How is it calculated?

Add the month, day and year of your birth (for example 10 + 16 + 1990 = 2016), then add the digits together (2 + 0 + 1 + 6 = 9) until you reach a number from 1 to 22. That number is a Major Arcana card, and 22 is The Fool. If the number is 10 or more, adding its digits once more gives your second birth card.