Inches of water to millimeters of mercury converter on this page calculates how many millimeters of mercury are in 'X' inches of water (where 'X' is the number of inches of water to convert to millimeters of mercury). In order to convert a value from inches of water to millimeters of mercury (from inH2O to mmHg) just type the number of inH2O to be converted to mmHg and then click on the 'convert' button.