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. Inches of water can be converted to a pressure unit using the formula for pressure head. It is defined as the pressure exerted by a column of water of 1 inch in height at defined conditions for example \(39 ^\circ F\) at the standard acceleration of gravity; 1 inAq is approximately equal to 249 pascals at \(0 ^\circ C\).