In the Western/Pythagorean system, the alphabet is laid out in a repeating 1-through-9 cycle: A, J, and S are all 1; B, K, and T are all 2; and so on, until Z lands on 8.
Once you have that chart, calculating a destiny number is just a matter of looking up every letter in your full birth name, writing down its value, and adding the whole list together — then reducing, the same way you'd reduce any large numerology total.
