Millimeters of mercury to centimeters of water conversion calculator shown above calculates how many centimeters of water are in 'X' millimeters of mercury (where 'X' is the number of millimeters of mercury to convert to centimeters of water). In order to convert a value from millimeters of mercury to centimeters of water (from mmHg to cmH2O) simply type the number of mmHg to be converted to cmH2O and then click on the 'convert' button.