What Is A Healthy Body Mass Index?

Body mass index, or BMI, is a system devised by Belgian scientist Adolphe Quetlet and first noted in 1832. Body mass indexing uses an individual's height and weight as a screening tool for obesity, as noted by the Centers for Disease Control and Prevention. Although this method of calculating body fat has drawbacks and limitations, it is a quick, noninvasive and cost-effective diagnostic tool used by medical professionals.

Calculation

BMI is calculated by dividing the person's weight in pounds by the person's height in inches squared. This number is then multiplied by 703, a conversion factor to correct the calculation, which was initially devised in metric units. This provides health care professionals with a number that correlates to standards devised for measuring a person's health using their BMI. To use metric units to calculate BMI, divide the person's weight in kilograms by the person's height in meters squared. The method of calculating BMI is the same for men and women, adults and children and people of all ages.

Interpretation

A BMI number between 18.5 an 24.9 is considered normal, notes the National Institutes of Health. A person with a BMI less than 18.5 is considered underweight, while a person with a BMI of 25.0 to 29.9 is considered overweight. Individuals with a BMI of more than 30 are considered obese. The CDC notes that these figures are appropriate measures for adults over the age of 20. The BMI of children aged two through 19 is interpreted differently than adults. Age- and sex-specific percentile charts are used to account for differences between age and gender, factors that are not considered when interpreting an adult's BMI.

Risk Factors

The National Institutes of Health state that persons with higher BMI scores are at an increased risk for obesity-related diseases, such as high blood pressure, Type 2 diabetes, breathing problems, heart disease and some cancers. The CDC adds sleep apnea, gallbladder disease, stroke and osteoarthritis as risk factors for individuals classified as overweight or obese as determined by the BMI scale.

Limitations

A BMI calculation is not always an accurate way to determine a person's health or fitness. Other factors must be considered. An athletic individual with a lot of muscle will still have a high BMI score because muscle weighs more than fat. An elderly person may have a lower BMI due to loss of muscle. The CDC cautions that BMI is not a direct measure of body fatness and that weight is just one of the many factors related to your individual risk for a disease.

Considerations

The CDC states that BMI correlates to body fatness with solid evidence, though certain variations exist by sex, age and race, noting as an example that women tend to carry more body fat than men. A person's waist circumference, physical activity and family history should also be taken into consideration when determining risk factors for obesity-related diseases. While it is possible to measure your BMI effectively at home, speak to your health care provider about concerns relating to your physical condition.