Inch of water

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Inches of water, wc, inch water column (inch WC), inAq, Aq, or inH2O is a non-SI unit for pressure. The units are by convention and due to the historical measurement of certain pressure differentials. It is used for measuring small pressure differences across an orifice, or in a pipeline or shaft.[1] Inches of water can be converted to a pressure unit using the formula for pressure head. An inch of water column (iwc) is synonymous with an inch of water gauge (iwg).

It is defined as the pressure exerted by a column of water of 1 inch in height at defined conditions. For example, 39 °F (4 °C) at the standard acceleration of gravity; 1 inAq is approximately equal to 249 pascals at 0 °C.

Alternative standard conditions in uncommon usage are 60 °F, 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 psi 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 psi.

1 inAq ≈ 0.036 psi, or 27.7 inAq ≈ 1 psi.

1 inH2 = 248.84 pascals (60 °F)[2]
= 2.4884 mbar or hectopascals (60 °F)
= 2.54 cmH2O (4 °C)
≈ 0.0024558598569 atm
≈ 1.86645349124 torr or mmHg (0 °C)
≈ 0.0734824209149 inHg (0 °C)
≈ 0.0360911906567 psi

See also[edit]


  1. ^ Inches of something - Link broken as of 4-8-2014. The UK NPL FAQ pages have issues
  2. ^ Perry, Robert H.; Green, Don W., eds. (1997). Perry's Chemical Engineer's Handbook (7th ed.). McGraw-Hill. p. 1-5.