Convert Centimeters of Water to Millibars

Formula

mbar = cmH₂O × 0.980665

or mbar = cmH₂O ÷ 1.019716212978

Derivation from SI units:

• 1 cmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.01 m = 98.0665 Pa
• 1 mbar = 10⁻³ bar = 100 Pa = 1 hPa
• 1 cmH₂O = 98.0665 Pa ÷ 100 Pa per mbar = 0.980665 mbar

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

Worked examples

1. A reading of 6 cmH₂O converts to 5.88399 mbar.
   6 cmH₂O × 0.980665 = 5.88399 mbar
2. A value of 30 cmH₂O is 29.41995 mbar.
   30 cmH₂O × 0.980665 = 29.41995 mbar

Where it's used

Centimeters of Water are used for mechanical ventilation (PEEP 5–10 cmH₂O, plateau pressure under 30 cmH₂O), while millibars appear in weather maps and barometers (sea level ≈ 1013 mbar). 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, "millibar".

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