Is obesity always defined by a person's body mass index?

Yes. Currently that is the standard for the definition. It is not 100% but the best available means to measure, at least for children over 2 y/o.

Related Questions

What is the normal body mass index of obese people?

BMI depends on you. On average, when you look at a population, bmi should be less than 25. Between 25-30 is called overweight, and over 30 is obese. However, some people are different. Muscular and "stocky" people can have a high bmi but have low body fat. For many people, it is better to strive for overall health and fitness and not obsess over a precise bmi. Read more...