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