Convert Millimeters of Water to Bar

Formula

bar = mmH₂O × 0.0000980665

or bar = mmH₂O ÷ 10197.16212978

Derivation from SI units:

• 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
• 1 bar = 10⁵ Pa (exact, by definition)
• 1 mmH₂O = 9.80665 Pa ÷ 100000 Pa per bar = 0.0000980665 bar

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

Worked examples

1. A reading of 25 mmH₂O converts to 0.0024516625 bar.
   25 mmH₂O × 0.0000980665 = 0.0024516625 bar
2. A value of 250 mmH₂O is 0.024516625 bar.
   250 mmH₂O × 0.0000980665 = 0.024516625 bar

Where it's used

Millimeters of Water are used for fan static pressure and duct design in Asia and Europe, while bar appear in compressed air and pneumatics (6–8 bar). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Millimeter 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, "millimeter of water, conventional (mmH2O)".
• 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, "bar".

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