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