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