Millimeters of water to inches of water conversion calculator shown above calculates how many inches of water are in 'X' millimeters of water (where 'X' is the number of millimeters of water to convert to inches of water). In order to convert a value from millimeters of water to inches of water (from mmH2O to inH2O) type the number of mmH2O to be converted to inH2O and then click on the 'convert' button.