Convert Inches of Water to Atmospheres
1 inch of water (inH₂O) equals 0.002458316408 atmospheres (atm).
1 inH₂O = 0.002458316408 atm
Type in either box. Scientific notation works: 1.5e-3.
Converting a list? Use the bulk inch of water to atmosphere converter to paste a column from Excel.
How to Convert Inch of Water to Atmosphere
- 1 inH₂O = 0.002458316408 atm
- 1 atm = 406.7824617 inH₂O
- Example: convert 15 inH₂O to atm: 15 inH₂O = 15 × 0.002458316408 atm = 0.03687474611 atm
Reference conditions. inH₂O: Conventional water column: ρ = 1000 kg/m³ and gₙ = 9.80665 m/s² (≈ water at 4 °C).
Formula
atm = inH₂O × 0.002458316407599
or atm = inH₂O ÷ 406.7824617322
Derivation from SI units:
- 1 inH₂O = 1000 kg/m³ × 9.80665 m/s² × 0.0254 m = 249.08891 Pa
- 1 atm = 101 325 Pa (exact, by definition)
- 1 inH₂O = 249.08891 Pa ÷ 101325 Pa per atm = 0.002458316407599 atm
Both units are defined exactly, so the factor is exact. Inch of Water definition: inch of water; Atmosphere definition: atmosphere.
Worked examples
- A reading of 0.3 inH₂O converts to 0.0007374949223 atm.
0.3 inH₂O × 0.002458316408 = 0.0007374949223 atm - A value of 3 inH₂O is 0.007374949223 atm.
3 inH₂O × 0.002458316408 = 0.007374949223 atm
Inch of Water to Atmosphere conversion table
| Inches of Water (inH₂O) | Atmospheres (atm) |
|---|---|
| 0.01 inH₂O | 0.00002458316408 atm |
| 0.02 inH₂O | 0.00004916632815 atm |
| 0.03 inH₂O | 0.00007374949223 atm |
| 0.05 inH₂O | 0.0001229158204 atm |
| 0.1 inH₂O | 0.0002458316408 atm |
| 0.15 inH₂O | 0.0003687474611 atm |
| 0.2 inH₂O | 0.0004916632815 atm |
| 0.25 inH₂O | 0.0006145791019 atm |
| 0.3 inH₂O | 0.0007374949223 atm |
| 0.4 inH₂O | 0.000983326563 atm |
| 0.5 inH₂O | 0.001229158204 atm |
| 0.75 inH₂O | 0.001843737306 atm |
| 1 inH₂O | 0.002458316408 atm |
| 1.5 inH₂O | 0.003687474611 atm |
| 2 inH₂O | 0.004916632815 atm |
| 2.5 inH₂O | 0.006145791019 atm |
| 3 inH₂O | 0.007374949223 atm |
| 4 inH₂O | 0.00983326563 atm |
| 5 inH₂O | 0.01229158204 atm |
| 7 inH₂O | 0.01720821485 atm |
| 10 inH₂O | 0.02458316408 atm |
| 11 inH₂O | 0.02704148048 atm |
| 14 inH₂O | 0.03441642971 atm |
| 20 inH₂O | 0.04916632815 atm |
| 27.7 inH₂O | 0.06809536449 atm |
Where it's used
Inches of Water are used for HVAC total external static pressure (residential ≈ 0.5 inH₂O), 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.
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
How many atmospheres are in 1 inch of water?
1 inH₂O = 0.002458316408 atm.
How do I convert inches of water to atmospheres?
Multiply the value in inH₂O by 0.002458316408, or divide it by 406.7824617. For example, 15 inH₂O × 0.002458316408 = 0.03687474611 atm.
Is the inH₂O 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, "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, "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 Inches of Water to Atmospheres. Retrieved October 17, 2026, from https://scientificconverters.com/pressure/inch-of-water-to-atmosphere/
IEEE
Scientific Converters, "Convert Inches of Water to Atmospheres," scientificconverters.com. Accessed: Oct. 17, 2026. [Online]. Available: https://scientificconverters.com/pressure/inch-of-water-to-atmosphere/