The most basic definition of overweight and obesity is having too much body fat-so much so that it “presents a risk to health.” A reliable way to determine whether a person has too much body fat is to calculate the ratio of their weight to their height squared. This ratio, called the body mass index (BMI), accounts for the fact that taller people have more tissue than shorter people, and so they tend to weigh more.
You can calculate BMI on your own, or use an online calculator such as this one.
BMI is not a perfect measure, because it does not directly assess body fat. Muscle and bone are denser than fat, so an athlete or muscular person may have a high BMI, yet not have too much fat. But most people are not athletes, and for most people, BMI is a very good gauge of their level of body fat.
Research has shown that BMI is strongly correlated with the gold-standard methods for measuring body fat. And it is an easy way for clinicians to screen who might be at greater risk of health problems due to their weight.