Megacalories to gigajoules conversion calculator shown above calculates how many gigajoules are in 'X' megacalories (where 'X' is the number of megacalories to convert to gigajoules). In order to convert a value from megacalories to gigajoules (from Mcal to GJ) just type the number of Mcal to be converted to GJ and then click on the 'convert' button.