Convert Inches of Water to Inches of Mercury

Formula

inHg = inH₂O × 0.07355591352767

or inHg = inH₂O ÷ 13.5951

Derivation from SI units:

• 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
• 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
• 1 inH₂O = 249.08891 Pa ÷ 3386.388640341 Pa per inHg = 0.07355591352767 inHg

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

Worked examples

1. A reading of 0.3 inH₂O converts to 0.02206677406 inHg.
   0.3 inH₂O × 0.07355591353 = 0.02206677406 inHg
2. A value of 3 inH₂O is 0.2206677406 inHg.
   3 inH₂O × 0.07355591353 = 0.2206677406 inHg

Where it's used

Inches of Water are used for HVAC total external static pressure (residential ≈ 0.5 inH₂O), while inches of mercury appear in aircraft altimeter settings (standard 29.92 inHg). Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Inch 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, "inch of water, conventional (inH2O)".
• 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)".

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