Standard Atmospheres to Inches Of Water (atm to inH₂O)

1 standard atmosphere equals 406.7824617 inches of water (rounded).

1 standard atmosphere = 406.7824617 inches of water

How to convert standard atmospheres to inches of water

To convert standard atmospheres to inches of water, multiply the number of standard atmospheres by 406.7824617. The factor is rounded to 10 significant digits. To go the other way, multiply inches of water by 0.002458316408 (rounded).

inches of water = standard atmospheres × 406.7824617

Example: 10 standard atmospheres = 4,067.824617 inches of water.

About these units

Standard Atmosphere (atm)

Metric (SI)Current unit

Exactly 101,325 pascals, the average pressure of the atmosphere at sea level.

A column of mercury about 76 cm tall was Torricelli's measure of the air, and the standard atmosphere began as exactly that: the pressure of 760 mm of mercury at 0 degrees Celsius under standard gravity. It was used as a reference for physical and chemical properties, and the Celsius scale set 100 degrees as the boiling point of water at that pressure.

In 1954 the 10th CGPM confirmed the standard atmosphere as exactly 1,013,250 dynes per square centimeter, which is 101,325 pascals, a definition that no longer depends on the properties of any substance.

In practice one atmosphere is about 1.01325 bar, 760 mmHg, 14.696 psi or 29.92 inHg, and roughly the pressure of 10.33 meters of water. Chemistry splits on it: until 1982 standard temperature and pressure used 1 atm, but that year IUPAC recommended a standard pressure of exactly 100 kPa instead.

1 standard atmosphere = 101,325 pascals (exactly).

Sources: Wikipedia: Standard atmosphere (unit); Wikipedia: Atmospheric pressure; Wikipedia: Evangelista Torricelli

Inch Of Water (inH₂O)

US & ImperialCurrent unit

About 249.089 pascals (the conventional value, 25.4 mm × 9.80665 Pa). Used for air pressure in HVAC and gas appliances.

Building services and gas fitting rely on this unit. Manometers filled with water read small differences in air or gas pressure, and in North America the result is stated in inches of water column. Natural gas supplied to appliances is typically 6 to 7 inches WC, and HVAC technicians use it for pressure across coils, filters and ducts. A pipe organ's wind pressure is quoted the same way.

Which water is meant matters a little. Wikipedia gives about 249.082 Pa for water at 4 degrees Celsius, equal to 39.2 degrees Fahrenheit, and also the conventional 249.08891 Pa, which is 25.4 mm times 9.80665 and is the figure used here. NIST lists 249.082 Pa at 39.2 degrees Fahrenheit and 248.84 Pa at 60 degrees Fahrenheit, and some industry standards use 60 or 68 degrees Fahrenheit. About 27.7 inches of water equal 1 psi.

1 inch of water = 249.08891 pascals (exactly).

Sources: Wikipedia: Inch of water; NIST SP 811, Appendix B.8

Related conversions

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