Convert Millibars to Millimeters of Water

Formula

mmH₂O = mbar × 10.19716212978

Derivation from SI units:

• 1 mbar = 10⁻³ bar = 100 Pa = 1 hPa
• 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
• 1 mbar = 100 Pa ÷ 9.80665 Pa per mmH₂O = 10.19716212978 mmH₂O

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

Worked examples

1. A reading of 20 mbar converts to 203.9432426 mmH₂O.
   20 mbar × 10.19716213 = 203.9432426 mmH₂O
2. A value of 900 mbar is 9177.445917 mmH₂O.
   900 mbar × 10.19716213 = 9177.445917 mmH₂O

Where it's used

Millibars are used for weather maps and barometers (sea level ≈ 1013 mbar), while millimeters of water appear in fan static pressure and duct design in Asia and Europe. Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Millibar 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, "millibar".
• 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, "millimeter of water, conventional (mmH2O)".

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