Inches of water is a non-SI unit for pressure. It is also given as inches of water gauge (iwg or in.w.g.), inches water column (inch wc or just wc), inAq, Aq, or inH2O. The units are conventionally used for measurement of certain pressure differentials such as small pressure differences across an orifice, or in a pipeline or shaft.
It is defined as the pressure exerted by a column of water of 1 inch in height at defined conditions. At a temperature of 4 °C (39.2 °F) pure water has its highest density (1000 kg/m3). At that temperature and assuming the standard acceleration of gravity, 1 inAq is approximately 249.082 pascals.
Alternative standard conditions in uncommon usage are 60 °F (15,6 °C), or 68 °F (20 °C), and depends on industry standards rather than on international standards.
In North America, air and other industrial gasses are often measured in inches of water when at low pressure. This is in contrast to inches of mercury or pounds per square inch (psi, lbf/in2) for larger pressures. One usage is in the measurement of air ("wind") that supplies a pipe organ and is referred simply as inches. It is also used in natural gas distribution for measuring utilization pressure (U.P., i.e. the residential point of use) which is typically between 6 and 7 inches WC (6~7″ WC) or about 0.25 lbf/in2.
1 inAq ≈ 0.036 lbf/in2, or 27.7 inAq ≈ 1 lbf/in2.
1 inH2O = 248.84 pascals  = 2.4884 mbar or hectopascals = 2.54 cmH2O ≈ 0.0024558598569 atm ≈ 1.86645349124 torr or mmHg ≈ 0.0734824209149 inHg ≈ 0.0360911906567 lbf/in2
- "My pressure gauge is scaled in 'inches' - what does this mean?" http://www.npl.co.uk/science-technology/mass-and-force/faqs/
- "The International System of Units (SI) – Conversion Factors for General Use" (PDF). 2006. p. 12. Archived from the original (PDF) on 2017-10-05. Retrieved 2017-10-05.
- Perry, Robert H.; Green, Don W., eds. (1997). Perry's Chemical Engineer's Handbook (7th ed.). McGraw-Hill. p. 1-5.