Half-demon Age x 9 = Human Age
Full Demon Age x 18 = Human Age
ex. Inuyasha's approximate half-demon age is 17. So 17 x 9 = 153. Adding the 50 years he was sealed makes him roughly 203 human years old.
ex. Sesshomaru's approximate full demon age is 25. So 25 x 18 = 450. That makes him roughly 450 human years old.