Convert Millimeters of Water to Atmospheres
1 millimeter of water (mmH₂O) equals 0.00009678411054 atmospheres (atm).
1 mmH₂O = 0.00009678411054 atm
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk millimeter of water to atmosphere converter to paste a column from Excel.
How to Convert Millimeter of Water to Atmosphere
- 1 mmH₂O = 0.00009678411054 atm
- 1 atm = 10332.27453 mmH₂O
- Example: convert 15 mmH₂O to atm: 15 mmH₂O = 15 × 0.00009678411054 atm = 0.001451761658 atm
Reference conditions. mmH₂O: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).
Formula
atm = mmH₂O × 0.00009678411053541
or atm = mmH₂O ÷ 10332.274528
Derivation from SI units:
- 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
- 1 atm = 101 325 Pa (exact, by definition)
- 1 mmH₂O = 9.80665 Pa ÷ 101325 Pa per atm = 0.00009678411053541 atm
Both units are defined exactly, so the factor is exact. Millimeter of Water definition: millimeter of water; Atmosphere definition: atmosphere.
Worked examples
- A reading of 25 mmH₂O converts to 0.002419602763 atm.
25 mmH₂O × 0.00009678411054 = 0.002419602763 atm - A value of 250 mmH₂O is 0.02419602763 atm.
250 mmH₂O × 0.00009678411054 = 0.02419602763 atm
Millimeter of Water to Atmosphere conversion table
| Millimeters of Water (mmH₂O) | Atmospheres (atm) |
|---|---|
| 1 mmH₂O | 0.00009678411054 atm |
| 2 mmH₂O | 0.0001935682211 atm |
| 5 mmH₂O | 0.0004839205527 atm |
| 10 mmH₂O | 0.0009678411054 atm |
| 15 mmH₂O | 0.001451761658 atm |
| 20 mmH₂O | 0.001935682211 atm |
| 25 mmH₂O | 0.002419602763 atm |
| 30 mmH₂O | 0.002903523316 atm |
| 50 mmH₂O | 0.004839205527 atm |
| 75 mmH₂O | 0.00725880829 atm |
| 100 mmH₂O | 0.009678411054 atm |
| 150 mmH₂O | 0.01451761658 atm |
| 200 mmH₂O | 0.01935682211 atm |
| 250 mmH₂O | 0.02419602763 atm |
| 300 mmH₂O | 0.02903523316 atm |
| 500 mmH₂O | 0.04839205527 atm |
| 1000 mmH₂O | 0.09678411054 atm |
| 2000 mmH₂O | 0.1935682211 atm |
| 5000 mmH₂O | 0.4839205527 atm |
| 10000 mmH₂O | 0.9678411054 atm |
Where it's used
Millimeters of Water are used for fan static pressure and duct design in Asia and Europe, while atmospheres appear in chemistry and gas laws (STP and NTP conditions). 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 atmospheres are in 1 millimeter of water?
1 mmH₂O = 0.00009678411054 atm.
How do I convert millimeters of water to atmospheres?
Multiply the value in mmH₂O by 0.00009678411054, or divide it by 10332.27453. For example, 15 mmH₂O × 0.00009678411054 = 0.001451761658 atm.
Is the mmH₂O to atm 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, "atmosphere, standard (atm)".
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 Atmospheres. Retrieved October 17, 2026, from https://scientificconverters.com/pressure/millimeter-of-water-to-atmosphere/
IEEE
Scientific Converters, "Convert Millimeters of Water to Atmospheres," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/pressure/millimeter-of-water-to-atmosphere/