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