 inHg). 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 mercury, conventional (inHg)".

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