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