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