Convert Millimeters of Mercury to Inches of Water

Formula

inH₂O = mmHg × 0.5352401574803

or inH₂O = mmHg ÷ 1.868320203603

Derivation from SI units:

• 1 mmHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.001 m = 133.322387415 Pa
• 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
• 1 mmHg = 133.322387415 Pa ÷ 249.08891 Pa per inH₂O = 0.5352401574803 inH₂O

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

Worked examples

1. A reading of 80 mmHg converts to 42.8192126 inH₂O.
   80 mmHg × 0.5352401575 = 42.8192126 inH₂O
2. A value of 160 mmHg is 85.6384252 inH₂O.
   160 mmHg × 0.5352401575 = 85.6384252 inH₂O

Where it's used

Millimeters of Mercury are used for blood pressure (e.g. 120/80 mmHg), while inches of water appear in HVAC total external static pressure (residential ≈ 0.5 inH₂O). 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, "inch of water, conventional (inH2O)".

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