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