Convert Millimeters of Water to Inches of Mercury
1 millimeter of water (mmH₂O) equals 0.002895902107 inches of mercury (inHg).
1 mmH₂O = 0.002895902107 inHg
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk millimeter of water to inch of mercury converter to paste a column from Excel.
How to Convert Millimeter of Water to Inch of Mercury
- 1 mmH₂O = 0.002895902107 inHg
- 1 inHg = 345.31554 mmH₂O
- Example: convert 15 mmH₂O to inHg: 15 mmH₂O = 15 × 0.002895902107 inHg = 0.04343853161 inHg
Reference conditions. mmH₂O: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C). inHg: Conventional mercury column: ρ = 13 595.1 kg/m³ and gₙ = 9.80665 m/s² (≈ mercury at 0 °C / 32 °F).
Formula
inHg = mmH₂O × 0.002895902107389
or inHg = mmH₂O ÷ 345.31554
Derivation from SI units:
- 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
- 1 inHg = 13 595.1 kg/m³ × 9.80665 m/s² × 0.0254 m = 3386.388640341 Pa
- 1 mmH₂O = 9.80665 Pa ÷ 3386.388640341 Pa per inHg = 0.002895902107389 inHg
Both units are defined exactly, so the factor is exact. Millimeter of Water definition: millimeter of water; Inch of Mercury definition: inch of mercury.
Worked examples
- A reading of 25 mmH₂O converts to 0.07239755268 inHg.
25 mmH₂O × 0.002895902107 = 0.07239755268 inHg - A value of 250 mmH₂O is 0.7239755268 inHg.
250 mmH₂O × 0.002895902107 = 0.7239755268 inHg
Millimeter of Water to Inch of Mercury conversion table
| Millimeters of Water (mmH₂O) | Inches of Mercury (inHg) |
|---|---|
| 1 mmH₂O | 0.002895902107 inHg |
| 2 mmH₂O | 0.005791804215 inHg |
| 5 mmH₂O | 0.01447951054 inHg |
| 10 mmH₂O | 0.02895902107 inHg |
| 15 mmH₂O | 0.04343853161 inHg |
| 20 mmH₂O | 0.05791804215 inHg |
| 25 mmH₂O | 0.07239755268 inHg |
| 30 mmH₂O | 0.08687706322 inHg |
| 50 mmH₂O | 0.1447951054 inHg |
| 75 mmH₂O | 0.2171926581 inHg |
| 100 mmH₂O | 0.2895902107 inHg |
| 150 mmH₂O | 0.4343853161 inHg |
| 200 mmH₂O | 0.5791804215 inHg |
| 250 mmH₂O | 0.7239755268 inHg |
| 300 mmH₂O | 0.8687706322 inHg |
| 500 mmH₂O | 1.447951054 inHg |
| 1000 mmH₂O | 2.895902107 inHg |
| 2000 mmH₂O | 5.791804215 inHg |
| 5000 mmH₂O | 14.47951054 inHg |
| 10000 mmH₂O | 28.95902107 inHg |
Where it's used
Millimeters of Water are used for fan static pressure and duct design in Asia and Europe, 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.
Millimeter of Water to other pressure units
| Conversion | 1 mmH₂O equals |
|---|---|
| millimeter of water to pascal | 9.80665 Pa |
| millimeter of water to kilopascal | 0.00980665 kPa |
| millimeter of water to megapascal | 0.00000980665 MPa |
| millimeter of water to psi | 0.001422334331 psi |
| millimeter of water to bar | 0.0000980665 bar |
| millimeter of water to millibar | 0.0980665 mbar |
| millimeter of water to decibar | 0.000980665 dbar |
| millimeter of water to torr | 0.07355592401 Torr |
| millimeter of water to millimeter of mercury | 0.07355591353 mmHg |
| millimeter of water to centimeter of water | 0.1 cmH₂O |
| millimeter of water to inch of water | 0.03937007874 inH₂O |
| millimeter of water to inch of water (60 °F) | 0.03940945989 inH₂O (60 °F) |
| millimeter of water to inch of mercury | 0.002895902107 inHg |
| millimeter of water to atmosphere | 0.00009678411054 atm |
| millimeter of water to barye | 98.0665 Ba |
Frequently asked questions
How many inches of mercury are in 1 millimeter of water?
1 mmH₂O = 0.002895902107 inHg.
How do I convert millimeters of water to inches of mercury?
Multiply the value in mmH₂O by 0.002895902107, or divide it by 345.31554. For example, 15 mmH₂O × 0.002895902107 = 0.04343853161 inHg.
Is the mmH₂O to inHg conversion exact?
Yes. Both units are defined exactly, so the ratio is exact; its decimal form does not terminate and is shown here to 10 significant figures (more with the precision selector).
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 mercury, conventional (inHg)".
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
Cite this page
APA
Scientific Converters. (2026, October 15). Convert Millimeters of Water to Inches of Mercury. Retrieved October 17, 2026, from https://scientificconverters.com/pressure/millimeter-of-water-to-inch-of-mercury/
IEEE
Scientific Converters, "Convert Millimeters of Water to Inches of Mercury," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/pressure/millimeter-of-water-to-inch-of-mercury/