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