Millimeters of water to centimeters of mercury conversion calculator above calculates how many centimeters of mercury are in 'X' millimeters of water (where 'X' is the number of millimeters of water to convert to centimeters of mercury). In order to convert a value from millimeters of water to centimeters of mercury (from mmH2O to cmHg) simply type the number of mmH2O to be converted to cmHg and then click on the 'convert' button.