Look in any medical text book and, since books are intended to be helpful, it will tell you obesity is carrying too much extra weight. This means everyone is assumed to have a “healthy” weight. This is an ideal weight. People who carry 20% more than the ideal start to be obese. In the range 20-40%, the obesity is mild. In the range 40-100%, the obesity is moderate, and anything over 100% is severe or morbid obesity, i.e. it is likely to be fatal.