Convert Millimeters of Mercury to Millimeters of Water

Formula

mmH₂O = mmHg × 13.5951

Derivation from SI units:

• 1 mmHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.001 m = 133.322387415 Pa
• 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
• 1 mmHg = 133.322387415 Pa ÷ 9.80665 Pa per mmH₂O = 13.5951 mmH₂O

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

Worked examples

1. A reading of 80 mmHg converts to 1087.608 mmH₂O.
   80 mmHg × 13.5951 = 1087.608 mmH₂O
2. A value of 160 mmHg is 2175.216 mmH₂O.
   160 mmHg × 13.5951 = 2175.216 mmH₂O

Where it's used

Millimeters of Mercury are used for blood pressure (e.g. 120/80 mmHg), 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.

Millimeter 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, "millimeter of mercury, conventional (mmHg)".
• 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.