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