Convert Inches of Water to Millimeters of Water
1 inch of water (inH₂O) equals 25.4 millimeters of water (mmH₂O).
1 inH₂O = 25.4 mmH₂O
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk inch of water to millimeter of water converter to paste a column from Excel.
How to Convert Inch of Water to Millimeter of Water
- 1 inH₂O = 25.4 mmH₂O
- 1 mmH₂O = 0.03937007874 inH₂O
- Example: convert 15 inH₂O to mmH₂O: 15 inH₂O = 15 × 25.4 mmH₂O = 381 mmH₂O
Reference conditions. inH₂O: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C). mmH₂O: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).
Formula
mmH₂O = inH₂O × 25.4
Derivation from SI units:
- 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
- 1 mmH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.001 m = 9.80665 Pa
- 1 inH₂O = 249.08891 Pa ÷ 9.80665 Pa per mmH₂O = 25.4 mmH₂O
Both units are defined exactly, so the factor is exact. Inch of Water definition: inch of water; Millimeter of Water definition: millimeter of water.
Worked examples
- 1 inH₂O is exactly 25.4 mmH₂O.
1 inH₂O × 25.4 = 25.4 mmH₂O - An external static pressure of 0.5 inH₂O equals 12.7 mmH₂O.
0.5 inH₂O × 25.4 = 12.7 mmH₂O
Inch of Water to Millimeter of Water conversion table
| Inches of Water (inH₂O) | Millimeters of Water (mmH₂O) |
|---|---|
| 0.01 inH₂O | 0.254 mmH₂O |
| 0.02 inH₂O | 0.508 mmH₂O |
| 0.03 inH₂O | 0.762 mmH₂O |
| 0.05 inH₂O | 1.27 mmH₂O |
| 0.1 inH₂O | 2.54 mmH₂O |
| 0.15 inH₂O | 3.81 mmH₂O |
| 0.2 inH₂O | 5.08 mmH₂O |
| 0.25 inH₂O | 6.35 mmH₂O |
| 0.3 inH₂O | 7.62 mmH₂O |
| 0.4 inH₂O | 10.16 mmH₂O |
| 0.5 inH₂O | 12.7 mmH₂O |
| 0.75 inH₂O | 19.05 mmH₂O |
| 1 inH₂O | 25.4 mmH₂O |
| 1.5 inH₂O | 38.1 mmH₂O |
| 2 inH₂O | 50.8 mmH₂O |
| 2.5 inH₂O | 63.5 mmH₂O |
| 3 inH₂O | 76.2 mmH₂O |
| 4 inH₂O | 101.6 mmH₂O |
| 5 inH₂O | 127 mmH₂O |
| 7 inH₂O | 177.8 mmH₂O |
| 10 inH₂O | 254 mmH₂O |
| 11 inH₂O | 279.4 mmH₂O |
| 14 inH₂O | 355.6 mmH₂O |
| 20 inH₂O | 508 mmH₂O |
| 27.7 inH₂O | 703.58 mmH₂O |
Where it's used
Both units use the same conventional water density and standard gravity, so the conversion factor is exactly 25.4, the number of millimetres in an inch.
Inch of Water to other pressure units
| Conversion | 1 inH₂O equals |
|---|---|
| inch of water to pascal | 249.08891 Pa |
| inch of water to kilopascal | 0.24908891 kPa |
| inch of water to megapascal | 0.00024908891 MPa |
| inch of water to psi | 0.036127292 psi |
| inch of water to bar | 0.0024908891 bar |
| inch of water to millibar | 2.4908891 mbar |
| inch of water to decibar | 0.024908891 dbar |
| inch of water to torr | 1.86832047 Torr |
| inch of water to millimeter of mercury | 1.868320204 mmHg |
| inch of water to millimeter of water | 25.4 mmH₂O |
| inch of water to centimeter of water | 2.54 cmH₂O |
| inch of water to inch of water (60 °F) | 1.001000281 inH₂O (60 °F) |
| inch of water to inch of mercury | 0.07355591353 inHg |
| inch of water to atmosphere | 0.002458316408 atm |
| inch of water to barye | 2490.8891 Ba |
Frequently asked questions
Is 1 inH₂O exactly 25.4 mmH₂O?
Yes. Both conventional definitions use 1000 kg/m³ and standard gravity, so only the length changes.
How many millimeters of water are in 1 inch of water?
1 inH₂O = 25.4 mmH₂O.
How do I convert inches of water to millimeters of water?
Multiply the value in inH₂O by 25.4. For example, 15 inH₂O × 25.4 = 381 mmH₂O.
Is the inH₂O to mmH₂O conversion exact?
Yes. Both units are defined exactly, and 1 inH₂O is exactly 25.4 mmH₂O.
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, "millimeter of water, conventional (mmH2O)".
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 Inches of Water to Millimeters of Water. Retrieved October 17, 2026, from https://scientificconverters.com/pressure/inch-of-water-to-millimeter-of-water/
IEEE
Scientific Converters, "Convert Inches of Water to Millimeters of Water," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/pressure/inch-of-water-to-millimeter-of-water/