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