Convert Millimeters of Water to Inches of Water (60 °F)

Formula

inH₂O (60 °F) = mmH₂O × 0.03940945989391

or inH₂O (60 °F) = mmH₂O ÷ 25.37461824374

Derivation from SI units:

• 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
• 1 inH₂O (60 °F) ≈ 999.0 kg/m³ × 9.80665 m/s² × 0.0254 m ≈ 248.84 Pa
• 1 mmH₂O = 9.80665 Pa ÷ 248.84 Pa per inH₂O (60 °F) = 0.03940945989391 inH₂O (60 °F)

One unit is a measured value, so the factor is approximate. Millimeter of Water definition: millimeter of water; Inch of Water (60 °F) definition: inch of water (60 °F).

Worked examples

1. A reading of 25 mmH₂O converts to 0.9852364973 inH₂O (60 °F).
   25 mmH₂O × 0.03940945989 = 0.9852364973 inH₂O (60 °F)
2. A value of 250 mmH₂O is 9.852364973 inH₂O (60 °F).
   250 mmH₂O × 0.03940945989 = 9.852364973 inH₂O (60 °F)

Where it's used

Millimeters of Water are used for fan static pressure and duct design in Asia and Europe, while inches of water (60 °F) appear in older US gas-industry and flow-meter references. Both measure pressure, stress and differential pressure (force per unit area), so the conversion is a fixed factor.

Millimeter 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, "millimeter of water, conventional (mmH2O)".
• 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 (60 °F)".

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