A Body Mass Index calculator uses an individual's body weight and height to calculate his body mass, according to How Stuff Works. The Body Mass Index rate is used to determine if a person is underweight, overweight or has a healthy weight. BMI is calculated in inches and pounds in the United States, and it is calculated in meters and kilograms in countries using the metric system.
The Calorie Control Council states that BMI is one of the most accurate methods to assess the health risks associated with extra pounds. A BMI of 24 and below is considered a healthy weight. A higher BMI means greater risk of developing health problems. A person with a BMI of 26 to 27 is around 20 percent overweight, while an individual with a BMI of 30 and above is considered obese.
The Calorie Control Council explains that being overweight is linked to high blood pressure, diabetes and heart disease. A BMI of 30 and above raises a person's risk of death from any cause by 50 to 150 percent. Health experts maintain that overweight people without any other health risk factors should eat a healthy diet and exercise regularly to prevent gaining additional weight. Overweight people who have health risks are advised to lose weight actively through a weight-loss program, under consultation with a health professional.