Convert Inches of Mercury to Inches of Water

Formula

inH₂O = inHg × 13.5951

Derivation from SI units:

• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
• 1 inHg = 3386.388640341 Pa ÷ 249.08891 Pa per inH₂O = 13.5951 inH₂O

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

Worked examples

1. A reading of 15 inHg converts to 203.9265 inH₂O.
   15 inHg × 13.5951 = 203.9265 inH₂O
2. A value of 29 inHg is 394.2579 inH₂O.
   29 inHg × 13.5951 = 394.2579 inH₂O

Where it's used

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

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, "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.