Convert Inches of Mercury to Millimeters of Water

Formula

mmH₂O = inHg × 345.31554

Derivation from SI units:

• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
• 1 inHg = 3386.388640341 Pa ÷ 9.80665 Pa per mmH₂O = 345.31554 mmH₂O

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

Worked examples

1. A reading of 15 inHg converts to 5179.7331 mmH₂O.
   15 inHg × 345.31554 = 5179.7331 mmH₂O
2. A value of 29 inHg is 10014.15066 mmH₂O.
   29 inHg × 345.31554 = 10014.15066 mmH₂O

Where it's used

Inches of Mercury are used for aircraft altimeter settings (standard 29.92 inHg), 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.

Inch of Mercury 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, "inch of mercury, conventional (inHg)".
• 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.