I'm pretty much looking for an easy (or not) formula for calculating the mass and/or weight of fantasy creatures on an Earth-like planet with similar gravity. My world's dragons sometimes reach over 1 km long, magically supported regardless of weight, but I would still like to add a realistic number to the weight fields of the profiles of my draconic characters. I also have turtles the size of islands (inspired by Magi-Nation, NOT ATLA) and skyscraper-tall humanoids. My question is: How does one calculate the weight or mass of such incredibly large creatures?