Actually the way to calculate this is to first find out the number of 5 card combinations that exist (I assume you are talking about being dealt a Royal Flush is a five card game)

that is denoted as 52C5 (how many combinations of five cards can be made from a deck of 52 order does not matter) that answer is 2,598,960. Of those 2,598,960 possible hands only four would be Royal Flushes (there is only one Royal of each suit) so the answer is 4/2,598,960 = 1 in 649,740