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Know Your Numbers: What Is BMI?

Body mass index (BMI) is a calculation that uses a person’s height and weight to indicate levels of body fat and the associated health risks. While many factors go into evaluating health, BMI is a simple tool that anyone can use. Knowing this number may help a person understand the personal risks of certain chronic health conditions.

center for family medicine Understanding Body Mass Index Is Knowing Your BMI Helpful

How do you calculate BMI?

There are many online calculators where a person can get a BMI number quickly. The formula to calculate the number at home is BMI = 703 X (weight ÷ (height x height)). A doctor may use the resulting number to define terms such as overweight or obese.

Understanding categories

A person’s BMI can fall into a few categories. People should understand that, while being below or above a normal weight range can increase specific health risks, BMI is only one tool in measuring overall health. The BMI categories include:
  • Underweight: BMI < 18.5
  • Normal weight: 18.5-24.9
  • Overweight: 25.0-29.9
  • Obese: BMI > 30.0

What are the risk factors?

Underweight people may be eating too little or exercising too much. This could put these patients at higher risk of malnutrition, stress fractures, or even infertility. Likewise, patients in the overweight or obese category can have an increased risk for heart disease, sleep apnea, or diabetes.

An overall picture of health

Still, people who are within a normal weight range and participate in specific lifestyle habits are at higher risk of health problems. For example, people who smoke, do not participate in physical activity, or choose foods low in nutrients and high in sugar are at a higher risk. In contrast, people who have a BMI in the overweight or obese category but exercise regularly and have normal levels of blood sugar, blood pressure, and cholesterol are at overall lower risk.

What about children?

Although a child’s BMI is calculated with the same formula as for adults, the numbers are interpreted differently. Because body fat changes with age and gender, calculating a child’s body mass index must consider these factors. For children, there is a different chart that shows BMI as a percentile ranking.

Do I need to know my numbers?

Many factors can affect a person’s BMI. For tall people and people with more than average muscle mass, the BMI measurement will not be an effective calculation for determining a person’s health.
For the average person the BMI measurement is one measurement to consider. The best plan is to practice healthy lifestyle habits. Eat at least 5 servings of fruits and vegetables daily, exercise regularly, quit smoking, and practice moderation with alcohol. For more information about personalized health risks, speak with a family medicine healthcare provider.
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